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AND SEROUS

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Originally appearing in Volume V01, Page 670 of the 1911 Encyclopedia Britannica.
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AND SEROUS MEMBRANES). The intestine now grows very rapidly and is thrown into a series of coils; the caecum ascends and passes to the right ventral to the duodenum, and presses it against the dorsal wall -of the abdomen; then it descends toward its permanent position in the right iliac fossa. From the ventral surface on the hinder (caudal) closed end of the intestinal tube the allantois grows to form the placenta and bladder (see URINARY SYSTEM, REPRODUCTIVE SYSTEM and PLACENTA), and this region is the cloaca into which the alimentary, urinary and generative canals or ducts all open, but later two lateral folds appear which, by their union, divide the cloaca into a ventral and a dorsal part, the former being genito-urinary and the latter alimentary or intestinal. In this way the rectum or dorsal compartment is shut off from the genito-urinary. Later an ectodermal invagination at the hind end of the embryo develops and forms the anal canal; this is the proctodaeum, and for some time it is separated from the hind (caudal) end of the rectal part of the mesodaeum (or part of the intestinal canal formed from the mesoderm) by a membrane called the anal membrane. This is eventually absorbed and the digestive tract now communicates with the surface by the anus. F. Wood Jones (British Medical Journal, 17th of December 1904) has given a somewhat different description of the development of the cloaca and anus, which better explains the various abnormalities met with in this region but requires further confirmation before it is generally accepted. For the development of the mouth, pharynx, lungs, liver and pancreas from the primitive alimentary canal, the reader is referred to the special articles on those structures. (For further details, see W. His, Anatomie menschlicher Embryonen (Leipzig, 188o–1885); C. S. Minot's Embryology (New York, 1897); and J. P. M'Murrich, Development of the Human Body (London, 1906). (F. G. P.) Comparative Anatomy.—The primitive condition of the vertebrate alimentary canal may be described as a straight, simple tube, consisting of an anterior portion, the stomodaeum, formed by an ectodermal invagination, the mesenteron, a long median portion lined by endoderm, and a short posterior portion, the proctodaeum, formed by ectodermal invagination. In the lower vertebrates the primitive tube subserved also the purpose of respiration, and traces of the double function remain in the adult structure of all vertebrates (see MOUTH, PHARYNX). In fish, the pharynx, or branchial region, suddenly becomes narrower, posterior to the gill-slits, to form the oesophagus; in higher animals the oesophagus, in the adult, is separated from the primitive pharyngeal region and lies dorsal to it. Probably, in the primitive vertebrata, the entire alimentary canal was lined with ciliated cells. Traces of this ciliation persist in many living forms. In the Ammocoete, the larval form of Petromyzon (see CYCLOSTOMATA), the whole canal is ciliated except the pharynx and the rectum; in the Dipnoi the epithelium of the stomach and the intestines is ciliated; in Selachii that of the posterior part of the gullet, and the spiral valve, is ciliated; extensive ciliation may occur in almost any region of the gut of the lower teleostomes, but in the higher forms (Teleostei) it is generally absent. In the latter, however, and in higher groups of vertebrates, a peculiar striated border on the columnar cells lining the intestinal tract has been held to be a final trace of ancestral ciliation. The alimentary canal may be conveniently described in three divisions, the oesophagus or gullet, the passage by which food reaches the stomach, the stomach, typically an expanded region in which the food remains for a considerable time and is mechanically pulped, mixed with mucus and certain digestive juices (see NUTRITION) and partly macerated, the intestinal tract or gut, extending from the distal end of the stomach to the cloaca or anus, in which the food is subjected to further digestive action, but which is above all the region in which absorption of the products of digestion takes place, the refuse material together with quantities of waste matter entering the gut from the blood and liver being gradually passed towards the anus for discharge from the body. The oesophagus is essentially merely a passage, as straight as may be, from the pharynx to the stomach, varying in length with the length of the neck and thoracic regions in different animals, and in calibre with the nature of the food. It is almost invariably lined with a many-layered epithelium, forming a tough coating, readily repaired and not easily damaged by hard food masses. It is occasionally separated from the stomach by a slight constriction which may be capable of contraction so as to prevent regurgitation. There are few exceptions to this structural and functional simplicity. In fishes (see ICHTHYOLOGY, Anatomy) the swim-bladder is developed as a dorsal outgrowth of the oesophagus and may remain in open connexion with it. In certain Teleosteis (e.g. Lutodeira) it is longer than the length it has to traverse and is thrown into convolutions. In many other fish, particularly Selachiis, a set of processes of the lining wall project into the cavity near the stomach and have been supposed to aid in preventing food particles, or living creatures swallowed without injury, escaping backwards into the mouth. In some egg-eating snakes the sharp tips of the ventral spines (hypapophyses) of the posterior cervical vertebrae penetrate the wall of the oesophagus and are used for breaking the shells of the eggs taken as food. In some aquatic Chelonians, the food of which consists chiefly of seaweeds, the lining membrane is produced into pointed processes backwardly directed. In birds this region frequently presents peculiarities. In Opisthocomus it forms an enormously wide double loop, hanging down over the breast-bone, which is peculiarly flattened and devoid of a keel in the anterior portion. In many birds part of the oesophagus may be temporarily dilated, forming a " crop," as for instance in birds of prey and humming birds. In the flamingo, many ducks, storks, and the cormorant the crop is a permanent although not a highly specialized enlargement. Finally, in the vast majority of seed-eating birds, in gallinaceous birds,. pigeons, sandgrouse, parrots and many Passeres, particularly the finches, the crop is a permanent globular dilatation, in which the food is retained for a considerable time, mixed with a slight mucous secretion, and softened and partly macerated by the heat of the body. Many birds feed their young from the soft contents of the crop, and in pigeons, at the breeding season, the cells lining the crop proliferate rapidly and are discharged as a soft cheesy mass into the cavity, forming the substance known as pigeon's milk. Amongst Mammalia, in Rodentia, Carnivora, elephants and ruminants, the wall of the oesophagus contains a layer of voluntary muscle, by the contraction of which these animals induce anti-peristaltic movements and can so regurgitate food into the mouth. Stomach.—Where the oesophagus passes into the stomach, the lining wall of the alimentary tract changes from a many-layered epithelium to a mucous epithelium, consisting of a single layer of endodermal cells, frequently thrown into pits or projecting as processes; from being chiefly protective, it has become secretory and absorbing, and maintains this character to the distal extremity where it passes into the epiblast of the proctodaeum. In most cases the course of the alimentary canal from the distal end of the oesophagus to the cloaca or anus is longer than the corresponding region of the body, and the canal is therefore thrown into folds. The fundamental form of the stomach is a sac-like enlargement of the canal, the proximal portion of which is continuous with the line of the oesophagus, but the distal portion of which is bent in the proximal portion, the whole forming an enlarged bent tube. At the distal end of the tube the intestinal tract proper begins, and the two regions are separated by a muscular constriction. In fishes the stomach is generally in one of two forms; it may be a simple bent tube, the proximal limb of which is almost invariably much wider than the distal, anteriorly directed limb; or the oesophagus may pass directly into an expanded, globular or elongated sac, from the anterior lateral wall of which, not far from the oesophageal opening, the duodenum arises. In Batrachia and Reptilia the stomach is in most cases a simple sac, marked off from the oesophagus only by increased calibre. In the Crocodilia, however, the anterior portion of the stomach is much enlarged and very highly muscular, the muscles radiating from a central tendinous area on each ofthe flattened sides. The cavity is lined by a hardened secretion and contains a quantity of pebbles and gravel which are used in the mechanical trituration of the food, so that the resemblance to the gizzard of birds is well marked. This muscular chamber leads by a small aperture into a distal, smaller and more glandular chamber. In birds the stomach exhibits two regions, an anterior glandular region, the proventriculus, the walls of which are relatively soft and contain enlarged digestive glands aggregated in patches (e.g. some Steganopodes), in rows (e.g. most birds of prey) or in a more or less regular band. The distal region is larger and is lined in most cases by a more or less permanent lining which is thick and tough in birds with a muscular gizzard, very slight in the others. In many birds, specially those feeding on fish, the two regions of the stomach are of equal width, and are indistinguishable until, on opening the cavity, the difference in the character of the lining membrane becomes visible. In other birds the proventriculus is separated by a well-marked constriction from the posterior and larger region. In graminiferous forms the latter becomes a thick-walled muscular gizzard, the muscles radiating from tendinous areas and the cavity containing pebbles or gravel. In mammals, the primitive form of the stomach consists of a more or less globular or elongated expansion of the oesophageal region, forming the cardiac portion, and a forwardly curved, narrower pyloric portion, from which the duodenum arises. The whole wall is muscular, and the lining membrane is richly glandular. In the Insectivora, Carnivora, Perissodactyla, and in most Edentaia, Chiroptera, Rodentia and Primates, this primitive disposition is retained, the difference consisting chiefly in the degrees of elongation of the stomach and the sharpness of the distal curvature. In other cases the cardiac portion may be prolonged into a caecal sac, a condition most highly differentiated in the blood-sucking bat, Desmodeus, where it is longer than the entire length of the body. There are two cardiac extensions in the hippopotamus and in the peccary. In many other mammals one, two or three protrusions of the cardiac region occur, whilst in the manatee and in some rodents the cardiac region is constricted off from the pyloric portion. In the Artiodactyla the stomach is always complex, the complexity reaching a maximum in ruminating forms. In the Suidae a cardiac diverticulum is partly constricted from the general cavity, forming an incipient condition of the rumen of true ruminants; the general cavity of the stomach shows an approach to the ruminant condition by the different characters of the lining wall in different areas. In the chevrotains, which in many other respects show conditions intermediate between non-ruminant artiodactyles and true ruminants, the oesophagus opens into a wide cardiac portion, incompletely divided into four chambers. Three of these, towards the cardiac extremity, are lined with villi and correspond to the rumen or paunch; the fourth, which lies between the opening of the oesophagus and the pyloric portion of the stomach, is the ruminant reticulum and its wall is lined with very shallow " cells." A groove runs along its dorsal wall from the oesophageal aperture to a very small cavity lined with low, longitudinally disposed folds, and forming a narrow passage between the cardiac and pyloric divisions; this is an early stage in the development of the omasum, psalterium or manyplies of the ruminant stomach. The fourth or true pyloric chamber is an elongated sac with smooth glandular walls and is the abomasum, or rennet sack. In the camel the rumen forms an enormous globular paunch with villous walls and internally showing a trace of division into two regions. It is well marked off from the reticulum, the " cells" of which are extremely deep, forming the well-known water-chambers. The psalterium is sharply constricted off from the reticulum and is an elongated chamber showing little trace of the longitudinal ridges characteristic of this region; it opens directly into the relatively small abomasum. In the true ruminants, the rumen forms a capacious, villous reservoir, nearly always partly sacculated, into which the food is passed rapidly as the animal grazes. The food is subjected to a rotary movement in the paunch, and is thus repeatedly subjected tc moistening with the fluids secreted by the reticulum, as it is passed over the aperture of that cavity, and is formed into a rounded bolus. Most ruminants swallow masses of hairs, and these, by the rotary action of the paunch,.are aggregated into peculiar dense, rounded balls which are occasionally discharged from the mouth and are known as " hair-balls " or " bezoars." The food bolus, when the animal is lying down after grazing, is passed into the oesophagus and reaches the mouth by anti-peristaltic contractions of the oesophagus. After prolonged mastication and mixing with saliva, it is again swallowed, but is now passed into the psalterium, which, in true ruminants, is a small chamber with conspicuous longitudinal folds. Finally it reaches the large abomasum where the last stages of gastric digestion occur. In the Cetacea the stomach is different from that found in any other group of mammals. The oesophagus opens directly into a very large cardiac sac the distal extremity of which forms a long caecal pouch. At nearly the first third of its length this communicates by a narrow aperture into the elongated, relatively narrow pyloric portion. The latter is convoluted and constricted into a series of chambers that differ in different groups of Cetacea. In the Sirenia the stomach is divided by a constriction into a cardiac and a pyloric portion, and the latter has a pair of caeca. In most of the Marsupialia the stomach is relatively simple, forming a globular sac with the oesophageal and pyloric apertures closely approximated; in the kangaroos, on the other hand, the stomach is divided into a relatively small, caecal cardiac portion and an enormously long sacculated and convoluted pyloric region, the general arrangement of which closely recalls the large caecum of many mammals. Intestinal Tract.—It is not yet possible to discuss the general morphology of this region in vertebrates as a group, as, whilst the modifications displayed in birds and mammals have been compared and studied in detail, those in the lower groups have not yet been systematically co-ordinated. Fishes.—In the Cyclostomata, Holocephali and a few Teleostei the course of the gut is practically straight from the pyloric end of the stomach to the exterior, and there is no marked differentiation into regions. In the Dipnoi, a contracted sigmoid curve between the stomach and the dilated intestine is a simple beginning of the complexity found in other groups. In very many of the more specialized teleosteans, the gut is much convoluted, exhibiting a series of watchspring-like coils. In a number of different groups, increased surface for absorption is given, not by increase in length of the whole gut, but by the development of an internal fold known as the spiral valve. This was probably originally a longitudinal fold similar to the typhlosole of chaetopods. It forms a simple fold in the larval Ammocoete, and in its anterior region remains straight in some adult fish, e.g. Polypterus, but in the majority of cases it forms a complex spiral, wound round the inner wall of the expanded large intestine, the internal edge of the fold sometimes meeting to form a central column. It occurs in Cyclostomata, Selachii, Holocephali, Chondrostei, Crossopterygii, Amiidae, Lepidosteidae and Dipnoi. A set of organs peculiar to fish and known as the pyloric caeca are absent in Cyclostomata and Dipnoi, in most Selachii and in Amia, but present, in numbers ranging from one to nearly two hundred, in the vast majority of fish. These are outgrowths of the intestinal tract near the pyloric extremity of the stomach, and their function is partly glandular, partly absorbing. In a few Teleostei there is a single caecal diverticulum at the beginning of the " rectum," and in the same region a solid rectal gland occurs in most elasmobranchs, whilst, again, in the Dipnoi a similar structure opens into the cloaca. These caeca have been compared with the colic caeca of higher vertebrates, but there is yet no exact evidence for the homology. In the Batrachia the course of the intestinal tract is nearly straight from the pyloric end of the stomach to the cloaca, in the case of the perennibranchiates there being no more than a few simple loops between the expanded " rectum " and the straight portion that leaves the stomach. In the Caducibranchiata the anterior end of the enlarged rectum lies very close to the distalextremity of the stomach, and the gut, between these two regions, is greatly lengthened, forming a loop with many minor loops borne at the periphery of an expanse of mesentery, recalling the Meckelian tract of birds and mammals. In the tadpole this region is spirally coiled and is still longer relatively to the length of the whole tract. In Hyla and Pipa there is a small caecum comparable with the colic caecum of birds and mammals. In Reptilia the configuration of the intestinal tract does not differ much from that in Batrachia, the length and complexity of the minor coils apparently varying with the general configuration of the body, that is to say, in reptiles with a long, narrow, and snake-like body the minor loops of the gut are relatively short and unimportant, whilst in those with a more spacious cavity, such as chelonians, many lizards and crocodiles, the gut may be relatively long and disposed in many minor coils. There is comparatively little differentiation between the mid-gut and the gut in cases where the whole gut is long; in the others the hind-gut is generally marked by an increase of calibre. A short caecal diverticulum, comparable with the colic caecum of birds and mammals, is present in many snakes and lizards and in some chelonians. In fishes, batrachians and reptiles the intestinal tract is swung from the dorsal wall of the abdominal cavity by a mesentery which is incomplete on account of secondary absorption in places, and which grows out with the minor loops of the gut. There are also traces, more abundant in the lower forms, of the still more primitive ventral mesentery. Intestinal Tract in Birds and Mammals.—There is no doubt but that the similarity of the modes of disposition of the alimentary tract in birds and mammals points to the probability of the chief morphological features of this region in these animals having been laid down in some common ancestor, although we c.c. Colic caeca. p.v. Cut root of portal vein. d. Duodenum. r.v. Rectal vein. g. Glandular patch. s. Proventriculus. 1.1. Meckel's tract. y. Meckel's diverticulum, or 1.i. Hind-gut. Yolk-sac vestige. . have not yet sufficient exact knowledge of the gut in Pisces, Batrachia and Reptilia to find amongst these with any certainty the most Drobable survival from the ancestral condition. The primitive gut must be supposed to have run backwards from the stomach to the cloaca suspended from the dorsal wall of the body-cavity by a dorsal mesentery. This tract, in the course of phylogeny of the common ancestors of birds and mammals, became longer than the straight length between its extreme points and, consequently, was thrown into a series of folds. The mesentery grew out with these folds, but the presence of adjacent organs, the disturbance due to the outgrowth of the liver, and the secondary relations brought about between different portions of the gut, as the out-growing loops invaded each other's localities, disturbed the primitive simplicity. Three definite regions of outgrowth, however,became conspicuous and are to be recognized in the actual disposition of the gut in existing birds and mammals. The first of these is the duodenum. In the vast majority of birds, and in some of the simpler mammals, the portion of the gut im- mediately distal of the stomach grows out into a long and narrow loop (fig. 4, d), the proximal and distal ends of which are close together, whilst the loop itself may re- main long and narrow, or may develop minor loops on its course. In mammals generally, how- ever, the duodenum is complex and is not so sharply marked off from the distal portion of the gut as in birds. The r second portion is Meckel's tract. It consists of the part generally khown as the FIG. 5.-Intestinal Tract of Canis small intestines, the vulpes. S, cut end of duodenum; C, jejunum and ileum of caecum; R, cut end of rectum. human anatomy, and stretches from the distal end of the duodenum to the caecum or caeca. It is the chief absorbing portion of the gut, and in nearly all birds and mammals is the longest portion. It represents, however, only a very small part of the primitive straight gut, corresponding to not more than two or three somites of the embryo. This narrow portion grows out to form the greater part of what is called the pendent loop in mammalian embryology. Its anterior or proximal end lies close to the approximated 669 birds persists throughout life, forming a convenient point of orientation. In mammals, no doubt in association with the functional reduction of the yolk-sac, this diverticulum, which is known as Meckel's diverticulum, has less importance, and whilst it has been observed in a small percentage of adult human subjects has not been recognized in the adult condition of any lower Mammalia. In birds, Meckel's tract falls into minor folds or loops, the disposition of which forms a series of patterns remarkably different in appearance and characteristic of different groups. In fig. 4 an extremely primitive type is represented. In mammals Meckel's tract remains much more uniform; it may be short, or in-crease enormously in length, but in either case it falls into a fairly symmetrical shape, suspended at the circumference of a nearly circular expanse of mesentery. Where it is short it is thrown into very simple minor loops (figs. 5, 6 and 7); where it is long, these minor loops form a con-voluted mass (figs. 8 and q). The third portion of the gut should be termed the hind-gut and lies between the caecum or caeca and the anus, corresponding to the large intestines, colon and rectum of human anatomy. It is formed from a much larger portion of the primitive straight gut than the duodenum and Meckel's tract together, and its proximal portion, in consequence, lies very close to the origin of the duodenum. In the vast majority of birds, the hind-gut in the adult is relatively extremely short, often being only from proximal and distal ends of the duodenal loop, whilst its distal end passes into the hind-gut at the colic caecum or caeca. In the embryos of all birds and mammals, the median point of Meckel's tract, the part of the loop which has grown out farthest from the dorsal edge of the mesentery, is marked by the diverticulum caecum vitelli, the primitive connexion of the cavity of the gut with the narrowing stalk of the yolk-sac (fig. 4, y). Naturally, in birds where the yolk-sac is of great functional importance this diverticulum is large, and in a majority of the families of one-eighth to one-thirtieth of the whole length of the gut. A certain number of primitive birds, however, have retained a relatively long condition of the hind-gut (fig. 4), the greatest relative length occurring in struthious birds, and particularly in the ostrich, where the hind-gut exceeds in length the duodenum and Meckel's tract together. Mammals may be contrasted with birds as a group in which the hind-gut is always relatively long, sometimes extremely long, and in which, moreover, there is a strong tendency to differentiation of the hind-gut into regions 670 the characters of which are of systematic importance. The first region is the colon, which forms a very simple expansion in mammals such as Carnivora (fig. 5), where the whole hind-gut is relatively short, or a series of simple loops in mammals in which the whole gut has a primitive disposition (e.g. Marsupialia, fig. 6). In the odd-toed Ungulata, the colon (fig. 7) forms an enormously long loop, the two limbs of which are closely approximated and the calibre of which is very large. In Ruminantia (fig. 8) the colon is still more highly differentiated, displaying first a simple wide loop, then a complicated watchspring-like coil, and finally a very long, irregular portion. In the higher Primates (fig. 9) it forms one enormous very wide loop, corresponding to the ascending, transverse and descending colons of human anatomy, and a shorter distal loop, the omega loop of human anatomy. Other striking patterns are displayed in other mammalian groups. The second region of the hind-gut is usually known as the rectum, and although it is some-times lengthened it is typically little longer than the portion of the primitive straight gut that it represents. Adaptations of the Intestinal Tract to Function.—The chief business of the gut is to provide a vascular surface to which the prepared food is applied so that the nutritive material may be absorbed into the system. Overlying and sometimes obscuring the morphological patterns of the gut, are many modifications correlated with the nature of the food and producing homoplastic resemblances independent of genetic affinity. Thus in birds and mammals alike there is a direct association of herbivorous habit with great relative length of gut. The explanation of this, no doubt, is simply that the vegetable matter which such creatures devour is in a form which requires not only prolonged digestive action, but, from the intimate admixture of indigestible material, a very large absorbing surface. In piscivorous birds and mammals, the gut is very long, with a thick wall and a relatively small calibre, whilst there is a general tendency for the regions of the gut to be slightly or not at all defined. Fish, as it is eaten by wild animals, contains a large bulk of indigestible matter, and so requires an extended absorbing surface; the thick wall and relatively small calibre are protections against wounding by fish bones. In frugivorous birds the gut is strikingly short, wide and simple, whilst a similar change has not taken place in frugivorous mammals. Carnivorous birds and mammals have a relatively short gut. In birds, generally, the relation of the length and calibre of the gut to the size of the whole creature is striking. If two birds of similar habit and of the same group be compared, it will be found that the gut of the larger bird is relatively longer rather than relatively wider. The same general rule 'applies to Meckel's tract in mammals, whereas in the case of the hind-gut increase of capacity is given by increase of calibre rather than by increased length. The Colic Caeca.—These organs lie at the junction of the hind-gut with Meckel's tract and are homologous in birds and mammals although it happens that their apparent position differs in the majority of cases in the two groups. In most birds, the hind-gut is relatively very short, and the caecal position, accordingly, is at a very short distance from the posterior end of the body, whereas in most mammals the hind-gut is very long and the position of the caecum or caeca is relatively very much fartherfrom the anus. Next, in most birds, the caeca when present are paired, whereas in most mammals there is only a single caecum. On the other hand, in certain birds (herons) as a normal occur= rence, and in many birds as an individual variation, only a single caecum occurs. In some mammals, e.g. many armadillos, in Hyrax and the manatee, the caeca are normally paired; in many other (e.g. some rodents and marsupials) in addition to the normal caecum there is a reduced second caecum, whilst in quite a number of forms the relation of the caecum, ileum and colon at their junction is readily intelligible on the assumption that the caeca were originally paired. The origin and many of the peculiarities of the ileo-caecal valve find their best explanation on this hypothesis. The caeca are hollow outgrowths of the wall of the gut, the blind ends being directed forwards. The caecal wall is in most cases highly glandular and contains masses of lymphoid tissue. In birds and in mammals this tissue may be so greatly increased as to transform the caecum into a solid or nearly solid sac, the calibre of which is for the most part smaller than that of the unmodified caecum. In some birds, the whole area of the caecum may be modified in this way; in mammals, it is generally the terminal portion, which then becomes the vermiform appendix, familiar in the anthropoid apes, in man and in some rodents. It is difficult to see in this modification merely a degeneration; not improbably it is the formation of a new glandular organ. The caeca exhibit almost every gradation of development, from relatively enormous size to complete absence, and there is no definite, invariable connexion between the nature of the food and the degree of their development. In the case of birds, it may be said that on the whole the caeca are generally large in herbivorous forms and generally small in insectivorous, frugivorous, carnivorous and piscivorous forms, but there are many exceptions. Thus, owls and falcons have a diet that is closely similar, and yet owls have a pair of very long caeca, whilst in the Falconidae these organs are much reduced and apparently functionless. The insectivorous and omnivorous rollers,motmots and bee-eaters have a pair of large caeca, whilst in passerine birds of similar habit the caeca are vestigial glandular nipples. It is impossible to doubt that family history dominates in this matter. Certain families tend to retain the caeca, others to lose them, and direct adaptation to diet appears only to accelerate or retard these inherited tendencies. So also in mammals, no more than a general relation between diet and caecal development can be shown to exist, although the large size of the single caecum of mammals is more closely associated with a herbivorous as opposed to a carnivorous, frugivorous, piscivorous or omnivorous diet than is the case in birds. There is no relationship between diet and the complete or partial presence of both members of the primi-pair of caeca in mammals, the occurrence of the pair being rather an " accident " of inheritance than in any direct relation to function.
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