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Originally appearing in Volume V19, Page 59 of the 1911 Encyclopedia Britannica.
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MUSCLES OF THE LOWER EXTREMITY.—On the front of the thigh the quadriceps extensor muscles are the most important: there are four of these, the rectus femoris (fig. i) with its straight and reflected heads rising from just above the acetabulum, the crureus, deep to this, from the front of the femur, and the vastus externus and internus wrapping round the femur on each side from the linea aspera. All these are inserted into the patella, or rather the patella is a sesamoid bone developed where their common tendon passes round the lower a a, The extensor tendon. e, An interosseous muscle. b, Deep flexor. f, Tendinous expansion from the lum- c, Superficial flexor. brical and interosseous muscles d, A lumbrical muscle. joining the extensor tendon. end of the femur when the knee is bent. The distal part of this tendon, which passes from the patella to the tubercle of the tibia, is the ligamentum patellae. The sartorius is a long riband-like muscle running from the anterior superior spine of the ilium to the inner surface of the tibia, obliquely across the front of the thigh. It forms the outer boundary of Scar pa's triangle, the inner limit of which is the adductor longus and the base Poupart's ligament. The floor is formed by the sliacus from the iliac fossa of the pelvis, which joins the psoas, to be inserted with it into the lesser trochanter, and by the pectineus running from the upper ramus of the pubis to List below the insertion of the last muscles. The adductor muscles, longus, brevis and magnus, all rise from the subpubic arch, and are inserted into the linea aspera of the femur, so that they draw the femur toward the middle line. The gracilis (fig. so) is part of the adductor mass, though its insertion is into the upper part of the tibia. The extensor muscles of the front of the thigh are supplied by the anterior crural nerve, but the adductor group on the inner side from the obturator. The pectineus is often supplied from both sources. On the back of the thigh the gluteus maximus (figs. 5 and so) plays an important part in determining man's outline (see ANATOMY: Superficial and Artistic). It rises from the sacral region, and is inserted into the upper part of the femur and the deep fascia of the thigh, which is very thick and is known as the fascia lata; the muscle is a great extensor of the hip and raises the body from the stooping position. The gluteus medius rises from the ilium, above the hip joint, and passes to the great trochanter; it abducts the hip and enables the body to be balanced on one leg, as in taking a step for-ward. The gluteus minimus is covered by the last muscle, and passes from the ilium to the front of the great trochanter, thus rotating the hip joint inward. Some of its anterior fibres are sometimes separate from the rest, and are then called the scansorius (see Jowl's). When the gluteus maximus is removed, a number of short externally rotating muscles are seen, rising from the pelvis and inserted into the great trochanter (fig. so); these are, from above downward, the pyriformis, gemellus superior, obturator internus, gemellus inferior and quadratus femoris. They are all supplied by special branches of the sacral plexus. On cutting the quadratus femoris a good deal of the obturator externus can be seen, coming from the outer surface of the obturator membrane and passing to the digital fossa of the great trochanter. Unlike the rest of this group, it is supplied by the obturator nerve. Coming from the anterior part of the crest of the ilium is the tensor fasciae femoris, which is inserted into the fascia lata, as is part of the gluteus maximus, and the thickened band of fascia which runs down the outer side of the thigh from these to the head of the tibia is known as the ilio tibial band. The tensor fasciae femoris, gluteus medius and minimus, are supplied by the superior gluteal nerve, the gluteus maximus by the inferior gluteal. At the back of the thigh are the hamstrings rising from the tuberosity of the ischium (fig. to); these are the semimembranosus and semitendinosus, passing to the inner part of the upper end of the tibia and forming the internal hamstrings, and the biceps femoris or external hamstring, which has an extra head from the shaft of the femur and is inserted into the head of the fibula. These muscles are supplied by the great sciatic nerve and extend the hip joint while they flex the knee. In the leg, as distinguished from the thigh, are three groups of muscles, anterior, external and posterior. The anterior group (fig. ii) all come from the front of the tibia and fibula, and consist of the extensor longus digitorum, extending the middle and distal phalanges of the four outer toes, the extensor proprius hallucis, extending the From A. M. Paterson, Cunningham's Text Book of Anatomy., Fxo. so.—The Muscles on the Back of the Thigh. The posterior group is divided into a superficial and a deep set. The superficial is composed of the gastrocnemius, the two heads of which rise from the two condyles of the femur, the soleus, which rises from the upper parts of the back of the tibia and fibula, the plantaris, which comes from just above the external condyle of the femur, and the popliteus which, although on a deeper plane, really belongs to this group and rises by a tendon from the outer condyle while its fleshy part is inserted into the upper part of the back of the tibia. The gastrocnemius and soleus unite to form the tendo Achillis, which is attached to the posterior part of the calcaneum, while the plantaris runs separately as a very thin tendon to the same place. These muscles are supplied by the internal popliteal nerve. The deep set is formed by three muscles which rise from the posterior surfaces of the tibia and fibula, the flexor longus digitorum, the tibialis posticus, and the flexor longus hallucis from within outward. Their tendons all pass into the sole, that of the flexor longus digitorum being inserted into the terminal phalanges of the four outer toes, the flexor longus hallucis into the terminal phalanx of the big toe, while the tibialis posticus sends expansions to most of the tarsal bones. The nerve supply of this group is the posterior tibial. On the dorsum of the foot is the extensor brevis digitorum (fig. II). which helps to extend From A. M. Paterson, Cunningham's Text Book of Anatomy. the four inner toes, while in the sole are four layers of short muscles, the most superficial of which consists of the abductor hallucis, the flexor brevis digitorum, and the abductor minimi digiti, the names of which indicate their attachments. The second layer is formed by muscles which are attached to the flexor longus digitorum tendon; they are the accessorius, running forward to the tendon from the lower surface of the calcaneum, and the four lumbricales, which rise from the tendon after it has split for the four toes and pass between the toes to be inserted into the tendons of the extensor longus digitorum on the dorsum. The third layer comprises the flexor brevis hallucis, adductor obliquus and adductor transversus hallucis and the flexor brevis minimi digiti. The fourth layer contains the three plantar and four dorsal interosseous muscles, rising from the metatarsal bones and inserted into the proximal phalanges and extensor tendons in such awa.y that the plantar muscles draw the toes towards the line of the second toe while the dorsal draw them away from that line. Of these sole muscles the flexor brevis digitorum, flexor brevis hallucis, abductor hallucis and the innermost lumbrical are supplied by the internal plantar nerve, while all the rest are supplied by the external plantar. Embryology. The development of the muscular system is partly known from the results of direct observation, and partly inferred from the study of the part of the nervous system whence the innervation is derived. The unstriped muscle is formed from the mesenchyme cells of the somatic and splanchnic layers of the mesoderm (see EMBRYOLOGY). but never, as far as we know, from the mesodermic somites. The heart muscle is also developed from mesenchymal cells, though the changes producing its feebly striped fibres are more complicated. The skeletal or real striped muscles are derived either from the meso- dermic somites or from the branchial arches. As the mesodermic somites are placed on each side of the neural canal in the early embryo, it is obvious that the greater part of the trunk musculature spreads gradually round the body from the dorsal to the ventral side and consists of a series of plates called myotomes (fig. 12). The muscle fibres in these plates run in the long axis of the embryo, and are at first separated from those of the two neighbouring plates by thin fibrous intervals called myocommata. In some cases these From A. M. Paterson, Cunningham's Text Book of Anatomy. N, I, 2, 3, 4, Last persisting cephalic myotomes. C, T, L, S, Co., The myotomes of the cervical, thoracic, lumbar. sacral and caudal regions. I., II., III., IV., V., VI., VII., VIII., IX., X., XI., XIL, Refer to the cranial nerves and the structures with which they may be embryologically associated. myocommata persist and even become ossified, as in the ribs, but more usually they disappear early, and the myotomes then unite with one another to form a great muscular sheet. In the whole length of the trunk a longitudinal cleavage at right angles to the surface occurs, splitting the musculature into a dorsal and ventral part, supplied respectively by the dorsal and ventral primary divisions of the spinal nerves. From the dorsal part the various muscles of the erector spinae series are derived by further longitudinal cleavages either tangential or at right angles to the surface, while the ventral part is again longitudinally split into mesial and lateral portions. A transverse section of the trunk at this stage, therefore, would show the cut ends of three longitudinal strips of muscle: (I) a mesial ventral, from which the rectus, pyramidalis sterno-hyoid, omohyoid and sterno-thyroid muscles are derived ; (2) a lateral ventral, forming the flat muscles of the abdomen, intercostals and part of the sternomastoid and trapezius; and (3) the dorsal portion already noticed. The mesial ventral part is remarkable for the persistence of remnants of myocommata in it, forming the lineae transversae of the rectus and the central tendon of the omo-hyoid. The lateral part in the abdominal region splits tangentially into three Iayers the external and internal oblique and the transversalis, the fibres of which become differently directed. In the thoracic region the intercostals probably indicate a further tangential splitting of the middle or internal oblique layer, because the external oblique is continued headward superficially to the ribs and the transversalis deeply to them. The more cephalic part of the external oblique layer probably disappears by a process of pressure or crowding out owing to the encroachment of the serratus magnus, a muscle which its nerve supply indicates is derived from the lower cervical myotomes. The deeper parts of the lateral mass of muscles spread to the ventral surface of the bodies of the vertebrae, and form the hypaxial muscles--such as the psoas, longus colli and recti capitis antics. The nerve supply indicates that the lowest myotomes taking part in the formation of the abdominal walls are those supplied by the first and second lumbar nerves, and are represented by the cremaster muscle in the scrotum. In the perineum, however, the third and fourth sacral myotomes are represented, and these muscles are differentiated largely from the primitive sphincter which surrounds the cloacal orifice, though partly from vestigial tail muscles (see P. Thompson, Journ. Anat. and Phys., vol. xxxv; and R. H. Paramore, Lancet, May 21, 1910). In the head no distinct myotomes have been demonstrated in the mammalian embryo, but as they are present in more lowly vertebrates, it is probable that their development has been shirred over, a process often found in the embryology of the higher forms. Probably nine cephalic myotomes originally existed, of which the first gives rise to the eye muscles supplied by the third nerve, the second to the superior oblique muscle supplied by the fourth nerve, and the third to the external rectus supplied by the sixth nerve. The fourth, fifth and sixth myotomes are sup-pressed, but the seventh, eighth and ninth possibly form the muscles of the tongue supplied by the twelfth cranial nerve. Turning now to the branchial arches, the first branchiomere is innervated by the fifth cranial nerve, and to it belong the masseter, temporal, pterygoids, anterior belly of the digastric, mylo-hyoid, tensor tympani and tensor palati, while from the second branchiomere, supplied by the seventh or facial nerve, all the facial muscles of expression and the stylo-hyoid and posterior belly of the digastric are derived, as well as the platysma, which is one of the few remnants of the panniculus carnosus or skin musculature of the lower mammals. From the third branchiomere, the nerve of which is the ninth or glossopharyngeal, the stylo-pharyngeus and upper part of the pharyngeal constrictors are formed, while the fourth and fifth gill arches give rise to the muscles of the larynx and the lower part of the constrictors supplied by the vagus or tenth nerve. It is possible that parts of the sterno-mastoid and trapezius are also branchial in their origin, since they are supplied by the spinal accessory or eleventh nerve, but this is unsettled. The limb musculature is usually regarded as a sleeve-like outpushing of the external oblique stratum of the lateral ventral musculature of the trunk, and it is believed that parts of several myotomes are in this way pushed out in the growth of the limb bud. This process actually occurs in the lower vertebrates, and the nerve supplies provide strong presumptive evidence that this is the real phylogenetic history of the higher forms, though direct observation shows that the limb muscles of mammals are formed from the central mesoderm of the limb and at first are quite distinct from the myotomes of the trunk. A possible explanation of the difficulty is that this is another example of the slurring over of stages in phylogeny, but this is one of many obscure morphological points. The muscles of each limb are divided into a dorsal and ventral series, supplied by dorsal and ventral secondary divisions of the nerves in the limb plexuses; and these correspond to the original position of the limbs as they grow out from the embryo, so that-in the upper extremity the back of the arm, forearm and dorsum of the hand are dorsal, while in the lower the dorsal surface is the front of the thigh and leg and the dorsum of the foot. For further details see Development of the Human Body, by J. P. McMurrich (London, 1906), and the writings of L. Bolk, Morphol. Jahrb. vols. xxi-xxv. Compar'ttive Anatomy. In the acrania (e.g. amphioxus) the simple arrangement of myotomes and myocommata seen in the early human embryo is permanent. The myotomes or muscle plates are < shaped, with their apices pointing towards the head end, each being supplied by its own spinal nerve. In the fishes this arrangement is largely persistent, but each limb of the < is bent on itself, so that the myotomes have now the shape of a the central angle of which corresponds to the lateral line of the fish. In the abdominal region, however, the myotomes fuse and rudiments of the recti and obliqui abdominis muscles of higher types are seen. In other regions too, such as the fins of fish and the tongue of the Cyclostomata (lamprey), specialized muscular bundles are separated off and are coincident with the acquirement of movements of these parts in different directions. In the Amphibia the limb musculature becomes much more complex as the )joints are formed, and many of the muscles can be homologized with those of mammals, though this is by no means always the case, while, in the abdominal region, a superficial delamination occurs, so that in many forms a superficial and deep rectus abdominis occurs as well as a cutaneus abdominis delaminated from the external oblique. It is probable that this delamination is the precursor ofthe panniculus carnosus or skin musculature of mammals. The branchial musculature also becomes much more complex, and the mylo-hyoid muscle, derived from the first branchial arch and lying beneath the floor of the mouth, is very noticeable and of great importance in breathing. In the reptiles further differentiation of the muscles is seen, and with the acquirement of costal respiration the external and internal. intercostals are formed by a delamination of the internal oblique stratum. In the dorsal region several of the longitudinal muscles which together make up the erector spinae are distinct, and a very definite sphincter cloacae is formed round and cloacal aperture. In mammals certain muscles vary in their attachments or presence and absence in different orders, sub-orders and families, so that, were it not for the large amount of technical knowledge required in recognizing them, they might be useful from a classificatory point of view. There is, however, a greater gap between the musculature of Man and that of the other Primates than there is between many different orders, and this is usually traceable either directly or indirectly to the assumption of the erect position. The chief causes which produce changes of musculature are: (1) splitting, (2) fusion, (3) suppression either partial or complete, (4) shifting of origin, (5) shifting of insertion, (6) new formation, (7) transference of part of one muscle to another. In many of these cases the nerve supply gives an important clue to the change which has been effected. Splitting of a muscular mass is often the result. of one part of a muscle being used separately, and a.good example of this is the deep flexor mass of the forearm. In the lower mammals this mass rises from the flexor surface of the radius and ulna, and supplies tendons to the terminal phalanges of all five digits, but in man the thumb is used separately, and, in response to this, that part of the mass which goes to the thumb is completely split off into a separate muscle, the flexor longus pollicis. The process, however, is going farther, for we have acquired the habit of using our index finger alone for many purposes, and the index slip of the flexor Lapis digitorum is in us almost as distinct a muscle as the flexor longus pollicis. Fusion may be either collateral or longitudinal. The former is seen in the case of the flexor carpi ulnaris. In many mammals (e.g. the dog), there are two muscles inserted separately into the pisiform bone, one rising from the internal condyle of the humerus, the other from the olecranon process, but- in many others (e.g. man) the two muscles have fused. Longitudinal fusion: is seen in the digastric, where the anterior belly is part of the first (mandibular) branchial arch and the posterior of the second or hyoid arch; in this case, as one would expect, the anterior belly is supplied by the fifth nerve and the posterior by the seventh. Partial suppression of a muscle is seen in the rhomboid sheet; in the lower mammals this rises from the head, neck and anterior (cephalic) thoracic spines, but in man the head and most of the neck part is completely sup-pressed. Complete suppression of a muscle is exemplified in the omo-trachelian, a muscle which runs from the cervical vertebrae to the acromian process and fixes the scapula for the strong action of the triceps in pronograde mammals; in man this strong action of the triceps is no longer needed for progression, and the fixing muscle has disappeared. Shifting of origin is seen in the short head of the biceps femoris. This in many lower mammals (e.g. rabbit) is a muscle running from the tail to the lower leg; in many others (e.g. monkeys and man) the origin has slipped down to the femur, and in the great anteater it is evident that the agitator caudae has been used as a muscle slide, because the short head of the biceps or tenuissimus has once been found rising from the surface of this muscle. Shifting of an insertion is not nearly as common as shifting of an origin; it 1s seen, however, in the peroneus tertius of man, in. which part of the extensor longus digitorum has acquired a new attachment to the base of the fifth metatarsal bone. The new' formation of a muscle is seen in the stylo-hyoideus alter, an occasional human muscle; in this the stylo-hyoid ligament has been converted into a muscle. The transference of part of one muscle to another is well shown by the human adductor magnus; here the fibres which pass from the tuber ischii to the condyle of the femur have a nerve supply from the great sciatic instead of the obturator, and in most lower mammals are a separate part of the hamstrings known as the presemimembranosus. For further details see Bronn's Classen and Ordnungen des Thier-: reichs; " The Muscles of Mammals," by F. G. Parsons, Jour. Anat. and Phys, xxxii. 428; also accounts of the musculature of mammals, by Windle and Parsons, in Proc. Zool. Soc. (1894, seq.); I-Iumphry, Observations in Myology (1874). (F. G. P.)

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