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Originally appearing in Volume V25, Page 670 of the 1911 Encyclopedia Britannica.
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MID.VENTRAL LA 14A (From D. J. Cunningham in Cunningham's Text-Book of Anatomy.) Flo. 5.—Schema of a' Transverse Section through the Early Neural Tube (Young). The left side of the section shows an earlier stage than the right side. of the canal and shoot out long processes—the axons. The permanent central canal of the cord was formerly said only to represent the ventral end of the large embryonic canal, the dorsal part being converted into a slit by the gradual closing in of its lateral walls, thus forming the postero-median fissure. A, Robinson, however, does not believe that the posterior fissure is any remnant of the central canal, and there are many points which bear out his contention (Studies in Anatomy, Owens College, 1891). The most modern view (1908) is that the fissure is formed partly by an infolding and partly from the original central canal. The antero-median fissure is caused by the ventral part of the cord growing on each side, but not in the mid-line where no germinal cells are. The anterior nerve roots are formed by the axons of the ,neuroblasts in the developing anterior cornea, but the posterior grow into the cord from the posterior root ganglia (see NERVE: Spinal), and, as they grow, form the columns of Goll and Burdach. That part of the grey matter from which the ventral, anterior or motor nerve roots rise is known as the basal lamina of the cord, while the more dorsal part into which the posterior nerve roots enter is the alar lamina. These parts are important in comparing the morphology of the spinal cord with that of the brain. In the embryo up to the fifth month there is little difference in the appearance of the grey and white matter of the cord, but at that time the fibres in the columns of Burdach acquire their medullary sheaths or white substance of Schwann, the fatty matter of which is probably abstracted from the blood. Very soon after these the basis bundles myelenate and then, in the sixth month, the columns of Goll. Next follow the direct cerebellar tracts and, in the latter half of the eignch month the tracts of Gowers, while the fibres of the pyramidal, and Lissauer's tracts do. not gain their medullary sheaths, until just before or after birth. At first the spinal cord exends as far as the last mesodermal somite, but neuroblasts are only formed as far as the first coccygeal somite, so that behind that the cord is non-nervous and degenerates later into the filum terminate. After the fourth month the nervous portion grows more slowly than the rest of the body and so the long cauda equina and fdum terminate are produced. At birth the lower limit of the cord is opposite the third lumbar vertebra, but in post-natal development it recedes still farther to the, lower level of the first. For further details see Quain's Anatomy, vol. i. (London, 1908); J. P. McMurrich, Development of the Human Body (1906). Most modern descriptions are founded on the writings of W. His, references to which and to other literature will be found on p. 463 of McM urrich's book. CORP Comparative Anatomy.—In the Amphioxus. there is little difference between the spinal cord and the brain; the former reaches the whole length of the body and is of uniform calibre. It encloses a central canal from which, a dorsal fissure extends to the surface of the cord and it is composed of nerve fibres and nerve cells; most of the latter being grouped round the central canal or neurocoele, as they are in the human embryo. Some very large multipolar ganglion cells are present, and there are also large fibres known as giant fibres, the function of. which is. not clear. When the reptiles are reached the cord shows slight enlargements in the regions of the limbs and these become more marked in birds and mammals. In the lumbar region of birds the dorsal columns diverge and open up the central. canal, converting it into a diamond-shaped space which is only, roofed over by the membranes of the cord, and is known as the. sinus rhomboidalis. In all these lower vertebrates except the Anura (frogs and toads), the cord fills the whole length of the spinal canal, but in the higher mammals (Primates, Chiroptera and Insectivore) it grows less rapidly, and so the posterior part of the canal contains the cauda equina within its. sheath of dura mater. In mammals below the anthropoid apes there. are no direct, pyramidal tracts in the cord, since the decussation of the pyramids in the medulla is complete. Moreover, the crossed tracts vary very much in their proportional size to the rest of the cord in different animals. In man, for example, they form 11.87 % of the total cross area of the cord, in the cat 7.76%, in the rabbit 5.3 %, in the guinea-pig 3 %, and in the mouse 1.14 %. In the frog no pyramidal tract is found. It is obvious, there-fore, that in the lower vertebrates the motor fibres of the cord are not so completely gathered into definite tracts as they are in man. A good deal of interest has lately been taken in a nerve bundle which in the lower vertebrates runs through the centre of the central canal of the cord, and takes its origin in the optic reflex cells in close relation to the posterior commissure of the brain. More posteriorly (caudad) it probably acquires a connexion with the motor cells of the cord and is looked upon as a means by which the muscles can be made to actively respond to the stimulus of light. " It is known as Reissner'sfibre, and its morphology and physiology have been studied most carefully in cyclostomes and fishes. It is said to be present in the mouse, but hitherto no trace of it has been found in man. It was discovered in 186o, but for forty years has been looked upon as an artifact. See P. E. Sargent, " Optic Reflex Apparatus of Vertebrates," Bull. Mus. Comp. Zool. Harvard, vol. xlv. No. 3 (July, 1904) ; also for general details R, Wiedersheim, Comparative Anatomy of Vertebrates (London, 1907)";Lenhosrsek, Bau des Nervensystem.i (1895). (F. G. P.)
End of Article: MID
MICROTOMY (Gr. -mµ17; r ivsu', to cut)

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