. The comparative anatomy of the domesticated animals. Veterinary anatomy. 908 EMBRYOLOGY. Fi5. 434. it stops about the middle of the sacrum in the equine foetus, but ascends higher in the other species. During this apparently ascensional movement is developed the filum terminale, and the nerves of the cauda equina. The walls of the medullary canal, at first very thin, increase in thickness by the appearance of the nervous matter of the cord. Soon they divide into two layers : an internal, the epithelium of the central canal; the other external, the grey substance of the cord. Gradually the ca
. The comparative anatomy of the domesticated animals. Veterinary anatomy. 908 EMBRYOLOGY. Fi5. 434. it stops about the middle of the sacrum in the equine foetus, but ascends higher in the other species. During this apparently ascensional movement is developed the filum terminale, and the nerves of the cauda equina. The walls of the medullary canal, at first very thin, increase in thickness by the appearance of the nervous matter of the cord. Soon they divide into two layers : an internal, the epithelium of the central canal; the other external, the grey substance of the cord. Gradually the canal contracts, and the marrow is seen, with its longitudinal furrows. At the end of the first month, the iaferior roots are in existence, as well as the spinal ganglia ; the latter are developed at the expense of the proto- vertebrsB, as will be shown presently. The superior roots are formed some time afterwards. The envelopes of the cerebro-spinal centres are furnished by the proto- vertebral laminaB; they are developed after the sixth week, progressing with the growth of the parts they are destined to cover. 3. Development of the Nerves.— The development of the nerves is some- what obscure. The motor roots seem to arise from the cord; but the spinal gan- glia are formed separately in the pro- tovertebrse, and perhaps originate the sensitive roots. The nervous ramifica- tions begin by elongated ramifying cells, which become fused to each other by their extremities; the nuclei of the cells lying at the periphery becoming the nuclei of the white substance of Schwann, the proper nervous tissue being afterwards gradually deposited between the axis- cylinder and the envelope. The gi-eat sympathetic is early seen as a nodulated cord. It is probably de- lamina ; df, Intestinal fibrous lamina, yeloped in the Same manner as the other dd. Intestinal glandular lamina; dr, ^ ^^ Pi-imitive intestinal groove; h, Corneal lamina; mr, Medullary tube—spinal cord; m, Muscular lamina; p, Pleu
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