98 OBSTETRICAL PHYSIOLOGY. 



and the lateral ventricles. The middle vesicle forms the crura cerebri, 

 corpora quadrigcmina, and the aqueduct of Sylvius or middle ventricle. 

 The posterior vesicle gives rise to the medulla oblongata, p>ons varolii, 

 cerebellum, and fourth ventricle. The middle vesicle increases more 

 rapidly in volume at first than the others, but it soon stops and allows 

 the anterior cell to develop : from this time the encephalon assumes its 

 oval shape, with predominance of the anterior part. 



Towards the end of their first third of intra-uterine life, nearly all the 

 parts of the encephalon are distinct ; the two hemispheres are separated 

 by the development of the septum lucidem, and the convolutions are 

 apparent on their surface: while the corpora quad rig em i na emd crura 

 are well defined. At a later period the cerebellum is seen, as well as 

 the 2M US Varolii, corpora restiformia, and corpora pjiramidalia. 



"With regard to the development of the spinal cord, we have observed 

 that the medullary canal is the first trace of this part. It occupies the 

 whole length of the vertebral stalk, and its cavity communicates 

 anteriorly with the fourth ventricle. When the spine is developed the 

 cord only increases longitudinally to a certain degree, and appears to 

 ascend in the canal ; it stops at the middle of the sacrum in the Equine 

 foetus, but ascends higher in the other species. During this apparent 

 ascensional movement is developed the filum terminals, and the termi- 

 nating nerves of the cord (cauda equina). The parietes of the medullary 

 canal are at first very thin, but increase in thickness with the appear- 

 ance of the nervous substance of the cord, and soon divide into two 

 layers : an internal, the epithelium of the central canal ; and an 

 external, the grey substance of the cord. Gradually the canal contracts, 

 and the cord shows longitudinal furrows. At the end of the first month 

 the inferior roots of the nerves are in existence, as well as the spinal 

 ganglia, which are developed at the expense of the proto-vertebrae ; the 

 superior roots are not distinguishable for some time after. The envelopes 

 of the nervous centres are furnished by the proto-vertebral laminae, and 

 are developed after the sixth week, following the formation of the parts 

 they are destined to cover. 



The nerves are not so definite in their development, and some 

 obscurity prevails with regard to them. It would appear that the 

 motor roots originate in the cord, but that the gangha are formed 

 separately in the proto- vertebras, and perhaps become the point of 

 departure of the sensitive roots. The nerve ramifications grow from 

 elongated ramified cells, which are joined by their extremities. The 

 nuclei of the cells, joined to the periphery, become the nuclei of the 

 sheath of Schwann, and the nei^ve tissue is afterwards deposited 

 gradually between the axis-cylinder and the envelope. The great 

 sympathetic nerve is perceived at an early date as a nodulated cord ; it 

 is probably developed in the same manner as the other nerves. 



The Organs of Sense. 



The principal portion of the organs of sense belong to the nervous 

 system, and are, of course, developed with it ; the other portions belong^ 

 to the external epithelial layer, and to the derm or germinative layer. 

 With regard to the organs of vision, two tubulous prolongations arise 

 from the anterior cerebral vesicle and, passing forward, terminate in the 

 primary ocular vesicles, traces of the ocular globes ; the hollow prolong- 

 ments form the optic nerves, and the vesicles furnish the choroid layer 

 and retina. The crystalline lens, vitreous body, cornea, and sclerotica 



