go OBSTETRICAL PHYSIOLOGY. 



The cephalic lamincB always remain adherent to the vertebral laminae^ 

 and are inflected inwards with them to form the anterior part of the 

 cephalo-intestinal cavity, which is divided into two compartments — the 

 pharyngeal and oesophageal cavities. The pharyngeal cavity opens exter- 

 nally by the mouth, and is partly enclosed on the sides by the pharyngeal 

 arches. The oesophageal cavity soon shows a diverticulum, which is not 

 long in communicating with the pleuro-peritoneal cavity, and subse- 

 quently contains the heart ; it is therefore named the cardiac cavity. The 

 cephalic laminas also form the derm of the cranium, and the fibrous layer 

 in which are developed some of the cranial bones. 



The Nervous System. 



The development of the nervous system comprises the growth of the 

 brain, spifial cord, and nerves. The initial steps in the development of 

 the brain and cord have been already indicated. At each extremity of 

 the medullary cavity, which is a modification of the median furrow, is a 

 slight bulging. From the posterior, or rhomboidal sinus, the sacro-lumbar 

 nerves are given off, v/Iiile the anterior gives origin to the brain. This 

 anterior enlargement appears as three successive dilatations, named the 

 cerebral vesicles or cells, which are distinguished as anterior, middle, and' 

 posterior. They are filled with fluid, and the middle slightly surmounts 

 the other two, which gives the whole the figure of a small triangular mass. 

 The vesicles increase irregularly in volume, and their walls, in develop- 

 ing, form the nervous tissue ; while their cavity persists and becomes the 

 space observed in each portion of the encephalon. The a.fiterior vesicle^ 

 represents the cerebral hemispheres, the thalami optici and the lateral 

 ventricles. The middle vesicle forms the crura cerebri, corpora quadrigem- 

 ina, and the aqueduct of Sylvius or middle ventricle. The. posterior vesicle 

 gives rise to the medulla oblongata, pons 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 septufn lucidem, and the convolutions are ap- 

 parent on their surface ; while the corpora quadrigemina and crura are 

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

 pons varolii, corpora restiformia, and corpora pyramidalia. 



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 anteri- 

 orly with the fourth ventrical. 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 Jilum terminale, and the nerves of the tail of 

 the Horse {cauda equina). The parietes of the medullary canal are at 

 first very thin, but increase in thickness with the appearance of the ner- 

 vous 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 



