100 



GENERAL ANATOMY. 



loped into the fourth ventricle, and its walls form the cerebellum, pons Varolii, 

 medulla oblongata, and parts in the floor of the fourth ventricle. The antero- 

 posterior fissure which indicates the division of the brain into two halves 

 appears early, and the primary anterior and posterior cerebral vesicles are 

 also soon divided by a transverse fissure into two parts, so as to constitute five 

 permanent rudiments of the brain and medulla oblongata. The middle pri- 

 mary vesicle remains undivided. 



The anterior part of the anterior cerebral vesicle (Vorderhirn, fore brain) 

 constitutes the cerebral hemispheres, corpus callosum, corpora striata, fornix, 

 lateral ventricles, and olfactory nerves. These parts lie at first quite covered 

 and concealed by those formed from the middle vesicle, and by the optic 

 thalami, which, with the optic nerves, the third ventricle, and the parts in 

 its floor, 1 are furnished by the posterior portion of the anterior vesicle (Zwis- 

 chenhirn, intermediary brain). By the third month, however, the hemis- 

 pheres have risen above the 'Optic thalami, and by the sixth month above the 

 cerebellum. Fissures are seen on the surface of the hemispheres at the third 

 month, but all except one disappear. This one persists, and forms the fissure of 

 Sylvius. The permanent fissures for the convolutions do not form till about 

 the seventh or eighth month. The middle cerebral vesicle (Mittelhirn, middle 

 brain) is at first situated at the summit of the angle shown on Fig. 61. Its sur- 

 face, at first smooth, is soon divided by a median and transverse groove into 

 four tubercles (tubercula quadrigemina), which are gradually covered in by the 

 growth of the cerebral hemispheres. The cavity diminishes as its walls thick- 

 en, and contracts to form the aqueduct of Sylvius. The crura cerebri are also 

 formed from this vesicle. The third primary cerebral vesicle is divided at an 

 early period (between the ninth and twelfth week) into two, the anterior part 



(Hinterhirn, hinder brain), forming the cerebellum, and 

 a membrane (mernbrana obturatrix), which closes the 

 upper part of the fourth ventricle, and which disap- 

 pears as development progresses; its posterior part 

 (Nachhirn, after-brain) forms the medulla oblongata, 

 with the restiform bodies and auditory nerves. 



"When the medullary groove is closed, the fcetal 

 spinal marrow at first occupies the whole of the canal 

 so formed. It presents at first a large central canal, 

 which gradually contracts, and in after life is no 

 longer perceptible to the eye, though it is still visi- 

 ble on microscopic sections (p. 64). After the fourth 

 month the spinal column begins to grow in length 

 more rapidly than the medulla, so that the latter no 

 longer occupies the whole canal. The ganglia and 

 anterior roots of the nerves are perceptible at the 

 fourth week, the posterior roots at the sixth. The 



Central canal. 2. Its epithe- cord is composed at first entirely of uniform-looking 

 Hum. 3. Anteriorgmy matter cells, which soon separate into two layers, the inner 



of which forms the epithelium of the central canal, 

 while the outer forms the central gray substance of 

 the cord. The white columns are formed later ; their 

 rudiments can be detected about the fourth week. 

 The central canal of the spinal cord is at first unclosed 

 behind, except by the epithelial layer, but at the age 

 of nine weeks the medullary substance is united here 

 also. The ganglia appear to be developed from the 



protovertebral disks, and it is possible that the posterior roots also are ; the 

 anterior roots proceed from the medulla itself. The development of the nerves 



Fisr. 62. 



10 



7 5 



Section of the medulla in the 

 cervical region, at six weeks 

 (magnified 50 diameters). 1. 



4. Posterior gray matter. 5. 

 Anterior commissure. 6. Pos- 

 terior portion of the canal, 

 closed by the epithelium only. 

 7. Anterior column. 8. Late- 

 ral column. 9. Posterior col- 

 umn. 10. Anterior roots. 11. 

 Posterior roots. 



The development of the pituitary body is still a matter of question. 



