THE NERVOUS SYSTEM 1463 



The cavity of the mesencephalon is largely encroached upon by the thick- 

 enings undergone by its walls. A minute canal, however, is left, called the 

 aquedoet ol Sylvius, or aqueductus cerebri, which constitutes a part of the 

 cephalic prolongation of the central canal of the spinal cord. It is continuous 

 below with the fourth ventricle, and above with the third ventricle. 



Dieneephalon or Thalameneephalon. — The diencephalon represents the 

 second secondary cerebral vesicle, which is one of the subdivisions of the pros- 

 encephalon, fore-brain, or anterior primary cerebral vesicle. Its walls give 

 rise to the parts around the third ventricle — namely, (i) the greater part of 

 the ependymal roof, with the velum interpositum and pineal body; (2) the 

 optic thalami; and (3) the tuber cinereum, infundibulum, posterior lobe of 

 the pituitary body, and corpora mammillaria. 



The diencephalon also gives origin to each optic stalk, from which the optic 

 nerve is developed, the retina being developed from the optic vesicle. The 

 cavity of the diencephalon forms the greater part of the third ventricle. The 

 roof remains very thin, and consists merely of ependymal epithehum. This 

 forms the direct roof of the third ventricle, and it is covered by two layers 

 of pia mater, adherent to each other, and closely connected to the ependymal 

 roof. One of these layers of pia mater belongs to the diencephalon, whilst 

 the other belongs to the under surfaces of the developing cerebral hemispheres, 

 which have expanded backwards so as to overlap the diencephalon. The 

 two layers of pia mater constitute the velum interpositum, or tela choroidea 

 superior, and, with the ependymal lamina, upon which they lie, they form 

 the roof of the third ventricle. 



The pineal body, or epiphysis cerebri, is developed from the dorsal part of 

 the ependymal roof of the third ventricle. It appears as a diverticulum of 

 the ependymal roof, close to the mesencephalon, and it is directed backwards, 

 so that it comes to lie over the corpora quadrigemina. The distal end is 

 blind, and, in connection with it, a number of closed foUicles are formed 

 which contain calcareous particles, forming the acervulus cerebri, or brain- 

 sand. The proximal part of the diverticulum forms the stalk of the pineal 

 body, which contains the pineal recess and opens into the third ventricle. 



The lateral walls of the diencephalon become much thickened and give rise 

 to the optic thalami, the cavity of the third ventricle being thereby converted 

 into a narrow cleft. 



The floor forms the tuber cinereum, and from the tuber cinereum a diver- 

 ticulum, called the infundibulum, passes downwards. The lower part of the 

 infundibulum becomes solid and forms the posterior lobe of the pituitary 

 body, the anterior lobe of that body being developed from the poueh of Rathke, 

 which is an upward diverticulum of the stomodeBal ectoderm. 



Caudal to the infundibulum the floor forms the mammillary recess, from 

 the thickened walls of which the corpora mammillaria are formed. 



The optic vesicles are important appendages of the diencephalon, each 

 vesicle growing out from the lateral part of its floor, in front of the level of 

 the infundibulum. For a description of these vesicles see the Development 

 of the Eye. 



Telencephalon. — ^The telencephalon represents the first secondary cerebral 

 vesicle, which is one of the subdivisions of the prosencephalon, fore-brain, or 

 anterior primary cerebral vesicle. It extends forwards and downwards 

 from the diencephalon, and from it are developed the cerebral hemispheres 

 and their associated structures, including the lateral ventricles, as well as the 

 anterior portion of the third ventricle. 



The cerebral hemispheres make their appearance as hollow protrusions 

 which grow out from the dorsal zones or alar laminae of the lateral walls of 

 the telencephalon. These two lateral protrusions constitute the vesicles 

 of the cerebral hemispheres, and their cavities are lateral extensions of the 

 cavity of the telencephalon. This cavity, mesially placed, forms the anterior 

 portion of the third ventricle. 



The vesicular cerebral hemispheres grow with great rapidity, and are 

 separated from each other by a cleft called the interhemispherical fissure, 

 which is the rudiment of the great longitudinal Assure. The direction of their 



