202 ANIMAL PHYSIOLOGY. 



care suffices to turn the hemisphere-vesicles outwards, and 

 then come into view between them the optic thalami, in 

 this early stage united, like the corpora quadrigemina, one 

 with the other, over the mesial canal, and covered with pia 

 mater. Along by the anterior and outer margin of each 

 of the optic thalami, a fissure is seen between it and the 

 hemisphere-vesicle, at which, as in the turtle, a large choroid 

 plexus enters; and the ruptured margin of the vesicle has its 

 edge turned away from the middle line into 

 the interior of the cavity. At a later period 

 this everted part adheres, across the middle 

 line, with its fellow of the opposite side; 

 and thus is formed a structure peculiar to 

 mammals, the fornix, separated from the 

 corpora quadrigemina and optic thalami by 

 a transverse fissure, containing the invagi- 

 Fig. 106. BRAIN nated part of the pia mater supporting the 

 OF EMBRYO choroid plexus, and called the velum inter- 



I r AMB ' ^^ positum. At a still more advanced period 



above and the r ' . . ,1,1 ,. /* 



rio-ht side, a, * embryonic growth, the adjacent surfaces 



Medulla oblon- of th^ hemisphere-vesicles become joined 



gata; b, cerebel- together by development of the corpus cal- 



lum; c, corpora IQ which ig united posterior i y to the back 

 quadrigemina : ' .. p . ., , r i 



d, optic thalami; P art * tne ibrnix, then arches forwards at a 



el right hemi- higher level, and turns downwards in front so 



sphere - vesicle a s to enclose a mesial space above that body, 



reflected;/ cor- The ^ of the walls of the vesicles limit . 

 pus striatum. \ . . , , . 



feound the open- ln *^ 1S mesia l space continue very slender, 



ing into the la- and constitute the septum lucidum; and the 



teral ventricle space itself is termed the fifth ventricle. The 



is the rudimen- cav ities of the hemisphere-vesicles are called 



tarv fornix. rne , 17 . 7 .-TJI i. .1 



choroid plexus, * ne 6w* ventricles; the space between the 



which, at this velum interpositum and optic thalami, as 



period, is ex- W ell as between these bodies, is the third 



ceedingly large, ven f r { c l e fas fourth ventricle, as has already 



nfoved. 6 " ^ een s * a * ec ^ ^ s * ne space between the cere- 



bellum and medulla oblongata; and the 

 canal continued forwards from this, beneath the corpora 

 quadrigemina, is called the iter (a tertio ad quartum ven- 

 triculum) or aqueduct of Sylvius. 



