THE BRAiy A XI) ITS irKMBRAXES. 74o 



Superior Surface of Mid-brain. The superior surface of the mid-brain begins 

 just anterior to the anterior pair of the corpora quadrigemina. It is directed 

 downward and forward, and meets the upper extremity of the ventral surface at 

 quite an acute angle. The central portion of this surface is narrow and free. It 

 consists of tegmentum, and forms the first part (from behind forward) of the floor 

 of the thii-'l ventricle (Fig. 442). Of each lateral portion the area immediately 

 u,ljo<;-nt to the central portion is also tegmentum. and has resting on it and is 

 closely connected with the optic ?/<<//<//////. of the inter-brain. External to this 

 area is the "margin " of the superior surface, which, when the mid-brain is iso- 

 lated by dissection, is seen to consist of the upper ends of the fibres of the crusta, 

 cut across just as they are about to be continuous with those of the internal cap- 

 sule of the hemisphere (see Figs. 460 and 461). 



Posterior Perforated Lamina. In vertical transverse sections through the pos- 

 t- /!"/ iirt of the optic thalami and superior surface of the mid-brain the tegmen- 

 tum is clearly to be distinguished, both the portions beneath the optic thalami 

 and the central free portion between them (beginning of floor of third ventricle). 

 In all similar sections made anterior to this, however, the tegmentum is seen to 

 become less and less distinct, until it finally disappears, and we have only the 

 optic thalami lying dorsal to the substantia nigra, which last also bridges over 

 the interval < third ventricle) between them. Tli* portion of the substantia nigra 

 is the anterior part of that (already mentioned) which is seen in the interval 

 between the crustre. It is called the pout />/ perforated lamina, and is the 

 second structure, from behind forward, forming the floor of the third ventricle. 



Subthalamic Region. The gradual disappearance of the tegmentum in the 



cross-sections just referred to is due to the blending of the tegmental tissue with 



that of the superjacent portion of the optic thalamus, the central tegmeutal tissue 



also thinning out as the lateral parts are thus absorbed. To this tissue, thus 



made up of tegmentum and optic thalamus, the name subthalamic tegmental 



n is given. In it are seen the remnant of the red nucleus on each side, 



together with what corresponds to the lateral parts of the substantia nigra 



r ' Z, >'//.*. Dorsal to this is the zona incerta. a layer of reticular formation 



derived from that of the tegmentum proper, and still more dorsally is the stratum 



il>. a layer of longitudinal fibres derived from the posterior longitudinal 

 bundle and superior peduncle of the cerebellum (see page 742). 



THE INTER-BRAIN (Fig. 441). 



The inter-brain, or thalamencephalon ( /. < . thalamencephalon proper and prosen- 

 cephalon, see page 706), together with the hemispheres, constitutes the cerebrum. 



Anteriorly, the inter-brain is connected with the combined frontal lobes of each 

 hemisphere: laterally, it is connected with the inner aspect of each hemisphere; 

 /"////. it has resting on it. but with two layers of pia mater interposed, the 

 under surface of the combined hemispheres ; posteriorly, it is connected, mesially. 

 with the lamina quadrigemina of the mid-brain, beyond which connection, on 

 each side, it is free, this free portion being the rounded posterior extremity 

 pulvinar) of the optic thalamus. 



The cavity of the inter-brain is the third ventricle, a vertical antero-posterior 

 slit lying between the optic thalami. which are the thick side-walls of the inter- 

 brain (see also Figs. 447 and 451). The roof proper of the ventricle is a layer of 

 epithelium, like that of the lower half of the fourth ventricle, which stretches 

 between the optic thalami. and, together with their superior surfaces, constitutes 

 the upper surface of the inter-brain. Hence (see above) the under surface of the 

 combined hemispheres lies, in the middle line, on the roof of the third ventricle, 

 but with two layers of pia mater (velum interpositum) interposed. The floor of 

 the third ventricle almost meets the roof posteriorly, being separated from it only 

 by the orifice of the Sylvian aqueduct, and then proceeds downward and forward 

 until it attains its greatest distance from the roof (infundibulum), where it turns 

 upward and forward, and finally upward to meet the anterior end of the roof 



