424 BASE OF THE BRAIN. 



posite side. A probe passed into this fissure between the crus cerebri 

 and middle lobe would enter the middle cornu of the lateral ventricle. 



The Optic commissure is situated on the middle line ; it is the point 

 of communication between the two optic nerves. 



The Tutor cinereum is an eminence of grey substance situated im- 

 mediately behind the optic commissure, and in front of the corpora 

 mammillaria. From its centre there projects a small conical body of 

 grey substance, apparently a prolongation of the tuber cinereum, the 

 infundibulum. The infundibulum is hollow in its interior, enclosing a 

 short coecal canal, which communicates with the cavity of the third 

 ventricle, and below the termination of the canal the conical process 

 becomes connected with the pituitary gland. The infundibulum and 

 tuber cinereum form part of the floor of the third ventricle. 



The Pituitary gland (hypophysis cerebri), is a small, flattened, red- 

 dish-grey body situated in the sella turcica, and closely retained in that 

 situation by the dura mater and arachnoid. It consists of two lobes, 

 closely pressed together, the anterior lobe being the larger of the two 

 and oblong in shape, the posterior round. Both lobes are connected 

 with the infundibulum, but the latter is so soft in texture as to be ge- 

 nerally torn through in the removal of the brain. Indeed, for the 

 purposes of the student, it is better to effect this separation with the 

 knife, and leave the pituitary body in situ, to be examined with the 

 base of the cranium. 



The Corpora albicantia (mammillaria, pisiformia, bulbi fornicis), are 

 two white convex bodies, having the shape and size of peas, situated 

 behind the tuber cinereum, and between the crura cerebri. They are a 

 part of the crura of the fornix, which after their origin from the thalami 

 optici descend to the base of the brain, and making a sudden curve 

 upon themselves previously to their ascent to the lateral ventricles con- 

 stitute the corpora albicantia. When divided by section, these bodies 

 will be found to be composed of a capsule of white substance, contain- 

 ing grey matter, the grey matter of the two corpora being connected 

 by means of a commissure. 



The Locus perforatus is a layer of whitish grey substance, connect- 

 ed in front with the corpora albicantia, behind with the pons Varolii, 

 and on each side with the crura cerebri, between which it is situated. 

 It is perforated by several thick tufts of arteries, which are distributed 

 to the thalami optici and third ventricle, of which it assists in forming 

 the floor. It is sometimes called the pons Tarini. 



The Crura cerebri are two thick white cords which issue from the 

 anterior border of the pons Varolii, and diverge to each side to enter 

 the thalami optici. By their outer side the crura cerebri are continu- 

 ous with the corpora quadrigemina, and above they constitute the 

 lower boundary of the aqueduct of Sylvius. In their interior they 

 contain grey matter, which has a semilunar shape when the crus is di- 

 vided transversely, and has been termed the locus niger. The third 

 nerve will be observed to arise from the inner side of each, and the 

 fourth nerves wind around their outer border from above. 



