378 BASE OF THE BRAIN. 



cavernous sinuses. It consists of two lobes, but presents nothing 

 glandular, either in structure or function. 



The Corpora albicantia are two rounded white bodies, placed side 

 by side, of about the size of peas ; hence their synonyme, pisiformia. 

 They are the anterior extremities of the crura of the fornix, and are 

 connected with the thalami optici by two white cords, which may 

 be easily traced. 



The Locus perforatus is a layer of whitish gray substance, con- 

 nected in front with the corpora albicantia, 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 

 beneath the pons Varolii, and diverge to each side to enter the tha- 

 lami optici. The third nerve will be observed to arise from the inner 

 side of each, and the fourth nerves wind around them from above. 

 If the orus cerebri be cut across, it will be seen to present, in the 

 centre of the section, the locus niger. 



The Pons Varolii* (protuberantia annularis) is the broad transverse 

 band of white fibres, which arches like a bridge across the upper 

 part of the medulla oblongata; and, contracting on each side into a 

 thick rounded cord, enters the substance of the cerebellum under 

 the name of crus cerebelli. There is a groove along its middle 

 which lodges the basilar artery. The pons Varolii is the commis- 

 sure of the cerebellum, and associates the two lateral lobes in their 

 common function. Resting upon the pons, near its posterior border, 

 is the sixth pair of nerves. On the anterior border of the crus cere- 

 belli, at each side, is the thick bundle of filaments belonging to the 

 fifth nerve, and, lying on its posterior border, the seventh pair of 

 nerves. 



The Medulla oblongata is the upper enlarged portion of the spinal 

 cord. Upon its anterior surface are seen two narrow projecting 

 columns, the corpora pyramidalia. These bodies are broad above, 

 and narrow below; and, at the point where they enter the pons Va- 

 rolii, they become considerably constricted. They are connected to 

 each other in the middle fissure, at about an inch below the pons, 

 by a decussation of their fibres, which form small interlacing bands 

 crossing from side to side. 



Externally to the corpora pyramidalia are two oblong and rounded 

 bodies, supposed to resemble olives in their form, and hence called 

 corpora olivaria. If these bodies be divided by a longitudinal sec- 

 tion, a gray zigzag outline, resembling the corpus rhomboideum of 



* Constant Varolius, Professor of Anatomy in Bologna : died in 1578. He dissected 

 the brain in the course of its fibres, beginning- from the medulla oblongata : a plan 

 which has since been perfected by Vieussens, and by Gall and Spurzheim. The work 

 containing his mode of dissection, " De Resolutione Corporis Humani," was published 

 after his death, in 1591. 



