IO6 EQUINE ANATOMY. 



perior median and inferior median fissures, the latter separating 

 to form the fourth ventricle, the point of divergence being called 

 the calamus scriptorius. 



Columns. 



The inferior median columns are continued as the pyramids; 

 outside these are two oblong elevations (the olivary bodies of 

 man), in front of which arise the sixth, and behind, the twelfth 

 nerves. 



The superior median column is continued in the medulla as 

 the restijorm body. 



The lateral tracts are the continuation of the lateral columns. 



The Isthmus. 



Is the prolongation of the medulla, supporting the cerebellum, 

 and terminating in the cerebrum. Its component parts are the 

 pons Varolii or mesencephalon, crura cerebri and cerebelli, valve 

 of Vieussens, corpora quadrigemina, thalami optici, pineal and pitu- 

 itary glands. 



Pons varolii. 



The pons is an elevated mass of white transverse fibres thrown 

 between the two halves of the cerebellum; it rests in a depression 

 on the basilar process, and below presents a longitudinal median 

 groove for the basilar artery (see A rteries) . The crura pass above it . 



Crura cerebri. 



The crura are two white bands continuous with the fibres of 

 the medulla, entering the hemispheres of the cerebrum in front 

 and in contact with the optic thalami and quadrate bodies above, 

 the pons below. In front they pass under the optic tracts, which 

 here unite and form the optic commissure. The crura also sepa- 

 rate anteriorly to enclose the corpus albicans. 



Crura cerebelli. 



The cerebellar crura are composed of three fasciculi: 



1 . Crus cerebelli ad pontem the pons' transverse fibres. 



2. Crus ad medullam the restiform bodies of the medulla. 



3. Processus e cerebello ad testes; or anterior crura to the testes 

 of the corpora quadrigemina. 



The valve of Vieussens is a thin, transverse layer, which passes 

 between the anterior cerebellar crura. It forms part of the roof 

 of the fourth ventricle. 



The corpora quadrigemina are four small bodies which lie 



