92 EQUINE ANATOMY, 



suprasphenoid or pituitary fold, which circumscribes- 

 the sella turcica. The cranial dura mater is more or 

 less adherent to the bone, and is lined by the parietal 

 layer of the arachnoid ; it gives prolonged sheaths to 

 the nerves. 



The middle or arachnoid is a serous membrane 

 with a parietal and a visceral layer, enclosing a cavity 

 by its internal face, perforated by the roots of the 

 nerves and blood vessels. It shows iilaments uniting 

 it to the pia mater ; passes over the circumvolutions of 

 the encephalon, leaving spaces or confluents containing 

 the sub-arachnoid fluid, a slightly yellow or colorless 

 but limpid and transparent liquid. 



The internal or pia mater, a cellulo-vascular struc- 

 ture, which, by its external face, furnishes the neuril- 

 emma of the nerves and filamentous threads to the 

 dura mater. From its internal face rise blood vessels 

 dipping into the nervous substance. 



Encephalon. — Yolume, closely that of the cranium. 

 Weight, about twenty-two or twenty-three ounces. Di- 

 vision : Isthmus, cerebrum and cerebellum.*^ yL^>v^ 



Isthmus. — a. External surface : four faces and two 

 extremities. Inferior face presents annular protuber- 

 erance or pons Varolii or mesocephalon, crura-cerebri 

 or pedunculum cerebri, rachidian bulb or medulla ob- 

 longata ; superior face shows the upper face of the 



