244 THE ANATOMY OF THE HORSE. 



the optic commissure divides into the middle aliJ anterior cerebral 

 arteries. 



The Middle Cerebral vlrto-y passes outwards across the hemisphere, in 

 the fissure of Sylvius. 



Tlie Aiiterior Cerebral Artery unites in the mesial plane, above the 

 optic commissure, with the corresponding vessel of the opposite side. 

 The single vessel thus formed receives the meningeal branch of the 

 ophthalmic artery, and turns round the anterior end of the corpus callo- 

 sum to gain the great longitudinal fissure. Here it separates into a 

 right and a left branch, each of which passes backwards along the flat 

 face of the hemisphere. 



By the anastomosis of the two anterior cerebral arteries in front, and 

 the junction of the posterior communicating artery on each sfde with 

 the posterior cerebral, which results from the bi?irrcation of the basilar 

 arterj', a vascular circle is established around the pituitary body. This 

 is termed the Circle of Willis, and its object is to keep^up a free blood 

 supply to the cerebrum, even should there be an obstruction in one of the 

 main vessels forming the circle. Moreover, the internal carotid arteries 

 of opposite sides are, before tl>ey divide, connected by a large transverse 

 branch which further contributes to the freedom of the circulation. 



The Ophthaljiic Artbet is a collateral branch of the internal maxil- 

 lary. It enters the cranial cavity from the orbit by the internal orbital 

 foramen, along with the nasal branch of the ophthalmic nerve, and 

 divides into meningeal and nasal branches. 



The Meningeal Braiiches of opposite sides give off branches to the 

 dura mater, and then unite to form a single trunk which joins the 

 middle cerebral arteries. 



The Nasal Branch passes through the cribriform plate to gain the 

 nasal chamber. y 



The Sympathetic Nerve. Two branches from the superior cervical 

 ganglion accompany the internal carotid artery, and anastomose aroilnd 

 it to form the carotid plexxis. Within the'^ca.vernous sinus they form 

 another plexus — the cavernous plexus. From these plexuses filaments 

 pass to join the 3rd, 4th, 6th,^ and ophthalmic cranial nerves. A 

 twig also joins th^ large'^superficial petrosal nerve from tKe 7th, to 

 form the vidian nerve ; another passes to the lenticular ganglion, either 

 separately or with the ophthalmic nerve ; and some' filaments pass to 

 the Gasserian ganglion. 



The Brain, or Encephalon, consists of four principal parts, viz , the 

 medulla oblongata, the pons Varolii, the cerebellum, and the cerebrum. 

 The medulla is the division which is in direct continuity behind with 

 the spinal cord. The pons projects as a thick transverse bar, or ridge, 

 in front of the medulla. The cerebellum is superposed to both medulla 



