236 



THE ANATOMY OF THE HORSE. 



them over. The space is continuous with the same space Jn the spinal 

 meninges, and contains the limpid cerebrospinal fluid. 



The PiA Mater. This is the vascular membrane of the braia It 

 consists of delicate areolar tissue and bloodvessels. It invests the 

 brain closely, following all its surface irreg-ularities. Behind the cerebral 

 hemispheres it sends towards the interior of the cerebrum a wide process 

 • — the vehtm inter positum, ; and where the cerebellum is superposed to the 

 medulla oblongata, it forms on each side a thickened granular cord — the 

 choroid plexus of ilie Jfth ventricle. These will be exposed at a later stage. 



ARTEBIBS OF THE BEAIN (fIG. 30). 



Three vessels are concerned in supplying blood to the encephalon, 

 viz., the basilar, internal carotid, and ophthalmic arteries. 



Fig. 30. 

 The Arteries of the Brain. 

 1. Anterior branch of cerebro-spinal artery ; 2. Basilar artery ; 3, 3. Irregular branches to medulla 

 and^ cerebeUnm ; 4. Posterior cerebellar arteries ; 5. Bifurcation of the basilar ; 6. Anterior cere- 

 bellar arteries ; 7. Posterior cerebral arteries (more numerous and smaller than usual) ; 8. Internal 

 carotid ; 9. Posterior communicating branch ; 10. Anterior branch of internal carotid, which divides 

 to foim 11 and 12 — the middle and anterior cerebral arteries ; 13. Single vessel formed by the union 

 of iXand 12, disappearing into great longitudinal fissure ; A. Medulla .oblongaia ; B. Pons Varolii ; 

 C. Cerebelluni ; D. Cms cerebri ; E. Coi-pus albicans ; F. Optic commissure ; ^^. Olfactory bulb ; 

 H. Cerebral hemisphere. - - . v : 



The Basilar Artery is formed on the middle line of the lower face of 

 the medulla oblongata, by the union of two vessels. These are the 



