CIRCULATORY SYSTEM OF THE OrRASS-SNAKE. 677 



jaw. It soon gives off a branch to the inferior labial gland *, and 

 then runs on in the lower jaw to its anterior end. Here it comes 

 out through the anterioi' maxillai'y foramen as the Mentalis Artery 

 (Art. mentalis, Rathke) and supplies the anterior part of the 

 inferior labial gland and the skin in the region of the chin. 



Just anterior to this branch the internal carotid splits into two 

 bi-anches, the cerebral carotid and the facial carotid arteries. 



The Cerebral Carotid (Art. carotis cerebralis, Rathke and Hof- 

 mann) is a vessel of moderate calibre running downwards just 

 behind the infra-maxillary branch of the trigeminal nerve. 

 It then turns inwards under the skull and enters the cranial 

 ca.vity through a foramen in the basisphenoid. This vessel was 

 overlooked by Schlemm. In its course it gives off a small artery 

 just outside the skull going to the posterior pterygo-sphenoidalis 

 muscle. Inside the skull near the hinder end of the pituitary- 

 fossa it splits into three fairly equal vessels, the posterior, median, 

 and anterior cerebral carotids. 



I. The posterior branch (Ramus caudalis, Hofmann) runs back- 

 wards round the outside of the pituitary fossa, giving off branches 

 to the cerebellum and, about half-way to the foramen magnum, 

 unites with the similar vessel of the other side to form the median, 

 basilar artery. 



The Basilar Artery (Art. basilaris, Rathke and Hofmann) 

 passes backwards until just before the foramen magnum, where, 

 it splits up into a loop in the form of an isosceles triangle. Into 

 the corners of the base of this loop open the right and left first 

 spinal arteries, and thus it forms the anastomosis between these 

 two vessels. Before it divides to form the loop, the basilar artery 

 gives off on either side a well-marked internal auditory artery 

 (Art. auditiva interna, Rathke and Hofmann) that enters the 

 ear with the auditory nerve, and also a series of smaller branches, 

 some of which supply the medulla, and some run on to the small 

 choroid plexus of the fourth ventricle. 



II. The median branch f runs outwards behind the cerebral 

 hemisphere, to vvhich it gives some twigs, and a short distance 

 from its origin divides into two. 



A. The Anterior Choroid Artei-y (Art. choroides anterior, 



Hofmann) is the anterior branch. It passes around the 

 hemispheres, supplying them with small twigs, to the 

 dorsal side of the brain near the pineal stalk, where it 

 breaks up in the choroid plexus of the third and lateial 

 ventricles and anastomoses with a branch from the olfactoiy 

 artery. 



B. The posterior division passes behind the optic lobes and 



spreads itself out over their dorsal surface. 



* The uomenclatvire of these glands in the head of the snake is that given by West 

 (37&38). 



f In some examples this median brniicli does not arise at the jjoint where the 

 cerebral carotid splits into anterior and posterior branches, but a little way down the 

 latter ; consequently it appears as a branch of the posterior cerebral carolid alid is 

 described as such by Hofmann (24). 



