548 ANATOMY OF VERTEBRATES. 



layer of the dura mater, carries the blood from the f rete ' to the 

 brain, supplying principally the prosencephalon. Another in- 

 stance of plexiform disposition in the arteries of Artiodactyles 

 has been observed in the gastric branch of the crcliac axis of 

 the Hog, prior to its ramification in the dorsal parietes of the 

 stomach. 1 



The Perissodactyles have a recognisable entocarotid which, 

 however, has no proper bony canal, but enters the cranium by 

 the posterior fissura lacera : it forms strong bends as it converges 

 towards its fellow, with which it is united, behind the f sella,' by 

 an external flexuous e ramus communicans ' ; then, piercing the 

 dura mater, the eiitocarotids are again united by the internal 

 ' ramus communicans,' which completes the circle of Willis be- 

 hind and also receives the ( arteria basilaris.' A rudiment of the 

 ( rete mirabile ' of Artiodactyles is represented by small plexi- 

 form vessels given off from the hinder part of ' Willis's circle.' 



The entocarotid deeply grooves the apex of the petrosal in the 

 Elephant. Branches of the ectocarotid form a remarkable plexus 

 internal to the cheek-gland which opens between the eye and ear 

 in this animal. 



In most Carnivora the entocarotid traverses a curved canal in 

 the petrosal and makes a bend on emerging, which protrudes at 

 the foramen lacerum before entering the cranial cavity : the bend, 

 so exposed externally, receives in Canidce and Viverridoz (Ue?*- 

 pestes) a branch of the ectocarotid ; and, as it advances alongside 

 the ' sella,' also anastomoses with branches of the internal maxil- 

 lary and ophthalmic arteries. In Ursidce a smaller portion of 

 the loop projects into the cartilage occupying the ( foramen lace- 

 rum.' The entocarotid and its bony canal are smallest in Felines 

 and the Hyama. In these a maxillary plexus is formed which 

 supplies the internal maxillary, ciliary, ethmoidal, ophthalmic, and 

 anterior meningeal arteries. 



The condition of the mesentcric arteries in Man renders them, 

 in a degree, reservoirs as well as conveyers of blood : it facilitates 

 a more continuous or less interrupted supply to the intestinal 

 membrane. The fewer anastomotic arches in the Carnivora 

 favour a more rapid and direct supply of blood when the presence 

 of chyme in the intestines gives stimulus to such supply. The 

 human condition relates to the more regular and frequently re- 

 peated supplies of food ; to the more constant and continuous 

 presence of chyme upon the villous surface of the gut. In the 

 Dog, and more especially in wild Carnivora, the gorging of prey 



1 cci". p. 614, pi. 28, fig. 4. 



