THE ARTERIES. 



Fig. 383. 



external maxillary artery, which turns round the inferior border of the maxilla, in company 

 with its satellite vein, and terminates on the forehead, as in the Horse, after supplying the 

 coronary arteries. 



2. The maxillo -muscular artery is distributed to the two masseters — to the external, as well 

 as the internal. 



3. The transverse artery of the face does not form the coronary arteries, as these come from 

 the external maxillary ; it is altogether expended in the masseter muscle. 



4. The anterior auricular artery sends an enormous branch into the temporo-parietal canal, 

 by the orifice situated behind the supra-condyloid process. 



5. The ophthalmic artery and the generating arteries of the rete mirabile proceed from a 

 common trunk. 



6. The rete itself shows some diflferences. We do not find, as in the Sheep, two lateral 

 elongated lobes, almost independent of each other, but a circular mass surrounding the sella 



Turcica. Besides, the occipital arteries concur in its 

 formation, and pass into its posterior part (Fig. 383). 



(This rete mirabile of Galen would appear to be 

 formed on the carotid and vertebral arteries of animals, 

 which, in a state of nature, feed from the ground; the 

 object being to furnish an equable and prolonged supply 

 of blood without tiie risk of check or hindrance, and 

 thus to obviate the tendency to congestion of tlie brain 

 during the dependent position of the head. This 

 minute subdivision and subsequent reconstitution of 

 an artery, with a like intention, is also observed in 

 other creatures besides grazing animals. The vessels 

 in the arm of the sloth are so disposed that the animal 

 can remain suspended by it for long periods; and a 

 similar arrangement is noted in the legs of birds — such 

 as the Swan, Goose, etc., which stand for a long time. 

 Around the Horse's foot the arteries break up into 

 numerous divisions, and we know that this animal can 

 remain in a standinir attitmle for months, and even 

 years. The rete ophthahnicum of birds is arranged like 

 the rete mirabile. The same object is sometimes 

 attained by great tortuosity, as we have already seen 

 in the description of several of the arteries. Perhaps 

 the most marked example, however, is to be found in 

 the carotid artery of the Seal, which is nearly forty 

 times longer than the space it has to traverse. 



But it may be remarked that there is no rete mira- 

 bile in the Horse— though it is also a grazing animal 

 — at all to be compared with that of Ruminants, the 

 circle of Willis being its only representative.) 



THE RETE MIRABILE OF THE OX 

 (POSTERIOR face). 



1, Rete mirabile; 2, trunk of the 

 originating arteries of the rete mi- 

 rabile; 3, .spheno-spinous artery; 4, 

 trunk of the encephalic, or internal 

 carotid arteries ; 5, branches of the 

 occipital passing to the rete mira- 

 bile ; 6, interspinal arterial canal, 

 formed by the intervertebral spinal 

 branches. 



Comparison of the Carotid Arteries of Man 

 with those of animals. 



The common carotids of Man have a different origin, 



the right arising from the arteria innominata, the left 



from the arch of the aorta. At the inferior border of the 



thyroid cartilage, they terminate by only two branches — the external and internal carotids ; the 



occipital artery is but a division of the former. 



Internal Carotid Artery. — Contrary to what is observed in animals, the internal 

 carotid is a little larger than the external, a difference which is explained in Man by the 

 predominance of the cranium over the face. This vessel describes a flexuous course until it 

 reaches the carotid foramen in the petrous bone ; it forms two curves in the cavernous sinus, 

 penetrates the dura mater, and divides at the fissure of Sylvius into four branches, which are, 

 as in Solipeds: the posterior communicating, anterior cerebral, middle cerebral, and artery of 

 the choroid 'plexus. The internal carotid has an important collateral branch— the ophthalmic 

 artery — that arises from the convexity of the curve the carotid makes inside the anterior clinoid 

 process, at the bottom of the orbit. If it differs at its origin, yet this vessel has a distribution 

 analogous to that already described. 



