1054 THE VASCULAK SYSTEM. 



The inferior mesenteric artery may give hepatic, renal, or middle colic branches ; occasion- 

 ally it is absent, being replaced by branches of the superior mesenteric, and sometimes, as in 

 ruminants and some rodents, its left colic branch does not anastomose with the middle 

 colic artery. 



All these variations of the unpaired visceral branches of the abdominal aorta are merely due 

 to modifications of the usual processes by which the vessels are developed. 



The hepatic, splenic, and left gastric arteries may arise directly from the aorta, a condition 

 which is due to the retention of a greater number of the splanchnic arteries than usual. A 

 double superior mesenteric artery results from the persistence of both the right and left 

 splanchnic vessels from which the superior mesenteric artery is formed, these remaining separate 

 instead of fusing together. All the other variations are the results of the obliteration of the 

 usual channels, combined with the enlargement of anastomoses which exist both between the 

 splanchnic arteries of adjacent segments and between the splanchnic and intermediate visceral 

 arteries. 



THE ARTERIES OF THE HEAD AND NECK. 



Innominate Artery. From what has already been said, with reference to the branches 

 of the arch of the aorta, it will be noted that the innominate artery may be absent. On the other 

 hand there may be two innominate arteries, a right and a left, each ending in corresponding 

 common carotid and subclavian trunks, and the two vessels may themselves arise by a common 

 stem. 



The branches given off by the innominate artery may be increased in number, or the innomi- 

 nate may vary from the normal only as regards length. As a consequence of such modifications 

 in length, the origins of the right common carotid and right subclavian arteries may be situated 

 at a higher or lower level than usual, whilst, in the absence of the innominate artery, both these 

 branches may arise directly from the aorta. 



Common Carotid Arteries. When the right common carotid artery arises separately from 

 the arch of the aorta, it may be the first, or, much more rarely, the second branch. In the 

 former case the fourth right aortic arch has been obliterated, and the right subclavian artery 

 springs from the descending aorta; in the latter case either the innominate stem has been 

 absorbed into the arch of the aorta, or the ventral root of the fourth right aortic arch has fused 

 with part of an elongated fourth left arch. 



Whether arising as the first or second branch, the origin may be to the left of the median plane, 

 and the trunk may pass in front of the trachea, or behind the oesophagus, before it ascends into 

 the neck. 



The left common carotid artery varies, as regards its origin, much more frequently than the 

 right vessel ; not uncommonly, and apparently because of the fusion of the ventral roots of the 

 fourth aortic arches, it arises "from a stem common to it and to the right common carotid and 

 right subclavian arteries. 



Both common carotids may vary a's regards their termination. They may divide at a higher 

 or lower level than usual, the former more commonly than the latter ; whilst in a few exceptional 

 cases the common carotid does not divide, but is continued directly into the internal carotid, and 

 from this the branches usually given off by the external carotid are derived. 



This arrangement is probably due to obliteration of the ventral roots of the first and second 

 aortic arches, the arches persisting and being divided into the branches which generally arise 

 from their ventral extremities. 



Usually the common carotids give off no branches, but not infrequently one or more of the 

 branches of the external carotids arise from them. 



The external carotid artery may be absent, or it may, in rare cases, arise directly from the 

 arch of the aorta. The number of its branches may be diminished either by fusion of their roots 

 or by transference to the internal or common carotid arteries. On the other hand, the number of 

 its branches may be increased ; thus, the sterno-mastoid artery, the hyoid branch usually given off 

 by the superior thyreoid artery, or the ascending palatine branch of the external maxillary, may 

 arise from it. Sometimes the branches may arise in the usual way, but may deviate from the 

 course generally taken ; more particularly is this the case with the internal maxillary artery, 

 which may pass either between the heads, or entirely lateral or medial to both heads of the 

 external pterygoid muscle. 



The internal carotid artery is rarely absent, but its absence has been noted upon one side, 

 more commonly the left ; and upon both sides. Occasionally it springs from the arch of the 

 aorta, and in its course through the neck it may vary somewhat in length and in tortuosity. One 

 or more of the branches usually derived from the external carotid artery may arise from it, and it 

 sometimes gives off a large meningeal branch to the posterior fossa of the skull. Its posterior 

 communicating branch may replace the posterior cerebral artery ; on the other hand, the upper 

 part of the internal carotid may be absent, and the posterior communicating artery may become 

 the middle cerebral artery. The anterior cerebral branch of the internal carotid may be absent, 

 or rather it may arise from the corresponding artery of the opposite side ; or there may be three 

 anterior cerebral arteries, the third arising from the anterior communicating artery which connects 

 the two anterior cerebrals together. The ophthalmic artery, as it traverses the orbit, may pas* 

 either above or below the optic nerve. It is occasionally replaced by a branch of the rniddl* 

 meningeal artery. 



The vertebral artery may have a double origin one from the subclavian, and one from th< 

 inferior thyreoid artery or from the aorta. 



