900 THE VASCULAR SYSTEM. 
The branches given off by the innominate artery may be increased in number, or the innomi- 
nate may only vary from the normal 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 mesial plane, 
and the trunk may pass in front of the trachea, or behind the cesophagus, before it ascends in 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 as 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 he increased ; thus the sterno-mastoid artery, the hyoid branch usually given off 
by the superior thyroid artery, or the ascending palatine branch of the facial, 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 external or internal to both heads of the external 
pterygoid muscle. 
The internal carotid artery is rarely absent. 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 pass 
either over or under the optic nerve. It is occasionally replaced by a branch of the middle 
meningeal artery. 
The vertebral artery may have a double origin—one from the subclavian, and one from the 
inferior thyroid artery or from the aorta. 
The right vertebral may arise from the common carotid or from the arch of the aorta. 
Occasionally it springs from the descending aorta, an arrangement associated with the persistence 
of the dorsal roots of the fourth and fifth mght arches. 
The left vertebral artery not infrequently springs from the arch of the aorta, arising between 
the left common carotid and left subclavian arteries; this is evidently due to the absorption of 
the stem of the seventh segmental artery into the aortic arch. Very exceptionally the left 
vertebral is a branch of an intercostal artery. 
In its course upwards either vertebral artery may enter the vertebrarterial foramen of any of 
the lower six cervical vertebra. 
The cases in which it does not enter one of the lowest of these are apparently associated with 
its formation in part from the precostal instead of from the postcostal anastomosing channels. 
The artery may enter the spinal canal with the second instead of with the first cervical nerve, 
or, after leaving the foramen in the transverse process of the third vertebra, it may divide into 
two branches, one of which accompanies the second and the other the first cervical nerve ; the two 
branches unite together again in the spinal canal to form a single trunk. 
Sometimes, though rarely, it gives off superior intercostal and inferior thyroid branches. The 
upper end of one of the vertebrals is sometimes very small, or it may be entirely wanting ; in 
the latter case the basilar artery is formed by the direct continuation of the opposite vertebral. 
The basilar artery may be double in part or the whole of its extent, or its cavity may be 
divided by a more or less complete septum. It may terminate in one instead of two posterior 
cerebral arteries, the missing vessel being supplied by the enlargement of the posterior com- 
municating branch of the internal carotid. 
