504 



THE BLOOD- VASCULAR SYSTEM. 



cranial nerves to the anterior perforated spot at the inner extremity of the fissure 

 of Sylvius, where it gives off its terminal or cerebral branches. This portion 

 of the artery has the optic nerve on its inner side, and the third nerve externally. 



Peculiarities. The length of the internal carotid varies according to the length of the 

 aeck, and also according to the point of bifurcation of the common carotid. Its origin sonie- 



FIG. 289. The internal carotid and vertebral arteries. Right side. 



times takes place from the arch of the aorta ; in such rare instances this vessel has been found 

 to be placed nearer the middle line of the neck than the external carotid, as far upward as the 

 larynx, when the latter vessel crossed the internal carotid. The course of the vessel, instead of 

 being straight, may be very tortuous. A few instances are recorded in which this vessel was 

 altogether absent : in one of these the common carotid passed up the neck, and gave off the 

 usual branches of the external carotid, the cranial portion of the internal carotid being replaced 

 by two branches of the internal maxillary, which entered the skull through the foramen rotundum 

 and ovale and joined to form a single vessel. 



