THE VERTEBRAL ARTERY 



621 



be driven beyond the area of the part supplied by the particular vessel which is 

 the subject of the experiment. 



The Cortical Arterial System. The vessels forming this system are the ter- 

 minal branches of the anterior, middle, and posterior cerebral arteries, described 

 above. These vessels divide and ramify in the substance of the pia, and give off 

 nutrient arteries which penetrate the cortex perpendicularly. These n**trient ves- 

 sels are divisible into two classes the long and short. The long or, as they are 

 sometimes called, the medullary arteries pass through the gray substance to pene- 

 trate the centrum ovale to the depth of about an inch and a half, without intercom- 

 municating otherwise than by very fine capillaries, and thus constitute so many 



Capillary network in the white substance. (After Charcot.) 



independent small systems. The short vessels are confined to the cortex, where 

 they form with the long vessels a compact network in the middle zone of the gray 

 substance, the outer and inner zones being sparingly supplied with blood (Fig. 455). 

 The vessels of the cortical arterial system are not so strictly terminal as those of 

 the central ganglionic system, but they approach this type very closely, so that 

 injection of one area from the vessel of another area, though it may be possible, 

 is frequently very difficult, and is only effected through vessels of small caliber. 

 As a result of this, obstruction of one of the main branches or its divisions may have 

 the effect of producing softening in a very limited area of the cortex. 



The Vertebral Artery (A. Vertebralis). 



The vertebral artery (a. vertebralis) (Figs. 447 and 456) is generally the first and 

 largest branch of the subclavian; in rare instances it springs independently from 

 the arch of the aorta. It arises from the upper and back part of the first portion 

 of the vessel, and, passing upward, enters the foramen in the transverse process of 



