BLOOD SUPPLY OF THE PITUITARY BODY 147 



The circulation of the posterior lobe enters the gland at the 

 posterior pole of the pars nervosa and is entirely independent of 

 the systems heretofore described. Each internal carotid, shortly 

 after it enters the cranial chamber and turns forward (fig. 1) 

 in the carotid groove, gives off a small branch : these two vessels 

 unite in front of the posterior clinoid process to form the single 

 median trunk which enters the hypophyseal posterior lobe at the 

 point w^here a small area of firm dural attachment is appreciable 

 in the usual dissection to liberate the gland. It is well to bear 

 in mind that these branches of the carotid lie between the two 

 laj'ers of dura forming the circular 'sinus, and therefore really in 

 a sense lie in this sinus, just as the internal carotid itself lies in 

 the lateral sinus. 



The artery enters the posterior lobe near its centre (fig. 4) 

 dividing immediately into numerous large branches which stream 

 outward toward the periphery. There is, however, no marked 

 difference in the inherent vascularity of any part of the posterior 

 lobe until the capillary bed is reached at the extreme periphery 

 under the epithelial investment. 



The veins collect the blood and pass to the point of dural attach- 

 ment in much the same arrangement as the arteries. A single 

 large central vein enters the circular sinus at this point (cf. fig. 3, m. 

 v.) immediately above the entrance of the artery, the two being 

 closely adjacent. In addition, there are other smaller veins which 

 empty independently into the sinus. 



Collateral circulation 



Although the anterior and posterior lobes have independent 

 blood supplies, the question arises as to the possible vascular 

 communication between structures so intimately related. Is 

 the collateral circulation sufficient to preserve the glandular 

 function in case of occlusion of the blood supply to one or 

 another of the anatomical subdivisions of the gland? Is separa- 

 tion of the vessels of the stalk equivalent to a transplantation of 

 the anterior lobe? There is doubtless some collateral circulation 

 between the anterior and posterior lobes through the pars inter- 



