BLOOD-SUPPLY OF THE BttAIN. 



any one white column or group of cells in the grey matter. At the same time it 

 should be noted that within the cord itself (as within the brain), all the arteries are 

 " terminal arteries," in Cohnheim's sense, that is to say, they do not anastomose with 

 other arteries, but each one terminates in its own capillary area, and supplies no 

 other. There is however much variation in the extent of the capillary area supplied 

 by any of the arterial branches, and no definite statement can be made concerning 

 the exact region supplied by any set of arterioles (Kadyi). 



The origin and course of the veins of the spinal cord have been described 

 in Vol. II., pp. 533, 534. The most considerable are two longitudinal median 

 vessels, one running along the anterior median fissure along with the anterior spinal 

 artery, and another over the posterior median fissure, unaccompanied by any 

 considerable artery. Both of these median veins have a tortuous course, and the 

 posterior one is frequently broken up into a kind of venous plexus, with longitudinal 

 meshes, which extends over the whole posterior surface of the cord. There is also 

 a less perfect lateral anastomotic chain lying behind the line of exit of the anterior 

 nerve roots. All these vessels communicate freely with one another by lateral offsets. 

 They receive the venous blood from the cord on the one hand, and on the other hand 

 carry it away by veins which accompany, at frequent intervals, the nerve roots. In 

 the upper part they join the veins of the cerebellum and pons, and the venous 

 sinuses around the foramen magnum. Within the cord their branches anastomose 

 frequently. The vein accompanying the anterior spinal artery receives, like that 

 vessel, very numerous tributaries from the anterior median fissure (central venules) : 

 these carry away a large part of the blood from the grey matter ; the peripheral 

 venules, which enter the veins of the pia mater which covers the general 

 surface of the cord, chiefly carry away the blood from the capillaries of the white 

 matter. 



Blood-supply of the brain. The origin and course of the vessels which supply 

 the brain have already been described in the section Angeiology, Vol. II. pp. 411 to 

 415 (arteries), and 519 to 524 (veins). In passing to their distribution the several 

 arteries, having passed across the subdural space, enter the subarachnoid space and 

 then divide and subdivide into branches, which, in their further ramification on the 

 nervous centres, are supported by the pia mater, and, it may be remarked, are more 

 deeply placed in the various fissures and sulci than the small veins, which do not 

 accompany the arteries, but pursue a different course, and are chiefly seen upon the 

 surface of the pia mater. From the arteries in the pia mater of the hemispheres 

 very numerous small branches pass vertically into the grey matter of the convolutions. 

 Most of these (cortical arteries) at once break up into a close plexus of capillaries for 

 the grey matter ; but others (medullary arteries), larger but less numerous, pass 

 through the grey matter, giving off only a few small branches to it, and penetrate 

 for some distance into the medullary centre, where they divide into a long-meshed 

 capillary network. The smaller branches of arteries anastomose together to a 

 certain extent in the pia mater before penetrating into the superficial grey matter 

 (Huebner), but the branches which pass to the chief ganglia, such as the optic 

 thalamus or corpus striatum, do not anastomose with one another. 



Moreover, it is to be observed that, whilst the main branches of the arteries are 

 situated at the base of the brain, the principal veins tend towards the upper surface 

 of the hemispheres, where they enter the superior longitudinal sinus, most of them 

 looping forwards as they pass into the sinus, and often entering the dura mater 

 a short distance from the sinus, but more usually having a free course, sometimes 

 of considerable length, through the subarachnoid space in passing from the pia into 

 the dura mater. The veins of Galen, coming from the lateral ventricles and choroid 

 plexuses, run backwards to the straight sinus, in the subarachnoid tissue which lies 

 between the two layers of the velum interpositum. 



VOL. III. O 



