THE BRAIN. 



279 



The blood-vessels supplying- the spinal cord, derived from many 

 sources, form an arterial network within the pia mater that accompanies the 

 nervous cylinder throughout its length. The gray matter receives its 

 principal supply from the series of anterior fissural arteries, over two hun- 

 dred in number, that pass from the anterior spinal trunk backwards within 

 the anterior median fissure to its bottom and there divide into right and left 

 branches, which traverse the ante- 

 rior white commissure to gain the 

 gray matter on either side of the 

 central canal. These vessels, the 

 sulco- marginal arteries, divide into 

 ascending and descending branches 

 that provide a rich capillary net- 

 work for the entire gray matter, 

 with the exception of its most pe- 

 ripheral zone. The latter, together 

 with the white matter, receives its 

 supply from the penetrating arteri- 

 oles that come from the surrounding 

 intrapial trunks and enter the sub- 

 jacent surface of the cord. Unpaired 

 horizontal twigs, the posterior sul- 

 cal arteries, follow the posterior 

 median septum at different levels 

 for some distance, but before reach- 

 ing the posterior gray commissure 

 usually break up into terminal 

 ramifications, some of which pass 

 to the gray matter of the posterior 

 horns. Notwithstanding the capil- 

 lary anastomoses within the nervous tissue, each artery provides the sole 

 available supply for some definite territory; they are, therefore, "end- 

 arteries, ' ' a fact which explains the extensive and elaborate system of vessels 

 necessary to maintain the nutrition of the cord. The plexiform veins within 

 the spinal pia are formed by the union of the small radicles, that collect the 

 blood from the intraspinal capillaries and emerge at the surface of the cord. 

 Definite lymphatics within the spinal cord are unknown. 



The investing membranes or meninges of the spinal cord are 

 described in connection with those of the brain (page 311). 



THE BRAIN. 



Before entering upon the description of the histological details of the 

 more important parts of the brain, it is most desirable to have some notion 

 of their general position and relations. A brief survey of the gross anatomy 

 of the human brain, therefore, is an advantageous introduction to the study 

 of its structure. 



Denuded of its investing membranes and the attached cranial nerves, 

 and viewed from below (Fig. 327), the brain, or encephalon, is seen to con- 

 sist of a median brain-stem, that inferiorly is directly continuous with the 

 spinal cord, through the foramen magnum, and above divides into two diverg- 

 ing arms that disappear within the large overhanging mass of the cerebrum. 

 The brain-stem includes three divisions, the inferior of which, the medulla 



FIG. 326. Transverse section of injected cord, show- 

 ing- vascular supply of white and gray matter; a, 

 anterior spinal giving off anterior sulcal (b)\ c, ascend- 

 ing branch ; d, perforating arteries ; e, postero-lateral; 

 /, parasulcal ; g, posterior sulcal. X 10. 



