I202 



HUMAN ANATOMY. 



Fig. 1036. 



Gray matter 



White matter 



the communications which these cranial nerves have with the plexuses surrounding 

 the arteries or with the superior cervical ganglion. The sensory nerves of the dura 

 form a rich net-work of delicate twigs from which filaments have been traced to the 

 inner surface in relation to which some end in bulbous expansions. 



The Pia Mater. — This membrane (pia mater encephali) lies next the nervous 

 substance and, being the vascular tunic supporting the blood-vessels for the nutrition 

 of the brain, follows accurately all the inequalities of its exterior. It not only closely 

 invests the exposed surface of the cerebrum and cerebellum, but penetrates along the 

 sides and to the bottom of all the fissures as well, although within the small shallow 

 fissures of the cerebellum a distinct process of pia mater can not be demonstrated. 

 Additionally, in certain places where the wall of the brain-tube is very thin, the pia 

 pushes before it the attenuated layer and seemingly gains entrance into the ventricles. 

 Examples of such invagination are afforded in the relations of the velum interpositum 

 and the choroid plexuses to the lateral and third ventricles (page 1162) and of the 



similar plexuses in the roof of the 

 fourth ventricle (page iioo). The pia 

 also contributes a sheath to each nerve, 

 or to its larger component bundles, as 

 the nerve leaves the brain at its super- 

 ficial origin, which sheath surrounds 

 the nerve during its intracranial course 

 and for a variable distance beyo'nd its 

 emergence from the dural sac. 



The pia is so thin that the larger 

 vessels, especially at the base of the 

 brain, lie within the subarachnoid 

 space, although in most cases they are 

 enclosed within a delicate investment 

 of pial tissue. The smaller vessels, 

 however, ramify within the pia and in 

 this situation divide into the twigs 

 which directly enter the subjacent 

 nervous tissue. As they penetrate the 

 latter they are accompanied by a 

 sheath of pia, which thus gains the 

 nervous substance within which it fol- 

 lows the subdivisions of the arteriole, 

 even their smallest ramifications. 



Whilst within the pia the larger 

 arteries form frequent anastomoses, 

 the smaller twigs remain isolated and, 

 being " end-arteries," on entering the 

 subjacent gray matter break up into 

 terminal ramifications which furnish the only supply for a particular district. The 

 capillary net-work within the cortical gray matter is much closer than that within the 

 subjacent white matter (Fig. 1035), in which the vessels are comparatively meagre. 

 Here and there larger vicdullary branches are seen traversing the cortex, to which 

 they contribute but few twigs, to gain the white matter within which they find their 

 distribution. The contrast in richness between the supply of the gray substance and 

 that of the adjoining white matter is not limited to the cerebral cortex, but is also 

 well shown when the internal nuclei are examined (Fig. 1036). The veins emerge 

 from the surface of the brain, but do not retain a definite relation to the arteries, since, 

 instead of following the latter to their points of entrance, they for the most part seek 

 the dural sinuses into which they empty. 



The special invaginating layers of pia mater, the velum hiterpositiwi (page 1162) 

 and the choroid plexuses of the lateral and third ventricles, and the choroid plexus of 

 the fourth ventricle (page iioo) have been described in connection with the appro- 

 priate parts of the brain. Attention may be again called to the manner in which the 

 velum interpositum and the associated plexuses are formed (page 1194), and to the 



Portion of injected dentate nucleus of cerebellum, show- 

 ing capillarj- supply of internal nucleus. X 20. 



