THE MENINGES. 



313 



Vascular tuft 



choroid plexuses to the lateral and third ventricles and of the similar plexuses 

 in the roof of the fourth ventricle. The pia mater further contributes a 

 sheath to each nerve, or its larger component bundles, as the nerve leaves 

 the brain or spinal cord, which sheath surrounds the nerve as it crosses the 

 subarachnoid space and for a variable distance beyond its emergence from 

 the dural sac. 



The spinal pia consists of two layers, of which the dense outer is 

 composed of interlacing stout bundles of fibrous tissue mixed with elastic 

 fibres and covered externally by endothelium, and the looser inner one of 

 less closely packed fibro- elastic strands. These layers are separated here 

 and there by lymphatic clefts and enclose between them the blood-vessels. 

 The latter subdivide within the pia into numerous small arteries which, 

 although the larger trunks frequently anastomose, enter the subjacent 

 nervous substance as "end-arteries," each providing the entire available 

 blood-supply for a definite territory. 



The brain-pia consists of only a single layer which corresponds to the 

 inner one of the spinal membrane both in structure and relations. The 

 larger vessels lie in or on its outer part and in certain places where they are 

 of large diameter, as at the base 

 of the brain, project within the 

 subarachnoid space, although 

 covered by a thin envelope of 

 pial tissue. As the vessels pene- 

 trate the nervous tissue, they 

 carry with them a sheath of pia 

 mater, at first loosely but later 

 closely applied. These consti- 

 tute the perivascular lymph- 

 sheaths that follow the arteries 

 to their smallest ramifications 

 and communicate, through the 

 intrapial lymph-clefts, with the 

 subarachnoid space. Since the 

 arteries entering the nervous tis- 

 sue, especially the cerebral and 

 cerebellar cortex, are very 

 abundant, collectively a consid- 

 erable amount of connective 

 tissue 'is carried with the vessels 

 into gray matter, the larger vas- 

 cular septa containing fibro-elastic tissue as well as neuroglia. The ultimate 

 distribution of the arteries entering the spinal cord and the brain is described 

 in connection with those organs (pages 279 and 307). In certain locations, 

 particularly the base of the brain and over the cervical and lumbar enlarge- 

 ments of the cord, the pia sometimes, especially in aged subjects, contains 

 deeply pigmented branched connective tissue cells. 



The choroid plexuses of the ventricles comprise two morphologically 

 distinct parts the vascular pial tissue and the thin covering of brain-wall. 

 The vascular fringes consist of numerous capillary convolutions, the cho- 

 roidal glomeruli, from 1-2 mm. in diameter, embedded in the pial connec- 

 tive tissue stroma and covered with a single layer of cuboid ependymal 

 cells. The latter contain fat and pigment particles and during foetal life 

 bear cilia. 



FIG. 359. Small portion of injected choroid plexus of 

 lateral ventricle ; surface view. X 25. 



