150 ELEMENTS OF HISTOLOGY. [Chap, xvm, 



substance become thereby filled and enlarged, and 

 finally the injection matter enters the venous sinus 

 itself. The pia cerebralis is very rich in blood-vessels, 

 like that of the cord, which pass to and from the 

 brain substance. The capillaries of the pia mater 

 possess an outer endothelial sheath. The plexus 

 choroideus is covered with a layer of polyhedral epi- 

 thelial cells, which are ciliated in the embryo and in 

 the young subject. 



195. As was mentioned of the cord, so also in the 

 brain the sub-dural lymph space does not communicate 

 with the sub-arachnoidal spaces or with the ventri- 

 cles (Luschka, Key and Retzius). Nor does there 

 exist a communication between the sub-arachnoidal 

 space and a space described by His to exist between 

 pia mater and brain surface, but doubted by others. 

 The relations between the cerebral nerves and the 

 membranes of the brain, and the lymph-spaces of both, 

 are the same as those previously described in the case 

 of the cord and the spinal nerves. 



196. The pia mater passes with the larger blood- 

 vessels into the brain substance by the sulci of the 

 cerebrum and cerebellum. 



In the white and grey matter of the brain we find 

 the same kind of supporting tissue that we described 

 in the cord as neuroglia. In the brain also it is 

 composed of a homogeneous matrix, of a network of 

 neuroglia fibrils, and of branched, flattened neuroglia 

 cells, called Deiters' cells. 



In the white matter of the brain the neuroglia 

 contains between the bundles of the nerve-fibres rows 

 of small nucleated cells ; these form special accumula- 

 tions in the bulbi olfactorii, and in the cerebellum. 

 Lymph corpuscles may be met with in the neuroglia, 

 especially around the blood-vessels and ganglion cells. 



All the ventricles, including the aqueductus Sylvii, 

 are lined with a layer of neuroglia, being a direct 



