352 



HISTOLOGY. 



epithelium. It connects with the lymph spaces of the peripheral nerves, 

 with the lymph vessels of the nasal mucosa, and with the ventricles of the 

 brain through apertures in the roof of the fourth ventricle. It contains 

 an abundant fluid called the liquor cerebrospinalis. (The direct con- 

 nection of the subdural and subarachnoid spaces with both lymphatic 

 vessels and tissue spaces, is not in accord with recent embryological studies 

 and requires further investigation.) 



The pia mater of the cord and brain is a delicate vascular connective 

 tissue which extends into their substance along with its blood vessels. 

 Its nerves may remain outside. Pericellular lymphatic spaces around 

 the nerve cells, and the epicerebral space between the pia and the brain, do 

 not communicate directly with the lymphatic vessels. The blood vessels 

 form narrow-meshed capillaries in the gray substance and coarser ones 

 in the white. Capillaries in the cerebral cortex empty into veins which 



Epithelium. 



FIG. 407. PORTION OF THE PLEXUS CHORIOIDEUS OP AN ADULT MAN. X 80. 



x, Blood vessel in optical section. The large dots in the epithelium are not nuclei, but pigment and 



fat granules. 



arise in the white substance beneath, and from there pass through the 

 cortex to the pia; the blood in the capillaries therefore passes through the 

 entire cortex before emptying into the veins. The blood vessels generally 

 have a second so-called "adventitial sheath" consisting of a mesenchymal 

 epithelium. Within the sheath is an "adventitial lymph space" con- 

 necting with the subarachnoid space; outside of it is a peri vascular 

 tissue space. 



Chorioid plexuses. In certain places where the wall of the medullary 

 tube is very thin, as in the roof of the fourth ventricle, it becomes invagi- 

 nated into the central cavity by the vascular pia, thus forming a chorioid 

 plexus. The epithelial cells of the brain covering the plexus, contain 

 pigment granules and sometimes fat droplets. The chorioid plexuses 

 extending into the third, fourth, and both lateral ventricles, are essentially 

 similar in structure. A part of the network of blood vessels within them 

 is shown in Fig. 407. 



