THE MENINGES AND BLOOD SUPPLY 625 



one of which is distributed through the pia mater to the adjacent white 

 matter of the spinal cord and to the gray pallium of the brain ; the other 

 penetrates the spinal cord through the anterior median fissure by a 

 series of small fissured arteries to be distributed to the central gray mat- 

 ter, and in the brain is represented by the branches of the middle cere- 

 bral arteries which penetrate directly to the ganglionic gray matter in 

 the interior of the cerebrum. 



In the spinal cord the vessels of the former set are mostly distributed 

 to the white cortex, the larger branches, however, penetrate the white 

 matter and aid in the formation of the capillary network of the gray 

 medulla. In the brain their distribution is similar, the smaller pial 

 vessels, the cortical arteries, being distributed to the cortex, which in this 

 case is formed by the gray matter; the larger, the medullary arteries, 

 penetrating to the white medulla in which they break up into capillary 

 vessels. 



The veins trend in the opposite direction and in the pia mater col- 

 lect into large vessels, which in the brain open into the sinuses of the 

 dura mater, and which in the spinal cord form the ventral and dorsal 

 median veins. 



All the larger vessels receive thin fibrous investments from the pia 

 mater; the smaller vessels and capillaries are surrounded by neuroglia. 



There are frequent anastomoses between the larger veins ; the arteries, 

 however, are all terminal arteries according to Cohnheim's classification, 

 possessing no anastomoses with the capillary areas of other vessels. 



Neither brain nor cord possess true lymphatics. The sole lymphatic 

 representatives within the central nervous system are pericellular and 

 perivascular spaces communicating with subpial spaces and ultimately 

 through uncertain clefts and channels with the subarachnoid spaces. 



The bulk of the cerebro-spinal fluid is secreted by the cells of the 

 choroid plexuses directly into the cerebral ventricles. It escapes into the 

 subarachnoid spaces through the roof of the fourth ventricle. From here 

 it is absorbed into the venous sinuses by way of the arachnoid villi. A 

 small quantity of cerebro-spinal fluid is contributed also by the blood 

 capillaries of the central nervous system. The lymphlike fluid passes 

 from these capillaries directly into the pericapillary spaces and thence 

 to each nerve cell or outward through the perivascular channels to the 

 subarachnoid spaces. Besides the fluid which escapes through the venous 

 sinuses of the dura a small amount drains also by way of the perineural 

 spaces indirectly into the lymphatic system. (See L. H. Weed; Anat 

 Rec., Vol. 12, 1917.) 



