THE CENTRAL NEKVOUS SYSTEM. 297 



round nuclei ; here and. there are seen stomata marked by an ag- 

 gregation of nuclei, which are all located at the edge of the cells 

 surrounding a stoma ; secondly, irregularly disposed lymph- 

 spaces and vessels. The lymph-spaces appear as irregular, 

 transparent patches lying just under the endothelium, and 

 the lymph-vessels are 

 seen as varicose chan- 

 nels, which begin by 

 a blind extremity, an- , 

 astomose freely, re- 

 ceive tributaries, and, 

 iinally, empty by a 

 constricted orifice 

 called a stoma. Their 



' " ^- *"" " e, 



WallS are in places FIG. ISS. Diagram representing the internal surface of the 



i-i 1-4. fi- dura, mater treated with nitrate of silver. Shows endothelium 



Him, DUG more Olien with nuclei and intercellular masses of protoplasm and a lymph- 



, -i . -i -, , channel, L.c., lined by delicate endothelial cells, which termi- 



IlllCK anOL 11'regUlar nates at S, an opening called a stoma about which is an aggre- 



, -, . gation of nuclei, x 300. 



from the aggregation 



of masses of protoplasm along the sides. The capillary lymph- 

 atic radicles are lined by a very delicate endothelial layer, 

 which can only be demonstrated in completely successful sil- 

 ver preparations. Some are seen to contain lymph-corpuscles, 

 others are found empty. 



The spinal arachnoid is also a serous membrane, much more 

 delicate, however, than the dura mater. Its extreme thinness 

 allows it to be examined in the fresh state or stained by car- 

 mine or hsematoxylon. It is seen to be essentially a large- 

 meshed connective-tissue network, containing many elastic 

 fibres. Good silver preparations demonstrate an endothelial 

 coat and a lymph system similar to, but more delicate than 

 that of the dura mater. It is doubtful whether blood-vessels 

 exist in the arachnoid in its normal state. 



The spinal pia mater consists of a small amount of connec- 

 tive tissue, holding together a vascular plexus. It is firmly 

 adherent to the cord, dips into all its fissures, and is intimately 

 connected and continuous with the connective-tissue frame- 

 work of the cord. The pia mater is best studied by means of 

 fresh specimens stained in hsematoxylon, as this demonstrates 

 beautifully the different coats of the small vessels. 



The spinal fluid should be clear, and contain only a few 

 lymph-corpuscles ; but it usually, when examined post-rnor- 



