ORGANS OF THE BODY. 599 



of the latter with its external portion, which is more vascular and of less 

 dense texture than the internal laminae. In the spinal cord it is very 

 scantily supplied with vessels. The dura mater is very rich in lymphatics. 

 Some of these run along side by side with the blood-vessels, while others 

 ensheath the latter. It appears very probable, further, that they open 

 into the cavity between the dura mater and arachnoid. The same seems 

 to occur also on the external surface of the first-named membrane 

 (Michel}. As yet no nerves have been found in the dura mater of the 

 cord, in contrast to that of the brain, which receives twigs from the sym- 

 pathetic and trigeminus, The mode of termination of these nervous 

 elements, which have been observed to undergo subdivision, has not yet 

 been sufficiently cleared up. They appear to end in the blood-vessels and 

 bones. 



The dura mater is separated from the next membrane underneath it by 

 what is known as the " subdural space" 



The second membrane, the arachnoidea, was formerly described as form- 

 ing a shut serous sac, but erroneously so ; the parietal leaf being usually 

 repcesented as fused together with the outer layer of the dura mater, since 

 it could not be demonstrated separately. It presents for our consideration a 

 very thin delicate membrane on the spinal cord, investing the pia mater 

 quite loosely, and only connected with the latter and with the nervous 

 roots by means of numerous bands of connective- tissue, varying according 

 to locality. Consequently there is left a considerable interspace between 

 it and the pia mater, known as the sub-arachnoid space,. But the relations 

 of this envelope on. the brain are different in some respects. Here we find 

 it for the greater part firmly adherent to the pia mater, the latter, however, 

 dipping in between the convolutions, while the arachnoid stretches across 

 the depressions between the same, and also those larger ones situated at 

 the base of the brain. In this way a great number also of smaller sub- 

 arachnoid spaces are produced. In regard to the connective-tissue of the 

 arachnoid Key and Retzius have recently made some admirable observa- 

 tions. The retiform fibrillated bundles are covered with flat connective- 

 tissue cells, similar to those we have already considered in speaking of 

 connective-tissue (130), the lymphatics (223), and the testes (283). 

 These are united to each other to form a kind of membrane, and fill up 

 the interstices of the various layers. Under the action, further, of nitrate 

 of silver solution they present tha well-known mosaic appearance of the 

 " endolhelial cells" (98, &c.) 



"Within these spaces situated under the arachnoid of the brain and 

 cord, and communicating more or less with one another, as well as within 

 the ventricles of the brain, we find what is known as the cerebro-spinal 

 fluid. This contains, besides, about 99 per cent, of water, small quan- 

 tities of album 'mate of sodium, extractives, and the ordinary salts of the 

 body (C. Schmidt, Hoppe). 



The number of capillaries encountered in the arachnoid is extremely 

 small. Nerves, on the other hand, are not uncommon, but whether they 

 actually end here is a point not yet decided. The outer surface of the 

 arachnoid and internal of the dura mater are covered with a slightly 

 laminated flattened epithelium (p. 141). 



We come now, finally, to the third and last of the coverings, the pia 

 mater. In it we have before us a delicate connective-tissue tunic. Here 

 also we find those flat membranous cells just referred to, with connective- 

 tissue bundles and elastic fibres. The whole, however, is a continuous and 

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