TISSUES OF THE BODY. 



227 



which had been doubled in by the organs which they invest. But this 

 is by no means always the case, and only so at most in those which have 

 received the names of true serous sacs, among which may be reckoned the 

 pericardium, pleura, peritoneum, and tunica vafjinalis propria of the 

 testicle. The arachnoidea, 

 which has also been numbered 

 among these, has no parietal 

 layer. 



The synovial capsules of 

 joints only possess also the re- 

 quisites of a serous membrane 

 on theirlateral portions, namely, 

 a layer of connective - tissue 

 clothed by epithelium, while 

 the floor and roof of the cavity 

 are formed of the naked articu- 

 lar cartilage. 



But some other cavities, ar- 

 ranged also in this category, 

 are even more imperfectly de- 

 fined, namely, the synovial 

 bursse and sheaths of tendons. 

 Here we have frequently to 

 deal, not with a regular wall, but 

 with an extremely soft connec- 

 tive-tissue, saturated with fluid, 

 gradually becoming more solid 

 externally, instead of a distinct 

 cavity. But in those situations 

 where the sheaths and bursaB 

 in question are more sharply defined, we may encounter at points a simple 

 flattened epithelium on the con- 

 nective-tissue forming the walls of 

 the cavity. 



The formation of these " true " 

 and "false" serous sacs is explained 

 by the occurrence of those cavities 

 in connective-tissue mentioned at 

 98. By the formation in the 

 middle germinal plate of larger 

 hollows of this kind, which become 

 more and more defined, we pass 

 gradually from the mucoid sheath 

 to the true serous sac. The sub- 

 arachnoid spaces may be looked 

 upon to a certain extent as inter- 

 mediate forms. 



The same serous exudation which 

 saturates formless connective-tissue 

 retains the surfaces of these cavities 

 moist and slippery. Its amount is 

 normally but very smalL We have already met with this fluid in greater 

 abundance in the form of synovia (p. 155). 



Fig. 219. Human skin in vertical section, a, super- 

 ficial layers of the epidermis ; b, rete Idalpighii. Un- 

 derneath the latter is the cerium, forming papillae at r, 

 and passing below into the subcutaneous connective- 

 tissue, in which collections of fat-cells may be seen us 

 A ; ff, sweat glands with their ducts, e and /; rf, vessels; 

 t, nerves. 



Fig. 220. Two tactile papillae of the skin freed of 

 epithelium. Here may be seen the connective- 

 tissue entering into their composition, with the 

 tactile corpuscles in the interior and the nerves 

 ending in the latter. 



