106 SEROUS MEMBRANES. 



over the trahecnlge ; covering these, like the walls of the commencing 

 Ijmph-lacunje elsewhere, with a layer of flattened cells. 



It is not unreasonable to presume that, in tlie proper glandular substance, there 

 is a continual production of lymph-corpiiscles, most probably by iissiparous multi- 

 plication, which pass into the lymph-sinus, and that fresh cor[3uscles are thus 

 added to the lymph as it passes through a gland ; and this view is supported hy 

 the fact, that the coi-puscles are found to be more abimdant in the lymph or 

 chyle after it has passed through the glands. It has been alleged, moreover, that 

 the lymph, after i:)assing the glands, is richer in fibrin, and therefore coagulates 

 more fimily. In any case, it is plain that the numerous blood-capillaries distri- 

 buted in a gland must bring the blood into near relation -ndth the elements of 

 the lymph ; and the latter fluid, as it must move very slowly through the rela- 

 tively wide space within the gland, is thus placed in a most favourable condition 

 for some not improbable interchange of material with the blood. 



SEROUS MEMBRANES. 



The serous membranes are so named from tlie apparent nature of the 

 fluid with which their surface is moistened. They line cavities of the 

 body which have no outlet, and the chief examples of them are, the 

 peritoneum, the largest of all, lining the cavity of the abdomen ; the 

 two pleuros and pericardium in the chest ; the arachnoid membrane in 

 the cranium and vertebral canal ; and the tunica vaginalis surrounding- 

 each of the testicles within the scrotum. 



Form and arrangement. — In all these cases the serous membrane 

 •has the form of a closed sac, one part of which is applied to the walls of 

 the cavity which it lines, the 2)arietal portion ; whilst the other is re- 

 flected over the surface of the organ or organs contained in the cavity, 

 and is therefore named the reflccied or visceral portion of the membrane. 

 Hence the viscera in such cavities are not contained within the sac of 

 the serous membrane, but are really placed behind or outside of it ;: 

 merely pushing inwards, as it were, the part of the membrane which 

 immediately covers them, some organs receiving in this way a complete, 

 and others but a partial and sometimes very scanty investment. 



In passing from one part to another, the membrane frequently forms 

 folds Avhich in general receive the appellation of ligaments, as, for 

 example, the folds of peritoneum passing between the liver and the 

 parietes of the abdomen, but which are sometimes designated by 

 special names, as in the instances of the mesentery, meso-colon, and 

 omentum. 



The peritoneum, in the female sex, is an exception to the rule that 

 serous membranes are perfectly closed sacs, inasmuch as it has two 

 openings by which the Fallopian tubes communicate with its cavity. 



A serous membrane sometimes lines a fibrous membrane, as where 

 the arachnoid lines the dura mater, or where the serous layer of the peri- 

 cardium adheres to its outer or fibrous part. Such a combination is 

 often named ^Jihro-serous membrane. 



The inner surface of a serous membrane is free, smooth, and polished; 

 and, as would occur with an empty bladder, the inner surface of one 

 part of the sac is applied to the corresponding surface of some other 

 part ; a small quantity of fluid, usually not more than merely moistens 

 the contiguous surfaces, being interposed. The parts situated in a 

 cavity lined by serous membrane can thus glide easily against its 



