SEROUS MEMBRANES. cxci 



which serve to discharge the lymph into the veins. These organs, which are named 

 lymph-hearts, have now been found in all the different orders of reptiles. In frogs 

 and toads two pairs have been discovered, a posterior pair, situated in the sciatic 

 region, which pour their lymph into a branch of the sciatic or of some other neigh- 

 bouring vein, and an anterior more deeply seated pair, placed over the transverse 

 process of the third vertebra, and opening into a branch of the jugular vein. The 

 parietes of these sacs are thin and transparent, but contain muscular fibres of the 

 striated kind, freely ramifying, decussating in different layers, as in the blood-heart. 

 In their pulsations they are quite independent of the latter organ, and are not even 

 synchronous with each other. In salamanders, lizards, serpents, tortoises, and 

 turtles, only a posterior pair have been discovered, which, however, agree in all 

 essential points with those of the frog. In the goose, and in other species of birds 

 belonging to different orders, Panizza discovered a pair of lymph sacs opening into 

 the sacral veins, and Stannius has since found that these sacs have striated muscular 

 fibres in their parietes; but, although this observer, in some cases, exposed them 

 in the living bird, he was not able to discover any pulsation or spontaneous movement 

 in them. Nerve-fibres, both dark bordered and pale, have been observed in the 

 lymph-hearts of the frog, and also nerve-cells in those of the common tortoise. 

 (Waldeyer.) 



Development of lymphatic r&sels. Kb'lliker has observed the formation of lym- 

 phatics from ramified cells in the tails of young salamander-larvae. He states that the 

 process takes place nearly in the same manner as in the case of sanguiferous capillaries; 

 the only notable difference being, that whilst the growing lymphatics join the ramified 

 cells, and thus extend themselves, their branches very rarely anastomose or become 

 connected by communicating arches. The soundness of his conclusions has, however, 

 been called in question and the subject requires further elucidation. New-formed 

 lymphatics have been injected in adhesions between inflamed serous membranes. 



SEROUS MEMBRANES. 



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

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

 have uo outlet, and the chief examples of them are, the peritoneum, the 

 largest of all, lining the cavity of the abdomen ; the two pleurae and peri- 

 cardium in the chest ; the arachnoid membrane in the cranium and verte- 

 bral 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 w.Jls of the cavity 

 which it lines, the parietal portion ; whilst the other is reflected over the 

 surface of the organ or organs contained in the cavity, and is therefore 

 named the reflected 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 some- 

 times very scanty investment. 



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

 folds which 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 omeutum. 



The peritoneum, in the female sex, is au 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- 



