SECRETIONS OF SEROUS MEMBRANES. 193 



two heads : 1st. Membranes, such as the serous and mucous ; 2d. Glands, 

 as the liver, kidney. This division is, however, not founded either on 

 structural or functional differences, and is to be preserved merely for the 

 sake of convenience. 



A secreting membrane consists essentially of a tunic of connective tis- 

 sue, affording a nidus for vessels and nerves. Upon this, in the opinion 

 of many anatomists, a thin basement membrane is laid, the existence of 

 which is denied by others. Upon the surface of the basement membrane 

 there is a layer of cells, the form and arrangement of which differ in 

 different regions. In some places the cells are flat, in others cylindroid. 

 Their duration is temporary, one brood succeeding another from germs on 

 the basement membrane. The superficial, and, therefore, the older cells, 

 desquamate or deliquesce, and are replaced by others from beneath. It 

 is usually said that the serous membranes, with the exception O f serous mem 

 of the peritoneum, are all closed sacs, the peritoneum being branes and 

 perforated where the fimbriated extremities of the Fallopian their secretion ' 

 tubes open into the abdominal cavity in the mammalia, and in fishes 

 through the lateral anal openings. The generality of this view is now 

 called in question, both as regards the sy no vial sacs and bursse mucosas, 

 which all belong to this group. Thus Kolliker regards the synovial 

 structures as tubes open at both ends, and attached by their edges round 

 the articular surfaces of the bones. 



However this may be, even the peritoneum is practically a shut sac. 

 Accumulations of water within it do not escape through the apertures of 

 the Fallopian tubes, nor can air be injected the opposite way. 



The fluid exuding from the serous surfaces is a dilute albuminous so- 

 lution, more dilute as it is presented in the ventricles of the _ 



n T . , i Serous fluids. 



brain, and more concentrated in the synovia! cavities, its con- 

 sistency in the latter case being such that it may sometimes be drawn 

 out in tenacious threads. The mechanical qualities of these various ex- 

 udations permit a certain freedom of motion in the parts to which they 

 are applied. Thus the secretion of the peritoneum facilitates the move- 

 ments of the abdominal viscera ; those of the pericardium and pleura, of 

 the heart and lungs ; those of the synovial membranes and bursa3 mu- 

 cos03, of the joints and tendons. 



The nature of serous secretions may be illustrated by the cases of 

 fluids collected from the abdominal and thoracic cavities, &c. They 

 are usually of a faint yellowish color, clear or turbid, reaction alkaline, 

 and sometimes containing so much albumen as to coagulate readily on 

 heating. 



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