352 SECRETION. 



is to moisten the membranes so that the opposing surfaces can move over each other 

 without undue friction, only enough fluid is secreted to keep these surfaces in a proper 

 condition. The error frequently committed by authors, in describing the serous exhala- 

 tions as vaporous, is due to the fact that a vapor is generally given off when the serous 

 cavities are exposed, either in a living animal or in one recently killed. This vaporous 

 exhalation takes place after exposure of the parts ; but, if the cavities be observed with- 

 out exposing the serous surfaces to the air, a certain quantity of liquid can be detected. 

 Colin always found liquid in the peritoneal, pericardial, and pleural cavities of animals 

 recently killed or opened during life. In these cavities, the opposite surfaces of the 

 serous membrane were either in contact or the space between them was filled with 

 liquid. In one of the small ruminants, he removed the muscles and the elastic tunic 

 from the lower part of the abdomen, exposing the transparent peritoneum, and through 

 this membrane he could see liquid collected in the dependent parts. 



As far as has been ascertained, the secretions of the different serous membranes bear 

 a close resemblance to each other. They are either colorless or of a slight amber tinge, 

 alkaline in reaction, and have a specific gravity of from 1012 to 1020. Their composi- 

 tion resembles that of the serum of the blood, except that the proportion of water is 

 very much greater. They contain albumen, chlorides, carbonate and phosphate of soda, 

 and a little glucose. These facts are the result of observations upon the serous fluids of 

 some of the inferior animals ; and it is exceedingly difficult to obtain the normal fluids 

 from the human subject. The elaborate analyses which are sometimes given of the 

 fluids from the different serous cavities in the human subject are the results of examina- 

 tions of large morbid accumulations. 



The normal quantity of pericardial fluid in the human subject is generally estimated 

 at from one to two fluidrachms. Colin found that the pericardial sac of the horse con- 

 tained from two and a half to three and a half fluidounces, the cavity being exposed 

 immediately after the death of the animal from haemorrhage. 



The quantity of fluid found in the peritoneal cavity in horses killed in this way was 

 from ten to thirty-four fluidounces. 



The quantity of fluid in the pleural cavity in the same animal was from three and a 

 half to seven fluidounces. 



These estimates are simply approximative; but they give an idea of the normal 

 quantity of liquid which may reasonably be supposed to exist in the serous cavities of 

 the human subject. Judging from the weight of a man of ordinary size as compared 

 with that of a horse, it may be stated, in general terms, that the pericardial sac contains 

 from two and a half to three and a half fluidrachms ; the peritoneal cavity, from one to 

 four fluidounces; and the pleural sac, from three and a half to seven fluidrachms. 



The fluid in the cavity of the tunica vaginalis is small in quantity and resembles in 

 every respect the peritoneal secretion. The cephalo-rachidian, or subarachnoid fluid 

 will be described in connection with the anatomy of the cerebro-spinal nervous system. 



Synovial Fluid. Although there is a certain similarity between the serous and the 

 synovial membranes, their secretions differ very considerably in their physical and chemi- 

 cal characters. Like the serosities, the synovial fluid has simply a mechanical function ; 

 but it is more viscid and contains a larger proportion of organic matter than the serous 

 fluids. The quantity of fluid in the joints is sufficient to lubricate freely the articulating 

 surfaces. In a horse of medium size and in good condition, examined immediately after 

 death, Colin found 1-6 fluidrachm in the shoulder-joint; 1-9 drachm in the elbow- 

 joint; 1-6 drachm in the coxo-femoral articulation; 2-2 in the femoro-tibial articula- 

 tion ; and T9 in the tibio-tarsal articulation. 



When perfectly normal, the synovial fluid is either colorless or of a pale, yellowish 

 tinge. It is so viscid that it is with difficulty poured from one vessel into another. This 

 peculiar character is due to the presence of an organic substance called synovine. When 



