752 SPECIAL PHYSIOLOGY. 



and, lastly, the lungs, which may be viewed as excreting glandular 

 organs, destined to eliminate carbonic acid from the blood. 



In certain forms of secretion, the separated products closely resemble 

 those contained in the blood itself, such as the albumen of the serous 

 and synovial fluids. Thus, the serous and synovial secretions consist 

 of little more than the transuded materials of the plasma of the blood, 

 unaltered in chemical character, but modified in their relative propor- 

 tions. The casein of milk is also merely a modified form of albumen. 

 In other more special secreting processes, there are formed, not as mere 

 transudations, but as the result of peculiar assimilative actions, sub- 

 stances not present in the blood itself, but, nevertheless, little removed 

 in chemical character from its albuminoid constituents; such, e.g., as 

 the pepsin, pancreatin, and salivin of the gastric juice, pancreatic 

 fluid and saliva, and the mucin of the mucous glands. The three 

 former substances are, by some, regarded as examples of albuminoid 

 compound undergoing retrograde chemical changes, or in peculiar 

 states of hydration. In other cases, the substances formed by secret- 

 ing glands, though more remote in chemical constitution from that of 

 the materials of the blood, and not pre-existent in it, are of a highly 

 complex nature, and are only partially reduced or oxidized substances, 

 such as the tauro- and glyco-cholic acids of the bile, the butyrin of 

 the milk, and the fat of the sebaceous secretion. Extreme examples 

 of special secretive power, by which compounds not existing in the 

 blood, are formed from it, are afforded by the appearance of sulpho- 

 cyanogen in the saliva, and of hydrochloric acid in the gastric juice. 

 So, also, soda is withdrawn from the normal soda salts of the blood, 

 by the agency of the liver, to combine with the fatty acids of the bile. 



In the case of the excretions, however, the characteristic substances 

 eliminated from the blood pre-exist in that fluid, as the result of de- 

 composition, and are always much more chemically reduced by oxida- 

 tion than any product of secretion, or they are even completely oxid- 

 ized. They usually exhibit a comparatively simple atomic constitution, 

 are often crystallizable, and frequently take the form of bases or acids, 

 such as the lactic and uric acids, and the urea, formed in the urine, 

 together with the sulphates and phosphates resulting from the oxida- 

 tion of the albuminoid constituents of the body ; such also as the 

 lactate of ammonia, and the acetic and formic acids of the cutaneous 

 excretion ; and lastly, the perfectly oxidized carbonic acid, given off 

 in small quantities by the skin, but forming the characteristic product 

 excreted by the lungs. Such substances are manifestly incapable of 

 animal organization. They are even, if retained in the system, noxious, 

 or fatal. The purpose of excretion is, indeed, to rid the body of the 

 compounds which are formed during the action of the living tissues, by 

 the oxidation of their substance, or of the blood passing through them. 

 The successive stages of oxidation render such compounds more and 

 more removed from an organizable character, and necessitate their 

 removal. 



In all the secretions, if one excepts the peculiar albuminoid sub- 

 stances, the saline substances and other special compounds are either 

 crystallizable, such as the sulphocyanide of potassium in the saliva, 



