82 GENERAL VIEW OF f HE FUNCTIONS. 



from the posterior part of the body) passes through the Liver on its return to 

 the heart. 



93. All animal substances have a tendency, during their decomposition, to 

 throw off nitrogen, as well as carbonic acid ; and this nitrogen may take either 

 the form of cyanogen, by going off in combination with carbon, or of ammonia, 

 by uniting at the time of its liberation with hydrogen. The chief function of 

 the Kidneys is evidently to separate the azotized products of decay from the 

 circulating fluid ; for the secretion which is characteristic of them, namely, 

 urea, contains a larger proportion of nitrogen than is found in any other or- 

 ganic compound ; it is identical in its chemical nature with cyanide of ammonia, 

 and may be considered as the result of the union of these two products of ani- 

 mal decomposition. The action of the kidneys is equally essential to the con- 

 tinued performance of the other vital functions, with that of the lungs and 

 liver ; since death invariably follows its suspension, unless some other means 

 be provided by Nature (as occasionally happens), for the separation of its 

 characteristic excretion from the circulating blood. But death does not so 

 speedily ensue, when the functional action of the liver and the kidneys is 

 suspended, as when that of the lungs is checked ; and for this obvious reason, 

 that only apart of the whole current of blood flows through the former organs, 

 and that, although a disturbance of the usual course of the circulation must 

 ensue from a stagnation of the flow through them, it is not from this cause 

 brought to a stand; whilst, in the case of the lungs, the fact that the whole of 

 the blood is sent to them, before it can be again impelled through the body, 

 necessitates the immediate cessation of the systemic circulation, when the pul- 

 monary has been checked. In the class of Reptiles, the lungs are on some- 

 what of the same footing w r ith the liver and kidneys in warm-blooded Verte- 

 brata, that is, only a part of the blood which has returned from the system 

 is transmitted through them, before being again propelled through the body ; 

 and, accordingly, the interruption of the pulmonary circulation does not in 

 them involve immediate death. Indeed, in the naked-skinned Batrachia, the 

 cutaneous surface has enough respiratory power, to effect that degree of al j ra- 

 tion of the blood which is necessary, whilst the temperature is low, and the 

 vital actions thereby diminished in energy. 



94. There seems reason to believe, however, that, of the products of decom- 

 position which are set free in the various tissues and organs of the body, only 

 a part is destined to be immediately excreted ; and that it is this part which 

 is taken up by the Veins, and conveyed, by the general vascular apparatus, 

 to the several glands which are to separate it. The remainder, consisting of 

 substances which are fit to be re-assimilated, appears to be taken up by a dis- 

 tinct system of vessels, termed Lymphatics ; which may be considered as an 

 extension of the Lacteal system through the fabric at large. There is good 

 reason to believe, that the special functions of the Lymphatics is, like that of 

 the Lacteals, to minister to Nutritive absorption, (although other substances 

 may find their way into them, by the mere physical process of imbibition) ; 

 the latter being especially destined to take up assimilable matter from the di- 

 gestive cavity, whilst the former absorb the products of the secondary digestion 

 which is continually going on in every part of the body. (See 404 467.) 

 Of these, however, a portion may still be destined to immediate excretion. 



95. The various Secretions which have not already been adverted to, appear 

 for the most part to have for their object the performance of some special 

 function in the system, rather than the conveyance out of it of any substances 

 which it would be injurious to retain. This is the case, for example, in regard 

 to the secretion of the Lachrymal, Salivary, and Mammary Glands, as well as 

 with that of the Mucous and Serous Membranes. The Excretion of fluid from 

 the cutaneous surface, however, appears to answer two important purposes, 



