368 GENERAL VIEW OF THE HUMAN FUNCTIONS. 



drogen, sulphur, phosphorus, &c., undergo oxidation), it is the principal means 

 of sustaining the independent heat of the " warm-blooded" animal. There is in 

 the system a certain self-adjusting power, whereby the consumption of the 

 pabulum provided for the combustive process is regulated according to the 

 external temperature; so that whilst, the external temperature being the 

 same, the amount of carbonic acid excreted varies with the degree of mus- 

 cular exertion made by the individual, any depression of the external tempe- 

 rature, requiring an augmented production of heat, occasions an increased 

 combustion of the oxidizable solids of the body, which is indicated by an 

 increase in the exhalation of carbonic acid from the lungs. The interchange 

 of oxygen and carbonic acid between the atmosphere and the blood, can only be 

 kept up by a continual renewal of the air in the interior of the lungs, and of the 

 blood in their capillaries. The former is effected by a set of muscular move- 

 ments that depend on the "reflex" power of certain nervous centres, and not on 

 any exertion of the will of the individual. It is not even requisite that he 

 should be conscious of their performance ; the ordinary power of the stimulus 

 that excites the movement not being sufficient to cause itself to be felt, unless 

 attention be specially directed to it. But if the respiratory movements be sus- 

 pended for a short time, sensations of distress are soon experienced, which 

 rapidly augment with the continuance of the suspension; and no exertion of the 

 will can any longer prevent the performance of the movements which are appro- 

 priate to relieve them. Thus we see that these movements, although placed in 

 Man under the control of the Will to such an extent as to enable him to regu- 

 late them in the actions of speech, are in themselves quite as independent of that 

 will, as are those of the Heart, whose automatic power has been already alluded to. 



383. The function of the Liver as an excreting organ is, like that of the lungs, 

 twofold: it separates from the blood a large quantity of the superfluous hydro- 

 carbon, which it acquires in circulating through the tissues ; and it combines 

 this with other elements ( 67-71), into a secretion, which is of great impor- 

 tance in the digestive process. The hepatic circulation, however, is not kept up 

 by a distinct impelling organ; but the venous blood from the abdominal viscera 

 (and, in the lower Vertebrata, that from the posterior part of the body) passes 

 through the Liver on its return to the heart. But further, all animal sub- 

 stances have a tendency, during their decomposition, to throw off nitrogen, as 

 well as carbon; and this nitrogen, in combination with other elements, forms 

 those peculiar azotized compounds ( 51-63), which it is the special function 

 of the Kidney to eliminate from the circulating fluid. The most characteristic 

 of these in Man, namely urea, contains a larger proportion of nitrogen than is 

 found in any other organic compound; and is identical in its chemical nature 

 with cyanate of ammonia. Its production seems in great part to depend upon 

 the disintegration of the muscular tissue ; but there is also evidence that it may 

 result from the retrograde metamorphosis of albuminous or even of gelatinous 

 matters circulating in the blood. The action of the kidneys is equally essential 

 to the continued 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 cha- 

 racteristic excretion from the circulating blood. 



384. 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 func- 

 tion 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 the 

 removal from the body of a portion of its superfluous fluid, containing products 



