906 SPECIAL PHYSIOLOGY. 



solid excreta, whilst the nitrogenous bodies ultimately reach the chemi- 

 cal condition of urea. But the more obvious metamorphosis of the albu- 

 minoid bodies, is that which consists of a series of retrograde chemical 

 changes into more oxidized nitrogenous bodies, such as creatin, crea- 

 tinin, leucin, tjrosin, inosinic acid, sarcin, xanthin, hippuric acid, and 

 uric acid, by which path they ultimately reach the condition of urea, 

 a substance identical with cyanate of ammonia, and which has also 

 been regarded as a carbamide or a carbide of amidogen, which con- 

 tains carbon, hydrogen, nitrogen, and oxygen. The ammonia found 

 amongst the saline constituents is probably always derived from a 

 further breaking up of urea. 



Albumen may be artificially decomposed, by acids or alkalies, or 

 by spontaneous changes, into leucin, tyrosin, and glycocoll, all which 

 nitrogenous compounds are found in the body, especially in venous 

 blood, and in the liver and spleen ; whilst creatin, creatinin, and ino- 

 sinic acid, are found in actively exercised muscles, arid in the blood. 

 Creatinin is, of all these substances, the nearest to urea, and is readily 

 converted into it, by assumption of the elements of water. Urea itself 

 has been found in the muscles of certain fishes. Gelatin and the 

 gelatinoid substances, behave in their downward metamorphoses like 

 albuminoid bodies, yielding especially urea, but no sulphur compounds. 

 It is doubtful whether they ever undergo an upward metamorphosis 

 into albumen ; but they may spare the waste of this, and may save, 

 and even nourish, the gelatin-yielding tissues. Large quantities alone 

 are useful for this purpose ; when much gelatin is taken in the food, 

 the urea is increased in the urine, the specific gravity of which has 

 been known to rise to 1034. 



One important inference from our present knowledge concerning the 

 chemistry of the food in the body, is this: that all food may be either 

 oxidized after being merely absorbed or assimilated into the blood, as 

 well as after its constituents have been converted into tissue. This is 

 sufficiently obvious as regards carbonaceous and hydrogenous food, or 

 the respiratory food ; but it is equally true of the plastic albuminoid 

 and gelatinoid substances. The excretion of urea is not so much in- 

 creased by muscular exertion as was once supposed, but it is largely 

 augmented by an excess of nitrogen in the food. (E. Smith, Voit, 

 Lehmann, Fick and Wiscilenus, and others.) The excess of any sub- 

 stance in the food, beyond that which is necessary for the tissue and 

 for respiration, is known as the luxus consumption, or diet of luxury ; 

 it reappears in an increased excretion of urea, carbonic acid, arid water. 



Interesting deductions may be drawn from comparing the destina- 

 tion of the food in the Herbivorous and Carnivorous animals. In the 

 Herbivora, a very large proportion of the carbon, hydrogen, and nitro- 

 gen of the food passes off undigested from the alimentary canal ; 

 whilst in the Carnivora, naarly all the food constituents are absorbed 

 into the chyle or blood. Of the carbon which thus enters the blood, 

 the ratio of that given off by the lungs and skin, to that excreted by 

 the kidneys, is, in the Herbivora, about as 30 to 1, whilst in the 

 Carnivora, the proportion is only as 10 to 1. Of the hydrogen ab- 

 sorbed, a greater relative proportion is also found in the cutaneous 



