660 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



other, or as to the manner in which energy is set free and made use of. 

 Certain of these processes of conversion may, however, be followed, and 

 the fact that these processes may in certain instances be located in 

 special tissues, in the continuation of the plan of specialization of 

 function, we are, perhaps,- warranted in speaking of certain tissues as 

 set aside to elaborate the raw materials which result from the absorption 

 of digestive products and to so modify the waste products as to permit 

 of their ready removal from the bodj r . Such tissues are spoken of as 

 metabolic tissues, and the study of the chemical changes which occur in 

 these tissues furnishes us with one method of tracing the conversion of 

 the substances absorbed into the matters excreted. 



The other method of studying the nutritive phenomena of the 

 animal body is what is known as the statistical method. 



We may, by chemically examining the composition of the food- 

 stuffs, ascertain the total quantity of each constituent. By chemical 

 analysis we may also determine the total amount of these different con- 

 stituents removed from the body in the excretions. By the comparison 

 of the results obtained in these two examinations valuable conclusions 

 may be drawn as to the changes which occur in the body in the con- 

 version of the income into the outgo. These methods will be referred 

 to in turn. 



I. THE FATE OF THE ALBUMINOUS FOOD-CONSTITUENTS. 



It has been seen that the albuminous food-constituents in digestion, 

 through a process of hydration by the action of ferments, are converted 

 into peptone, which enters through absorption into the radicals of the por- 

 tal vein, but an insignificant portion being absorbed by means of the lac- 

 teals. When once within the blood-current, possibly in the process of 

 absorption itself, the peptone must apparently be reconverted to the form 

 of albumen, for the amount of peptone capable of detection in the portal 

 vein, even after an abundant albuminous diet, is too insignificant to repre- 

 sent the amount of albuminous matters absorbed. We may , therefore, state 

 that almost immediately on entering the blood-current the albuminous 

 food-constituents are converted mainly into serum-albumen. It has 

 further been stated that urea represents the end product in the series of 

 decompositions which the albuminous bodies of the tissues undergo. 

 The attempt to trace the changes, commencing with albumen and termi- 

 nating in the production of urea, is, however, shrouded with the greatest 

 obscurity. It is known that the amount of urea removed gives an index 

 of the amount of albuminous destruction occurring in the animal body. 

 Numerous experiments have proved that with the withdrawal of all food 

 the excretion of urea decreases, and that a small amount continues to be 

 removed through the urine until the occurrence of death through starva- 



