THE METABOLISM OF PBOI 619 



perfused, no urea is formed. The evidence shows thai the liver is an 

 important seal of urea formation, bn1 qo1 q< rily that other organs 



arc unable to form ii in the intacl animal, for there are many 

 of inaccuracy in perfusion experiments. Even though 

 greatesl care, we can not hope to maintain the organ in other than .1 

 slowly dying condition. It is certainly far removed from the normal 

 state, as [3 revealed no1 only by histological examination, bul by the fact 

 thai edema almosl Invariably sets in and the blood vessels become 

 tremely constricted, thus necessitating a gradual increase in tl ■ 

 fusion pressure as the perfusion goes on. Furthermore, the organ being 

 isolated from the nervous system, there can be no control of the rela- 

 tive blood supply of different parts. In the intact animal the circula- 

 tion is more or less distributed according to the particular needs of the 

 different viscera, and such conditions obviously can not be simulated in 

 a perfusion experiment. Another objection depends on the fact that 

 the well-being of the organs in the intact animal is largely dependent on 

 hormones conveyed to them from other organs. Such hormones 

 frequently quite labile in nature, and soon disappear from the perfusi 

 fluid. 



Notwithstanding these objections, there can be no doubt that many 

 of the functions of an organ are retained much longer than they would 

 be if the organ were not perfused; for example, the contractility of the 

 muscle or the power of forming urea in the liver. Perfusion experim< 

 are of value therefore when they yield positive results. Negative 

 sidts may indicate either that the organ does not perform the particular 

 function that is being investigated or that it has lost this function , - 

 resull of partial death. That a perfused muscle retains its power of 

 contraction does not necessarily indicate that it maintains all of its 

 metabolic functions; neither docs the fact that the liver forms urea 

 prove that it is capable of performing its other functions, it to 



show thai the liver dies piecemeal; some functions, such . 3 _ 

 formation, die early, while others, such as urea-formation, remain for a 

 long time intact. Th\ use of perfusion ments for t'< ■ settling 



questions of metabolism should tfo always h< very carefully 1 



trolled and never used as tfn soli line of evidence on which to h • 

 tani conclusions. 



Before leaving this subject it may be well to point out thai the 

 method which at firsl sighl mighl appea such 



questions namely, the examination of t)n inflowing <i>i<l outflowing l>' 

 of different parts or organs is not applicable in d si Tl 



cause of the extremely small changes in concentration which maj 



even although large amounts of the particular sul in qi 



