332 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the hind legs of a well-nourished animal, no increase in urea in the blood can 

 be detected ; but if the blood, after irrigation through the hind legs, is subse- 

 quently passed through the liver, a marked increase in urea results. Obviously, 

 the blood in this experiment derives something from the tissues of the leg 

 which the tissues themselves cannot convert to urea, but which the liver-cells 

 can. Finally, in some remarkable experiments upon dogs made by four in- 

 vestigators (Halm, Massen, Nencki, and Pawlow), which will be described 

 briefly in the next section in connection with urea, it was shown that when 

 the liver is practically destroyed there is a distinct diminution in the urea 

 of the urine. In birds uric acid takes the place of urea as the main 

 nitrogenous excretion of the body, and Minkowski has shown that in 

 them removal of the liver is followed by an important diminution 

 in the amount of uric acid excreted. From experiments such as these 

 it is safe to conclude that urea is formed in the liver and is then given to the 

 blood and excreted by the kidney. When we come to describe the physiological 

 history of urea (p. 334 ), an account will be given of the views held with regard 

 to the antecedent substance or substances from which the liver produces urea. 

 Physiology of the Spleen. — Much has been said and written about the 

 spleen, but we are yet in the dark as to the distinctive function or functions of 

 this organ. The few facts that are known may be stated briefly without going 

 into the details of theories that have been offered at one time or another. 

 The older experimenters demonstrated that this organ may be removed from 

 the body without serious injury to the animal. An increase in the size 

 of the lymph-glands and of the bone-marrow has been stated to occur after 

 extirpation ; but this is denied by others, and, whether true or not, it gives 

 but little clue to the normal functions of the spleen. Laudenbach 1 finds that 

 one result of the removal of the spleen is a marked diminution in the number 

 of red corpuscles and the quantity of haemoglobin. He infers, therefore, that the 

 spleen is normally concerned in some way in the formation of red corpuscles. 

 These facts are significant, but they need, perhaps, further confirmation. The 

 mosl definite facts known about the spleen are in connection with its move- 

 ments. It has been shown that there is a slow expansion and contraction of 

 the organ synchronous with the digestion periods. After a meal the spleen 

 begins to increase in size, reaching a maximum at about the fifth hour, and 

 then slowly returns to its previous size. This movement, the meaning of which 

 is not known, i~ probably due to :i slow vaso-dilatation, together, perhaps, with! 

 a relaxation of the tonic contraction of the musculature of the trabecular In 

 addition to this slow movement, Roy 2 has shown that there is a rhythmic 

 contraction and relaxation of the organ, occurring in cats and dogs at intervals 

 of about one minute. Roy supposes that these contractions are effected through 

 the intrinsic musculature of the organ — that is, the plain muscle-tissue present 

 in the capsule and trabecules — and he believes that the contractions serve to 

 keep up a circulation through the spleen and to make it< vascular supply more 



1 Centralblatt fiir Physiologie, 1895, Bd. ix. S. 1. 



2 Journal of I'lii/xinl,,,/*/, 1 SS 1 , vol. iii. p. 203. 



