272 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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

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

 of the urine, its place being taken by carbamic acid. 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. 274), 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 which 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 

 most 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, is probably due to a slow vaso-dilatation, together, perhaps, with 

 a relaxation of the tonic contraction of the musculature of the trabeculaB. 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 trabeculse and he believes that the contractions serve to 

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

 or less independent of variations in general arterial pressure. These .observa- 

 tions are valuable as indicating the importance of the spleen functions. The 

 fact that there is a special local arrangement for maintaining its circulation 

 makes the spleen unique among the organs of the body, but no light is thrown 

 upon the nature of the function fulfilled. The spleen is supplied richly with 

 nerve-fibres which when stimulated either directly or reflexly cause the organ 

 to diminish in volume. According to Schaefer, 3 these fibres are contained in 



1 Centralblatt fur Physiologic, 1895, Bd. ix. S. 1. * Journal of Physiology, 1881, vol. iii. p. 203. 

 3 Proceedings of the Royal Society, London, 1896, vol. lix., No. 355. 



