SECRETION AND EXCRETION 327 



We may assume that various colloid-tissue elements differ in their 

 ability to absorb different crystalloids which they remove from solu- 

 tion, just as fibers remove dyes, and that thus a different composition 

 is given to the ultrafiltrate. There is ample reason to believe that 

 certain crystalloids (e.g., urea, nitrates, etc.) open the paths through 

 the colloid membrane, while others (e.g., sulphates) close them just as 

 was shown in their diffusion experiments with jellies by H. BECHHOLD 

 and J. ZIEGLER (see p. 55). Side by side with this occur the specific 

 chemical activities of the gland elements which give them their par- 

 ticular character, the formation of ptyalin in the salivary glands, 

 and of bile in the liver, etc. 



E. PRIBRAM * x has formulated a theory according to which there 

 first occurs a coagulation of nutritive material (granule formation) 

 in the gland cells, which is followed by swelling, i.e., secretion. 1 



With the above systematization of glandular function, we may ob- 

 tain an explanation of a number of processes, and we may learn how 

 to study the remainder. We shall, therefore, regard every secretion 

 as an ultrafiltrate whose composition is changed by reabsorption and 

 specific adsorption, and to which, in the case of most glands 2 (e.g., 

 salivary, pancreas, liver, etc.), specified chemical products of glandu- 

 lar activity are added. In what order the change by adsorption 

 and ultrafiltration occurs remains for the present an open question. 



The presence of free water is an essential condition for the ultra- 

 filtration of the blood. This applies only to the glands. As was 

 mentioned in more detail elsewhere, we may assume with MARTIN H. 

 FISCHER that venous blood, rich in C0 2 , abstracts water, whereas 

 arterial blood can release it. We now know that all glands are 

 plentifully supplied with arterial blood so that the free water neces- 

 sary for ultrafiltration is supplied. The influence of the pulsations 

 of blood pressure, noted by H. BECHHOLD, is considered on page 332. 



1 Many authors distinguish between secretion and excretion (urine, sweat, 

 etc.); whereas the former contain colloidal ingredients, they are more or less 

 completely absent from the latter. Since there is no essential difference we shall 

 not strictly enforce this classification. 



2 It cannot be denied, that the consideration of the digestive glands from this 

 viewpoint offers very considerable difficulties, since their function is controlled 

 to a very great extent by nervous influences. The glands which are not directly 

 under the control of the nervous system, e.g., the kidneys, give clearer pictures. 

 Since we know that there is an excess of water in arterial blood and that venous 

 blood fixes water, the nervous control of secretion becomes a working hypothesis 

 or at least the question becomes shifted, and we must investigate the effect of 

 nervous influences on the supply of blood to the glands. 



