

SECRETION 343 



It is remarkable that the latter investigators found an increase of glucose in the venous 

 blood of the gland during the process of secretion itself ; they hold that it may be a constituent 

 ot some substance which breaks down in secretory activity. This does not seem a very 

 probable thing to happen, as it would not be capable of affording much energy It is also 

 difficult to understand why the glucose does not appear in the saliva. It seems, moreover 

 that the concentration of the venous blood, due to loss of water into saliva and lymph, has not 

 been sufficiently taken into account in the experiments referred to. For example, in experi- 

 ment 1 (p. 40 of the paper), during the two minutes necessary for collection of the 10 '57 c c 

 of venous blood for analysis, 4 c.c. of saliva were secreted ; adding this to the blood makes 

 14 -o7 c.c. and the percentage of glucose must be diminished in the same ratio, which makes 

 it 0-148 per cent., a value practically equal to that in the arterial blood supplied to the gland, 

 and no account has been taken of the lymph, which would make the value in the venous blood 

 less than that in the arterial blood. From the results of Barcroft and Piper, it is not to 

 be expected that there would be any very considerable consumption of glucose during the first 

 period of the activity of the gland. 



In Barcroft and Brodie's work (1905, p. 65) on the gaseous metabolism of the 

 active kidney, it was found that, taking all the experiments together, the output 

 of carbon dioxide was equivalent to that of the oxygen taken in. That is, the 

 respiratory quotient (see above, page 279) is practically unity, as it would be 

 from the oxidation of carbohydrate only. So far as it goes, this result suggests 

 that the substance oxidised is of carbohydrate nature and that it is completely 

 oxidised and its energy used for some process in connection with secretion. The 

 oxygen, moreover, could not be combined up in an intramolecular form in an 

 "explosible" substance, since, if this were the case, the respiratory quotient 



\ O/ wou ^ k e g reater than unity during the period of formation of this substance 

 and less than unity during its breaking up. 



The great sensitiveness of the salivary glands to slight diminution of oxygen 

 supply, as found by Heidenhain (1868, pp. 88-101), by Jonescu (1909, p. 68), and 

 by Liebermann (1911, p. 26), shows that the process of formation of the secretion 

 itself requires free oxygen in addition to the stored energy just referred to. 

 Ludwig, however (1851), obtained a slight secretion after the circulation had 

 nearly ceased, so that a current of blood is not absolutely necessary. This con- 

 sumption of oxygen during actual secretory activity suggests that the system of 

 high potential energy, formed previously, does not contain in itself the oxygen 

 necessary for its combustion, but chiefly consists of an oxidisable substance 

 capable of affording energy when supplied with oxygen. 



This immediate dependence on oxygen is shown still more strikingly by the higher nerve 

 centres and makes it probable that, for proper functional work, oxygen must be supplied, not 

 only in a certain amount, but at a tension not far below that in which it is present in the 

 atmosphere and in arterial blood. An organ may suffer from want of oxygen even when the 

 venous blood coming from it still contains oxygen, so that oxygen has passed the cells unused. 



Formation of Heat. It is scarcely to be supposed that the efficiency of the 

 gland machinery is so high that no free chemical energy is degraded to heat in the 

 secretory process. It was, in fact, found by Ludwig and Spiess (1857) that the 

 temperature of the saliva coming from the submaxillary gland was higher than that 

 of the blood in the carotid artery, but Bayliss and Hill (1894, 1) were unable to 

 detect any difference in this sense, if care was taken to obtain the actual 

 temperature of the blood in the flowing stream. Ludwig, himself, in a letter to 

 Prof. Schilfer, appears to have been prepared to admit these negative results. 

 Of course, the fact merely shows that, if heat was produced, the blood current was 

 sufficiently rapid to carry it away as fast as it was formed, as is very probable from 

 considerations of the actual amount of the combustion going on. It should be 

 stated also, however, that we were unable to detect any formation of heat in the 

 excised salivary glands of the grass snake, although a very delicate method was 

 used (p. 352 of the paper quoted). 



MODES OF EXCITATION 



There are two different ways in which glands can be made to secrete. The one 

 is by the agency of chemical substances contained in the blood with which they 



