342 PRINCIPLES OF GENERAL PHYSIOLOGY 



Mention has frequently been made of the use of intravenous injections for various purposes, 

 so that it may interest the reader to leani that, according to Sprat's "History of the Royal 

 Society" (1722, p. 317), it was Christopher Wren who was, as the author puts it, "the first 

 Author of the Noble Anatomical Experiment of Injecting Liquors into the Veins of Animal-. 

 An Experiment now vulgarly known ; but long since exhibited to the Meetings at Oxford, and 

 t In-nee carried by some (Germans, and published abroad. By this operation divers Creatures 

 \MTI- immediately purg'd, vomited, intoxicated, kill'd or reviv'd, according to the quality <>t 

 the Liquor infected. Hence arose many new Experiments, and chiefly that of Transfusing 

 i;i""<l, which the Society has prosecuted in sundry instances, that will probably end in 

 extraordinary Success." 



Secreting glands also require energy for the production of the chemical 

 constituents of their secretions, whenever these substances are not already present 

 in the blood. 



When we come into possession of more knowledge of the chemical changes 

 involved, it is possible that we may, by the application of Nernst's new 

 thermodynamic theorem (page 30 above), be able to calculate the energy changes 

 involved. For the present we must be content with indirect measurements by 

 determining the difference between the oxygen consumption of the resting and the 

 active organ. This knowledge we owe chiefly to the work of Barcroft and his 

 co-workers. The measurement is made by determining the oxygen content of 

 the blood supplied to the gland, that is, the ordinary arterial blood, and the 

 oxygen content of that leaving it by the vein, together with the amount of blood 

 passing in a given time. It is in the accuracy of the last estimation that the chief 

 difficulty lies, since the rate of flow increases in activity. The resting submaxillary 

 gland of the cat consumes about 0*02 c.c. of oxygen per gram per minute. When 

 excited to secretion, the consumption may rise to as much as l - 9 in the same units 

 (Barcroft and Piper, 1912, p. 362), that is, more than five times as much. By 

 taking the difference between the oxygen consumption of the resting and that of 

 the active gland, the same observers have calculated the oxygen necessary to form 

 0'30 c.c. of saliva to be 0'18 c.c. What this means in terms of energy naturally 

 depends on what chemical substance is oxidised. Taking it as glucose, it would 

 imply the use of 0*17 g., since 180 g. of glucose require 192 g'. of oxygen for 

 complete combustion. A small part only of the energy is required for osmotic 

 work, on account of the small volume of the saliva secreted. 



A very important result as regards the mechanism of the process was obtained 

 in the course of the experiments of Barcroft and Piper. When the time course of 

 the oxygen consumption was determined in relation to that of the flow of saliva, it 

 was found that the maximal rate of the former occurred considerably later than 

 that of the latter, and that the increased consumption might last for as long as 

 seven minutes after the formation of saliva had ceased. The length of this period 

 of increased consumption of oxygen was found to depend on the degree of 

 activity of the gland previously, and also on the functional capacity of the organ. 

 We shall meet with a similar state of affairs in the case of voluntary muscle. It 

 seems to imply that the chemical energy derived from oxidation is not used directly 

 in the process of secretion, but that potential energy is stored in some physico- 

 chemical system, from which it is given out for use in the actual process itself. 



It might perhaps be thought, by adherents of the "biogen" theory, that the oxygen itself 

 is taken up in combination in an explosive-like giant molecule, analogous to potassium chlorate, 

 for example. It seems possible that this view might be tested by simultaneous determination of 

 the carbon dioxide given out together with the oxygen consumption. If the two were found to 

 correspond, it would indicate that an oxidation process was giving energy to another independent 

 chemical or physical reaction. Want of parallelism between the oxygen and carbon dioxide would 

 not decide the question in either way. We shall find later, however, that there is no evidence 

 for the existence of "intramolecular" oxygen in the sense of the biogen hypothesis, and \M 

 have already seen reason for doubting the correctness of this point of view. 



It may be called to mind that Chauveau and Kaufmann (1886) found a 

 diminution of glucose in the blood after it had passed through the active 

 salivary gland of the horse. This was also found to be the case by Asher and 

 Karaulov (1910), so far as the period immediately succeeding the flow of saliva 

 is concerned. The fact suggests the possibility that the energy required to form 

 the system of high potential energy, which afterwards breaks down in the process 

 of secretion, may be derived from the oxidation of glucose. 



