386 



Mr. D. T. Harris. 



What are the possible chemical vaso-dilator substances which may exert 

 a " depressor " effect on the plain muscle or endothelium of the blood vessels 

 of an organ so as to ensure a greater blood supply during activity ? Two 

 possibilities are presented : — 



a. Acid Metabolites: The more or less completely oxidised products, e.g., 

 CO2 and lactic acid, derived from the energy-producing food-stuffs. 



y3. Basic split products resulting from the wear and tear of the cell 

 protoplasm. 



(iv) Functional hypercemia. — With this brief survey of the factors which 

 produce vaso-dilation, we may embark upon the controversial question of the 

 means whereby the functional hypereemia of an active organ is brought about. 

 Leaving out of the discussion the alterations in general blood pressure 

 {vide (i) ), the issue lies between neuro-muscular vaso-dilation and the 

 automatic vaso-dilation produced by metabolites. On the one hand, among 

 our best authorities we have some who regard functional vaso-dilation as a 

 simple local effect of the metabolites resulting from activity, just as the heat 

 evolved results from the chemical changes of activity, and there is, according 

 to them, as little need to presume the existence of special vaso-dilator nerves 

 as there is to suppose the presence of caloric nerves. 



Barcroft (1914, p. 145) found that during stimulation of the sympathetic 

 fibres in the submaxillary gland (cat) by means of adrenalin, the vaso- 

 constriction ordinarily produced by adrenalin on blood vessels is more than 

 counterbalanced by the action of the vaso-dilator substances produced when 

 the gland secretes. 



On the other hand it is well known that stimulation of the chorda tympani 

 in an atropised submaxillary gland causes a vaso-dilation in the total absence 

 of secretion ; Barcroft explains this by advancing the possibility of lower 

 grades of cellular activity, without external secretion occurring in such 

 cases, and he actually found that during stimulation there was an increased 

 oxygen intake in the atropised gland. An examination of his figures 

 (p. 147) shows, for example, equal metabolic rates for a 4- and 2-fold blood 

 flow : — 



Per cent, increase in oxygen 55 55 



Per cent, increase in blood flow 333 102 



Here there must be, as Bayliss (1920) points out, a factor in addition to 

 metabolites called into play; the hyperaemia in the last experiment can 

 hardly be caused by metabolites arising from activity of such " subliminal 

 degree " ; it would appear that the neuro-muscular mechanism is also excited 

 to activity. Anrep (1916) has shown that when secretin used to stimulate 

 the pancreas is free from the depressor substance, there is little or no 



