700 /'K/XC/rLKS- OF GENERAL PHYSIOLOGY 



cells, or is given off in their normal metabolism, is not known. It is interesting 

 to note that the vaso-dilatation, seen in the cat and dog, is replaced by v;is<> 

 constriction in the rabbit and guinea-pig, so that it would be rash to assert that 

 this substance plays the part of a general metabolite to bring about vascular 

 dilatation in active organs. 



In contradistinction to the usual dilator action of the products of tissue 

 metabolism, the secretion of the suprarenal bodies contains an extremely potent 

 substance, adrenalin*, which causes vaso-constriction in all cases where there is a 

 supply of sympathetic vaso-constrictor nerves. This " hormone " will come up 

 for further discussion in the next chapter. Reference is made to it here since, 

 in many cases in which reflex rise of blood pressure occurs, there is a turning out 

 of adrenaline into the blood vessels, which assists in the rise of pressure. The 

 secretory nerves to the suprarenals are excited simultaneously (see especially 

 the work of Anrcp, 1912, 1). 



It might occur to the reader that, perhaps, this production of adrenaline is the 

 cause of all rise of blood pressure in pressor reflexes : if so, the existence of a vaso- 

 constrictor centre would be superfluous. Hoskins and Wheelon (1914), however, 

 find that four to six hours after tying off' both suprarenal bodies, although there 

 is weakness of the skeletal muscle and the heart, the blood pressure is not lowered, 

 and pressor reflexes from a sensory nerve are still to be obtained. Thus, excita- 

 tion of vasoconstrictors occurs in the absence of stimulation of secretion of 

 adrenaline. 



Similarly, the vaso-dilator nerves have been supposed by some, Barcroft (1914, 

 p. 148), for example, to be of comparatively small importance, even if their 

 existence is not doubtful. The products of metabolism are supposed sufficient 

 to account for the functional dilatation. But when vaso-dilators are excited 

 through the depressor nerve in order to relieve the heart by fall of blood pressure, 

 it would seem a remarkable way of bringing it about if it were necessary to set a 

 multitude of organs into activity. It must be admitted, however, that the vaso- 

 dilators do not appear to play a great part in the reflex fall of blood pressure, since 

 inhibition of constrictor tone is usually sufliciently effective. The decision of the 

 question obviously rests on the proof that vaso dilatation can occur on stimulation 

 of a nerve apart from increase of metabolism. The effect of the chorda tympani 

 on the atropinised submaxillary gland, in which vaso-dilatation is obtained without 

 visible secretion, has been brought forward in evidence, but Barcroft (1914, p. 

 147) rightly points out that there may be stages of cell activity, preliminary to 

 extrusion of secretion, which are not paralysed by atropine. In fact, he obtains 

 increase of oxygen consumption. The data given seem to me to show that, 

 although metabolites are a partial cause, there is a nervous effect in addition, since 

 the degree of dilatation is not in proportion to the increase of oxygen consumption. 

 Thus a 109 per cent, increase in oxygen consumption coincides with a 488 per 

 cent, increase in blood flow, while a 50 per cent, increase coincides with an 

 increase in blood flow of 812 per cent., that is, a larger dilatation with a 

 smaller consumption of oxygen. Some recent experiments by Anrep and Evans 

 (unpublished) show that it is possible to obtain vaso-dilatation in the tongue, on 

 stimulation of the peripheral end of the lingual nerve, without any increase in 

 oxygen consumption. In some cases an increase was found, but this was probably 

 due to secretory activity of the glands in the tongue. 



Anrep's experiments, referred to on page 349 above, show that when the 

 secretin used to stimulate the pancreas is free from the depressor substance, 

 there is little or no sign of vaso-dilatation in the gland, associated with secretion. 

 The products of metabolism of active glands have not, therefore, universally 

 a vaso-dilator action. 



It was mentioned above (page 345) that the cervical sympathetic in the cat 

 gives abundant secretion of saliva. Disregard of the fact that metabolites cause 

 dilatation led some observers to state that the cervical sympathetic contains 

 vaso-dilators. Now the drug, ergotoxine, obtained by Dale (1906) from ergot, 

 paralyses sympathetic constrictors and secretory fibres, but does not affect vaso- 

 dilators. After ergotoxine, Stimulation of the cervical sympathetic fails to 



