176 THE CIRCULATION OF THE BLOOD AND LYMPH 



reaction to electrical stimuli (p. 173), it has been shown that vaso- 

 dilators are also present, and come to the front when the conditions are 

 rendered favourable for them and unfavourable for the constrictors. 



Vaso-motor fibres for the fore-limb (dog) issue from the cord in the 

 anterior roots of the third to the eleventh dorsal nerves, and for the 

 hind-limb in the anterior roots of the eleventh dorsal to the third lumbar. 

 Stimulation of most of these roots causes constriction of the vessels, 

 but stimulation of the eleventh dorsal may cause dilatation (Bayliss 

 and Bradford). 



The Vaso-Motor Nerves of Muscle. When the motor nerve of the thin 

 mylo-hyoid muscle of the frog, which can be observed under the micro- 

 scope, is cut, and the peripheral end stimulated, the vessels are seen to 

 dilate distinctly, and this effect is not abolished when contraction of 

 the muscle is prevented by a dose of curara insufficient to paralyze the 

 vaso-motor nerves. This indicates the existence in the nerve of vaso- 

 dilator fibres. But we must be cautious in transferring this result to 

 ordinary skeletal muscle, for the mylo-hyoid is more closely allied to 

 the muscles of the tongue than, for example, to the muscles of the limbs, 

 and in the mammal the tongue muscles are known to be better supplied 

 with vaso-dilator fibres than the limb muscles. The average flow of 

 blood through a mammalian muscle is indeed increased during volun- 

 tary contraction, and during rhythmically repeated artificial tetaniza- 

 tion of its motor nerve. The outflow of blood from the main vein of 

 the levator labii superioris, one of the muscles used in feeding in the 

 horse, was found to be in one experiment nearly eight times, in another 

 about seven times, and in a third three and a half times as great during 

 voluntary work with it (in chewing) as in rest. But as no increase in 

 the blood-flow through the skeletal muscles of a completely curarized 

 mammal during excitation of their nerves has ever been satisfactorily 

 demonstrated, we must conclude that they are very scantily provided 

 with vaso-dilator fibres or not at all. It is uncertain whether they are 

 supplied with vaso-constrictors. The undoubted increase in the blood- 

 flow in contraction may therefore be connected in some way with the 

 mechanical or chemical changes in the muscular fibres themselves. 



It has been suggested that the muscular vessels are widened by the 

 direct action of the acid products of the active muscle, since very dilute 

 acids (lactic acid, e.g.) cause general dilatation of the small vessels. 

 A similar explanation has been extended to the dilatation of the vessels 

 of the brain during cerebral activity by some of those who deny the 

 existence of vaso-motor nerves for that organ, but here the evidence 

 is by no means satisfactory. The vagus has been stated to contain 

 vaso-constrictor, and the annulus of Vieussens vaso-dilator, fibres for 

 the coronary arteries of the heart. But this question is far from being 

 settled. Adrenalin causes dilatation and not constriction of the 

 coronary vessels. There is some reason to believe that the metabolic 

 products liberated in the heart-muscle, e.g., carbon dioxide, govern the 

 changes in the calibre of the coronary arterioles. A close relationship 

 exists between the output of carbon dioxide and the rate of flow through 

 the coronary circulation. In asphyxia the flow through the coronary 

 vessels is notably increased; indeed, it is at its maximum just before 

 the heart fails altogether, as if an effort were being made to keep the 

 heart going to the last by making up to it in the quantity of the blood 

 supplied what it lacks in quality. As this increased flow is seen in the 

 isolated heart-lung preparation, it has been concluded that metabolites 

 produced in the cardiac muscle itself cause an increased coronary flow 

 when increased demands are made on the heart, a local regulative 



