1 62 A MANUAL OF PHYSIOLOGY 



of^temperature^of the feet, stimulation by paling and fall of 

 temperature. By taking advantage, however, of the unequal 

 excitability of dilators and constrictors in a degenerating nerve, 

 and of the differences between the two kinds of fibres in their 

 reaction to electrical stimuli (p. 159), 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 

 injthe 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 constric- 

 tion 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 

 microscope, is cut, and the peripheral end stimulated, the vessels are 

 seen to dilate distinctly, and this effect is not abolished when con- 

 traction 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 voluntary contraction, and during 

 rhythmically repeated artificial tetanization 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 demon- 

 strated, 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 vasoconstrictor, and the annulus of Vieussens 

 vaso-dilator, fibres for the coronary arteries of the heart. But this 

 question is far from being settled. There is some reason to believe 

 that the metabolic products liberated in the heart - muscle, and 

 especially 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 circula- 

 tion (Barcroft and Dixon). 



