THE CIRCULATION OF THE BLOOD AND LYMPH 153 



and constrictors in a degenerating nerve, and of the differ- 

 ences between the two kinds of fibres in their reaction to 

 electrical stimuli (p. 150), it has been shown that vaso- 

 dilators are also present, and come to the front when 

 the conditions are rendered favourable for them and un- 

 favourable for the constrictors. 



The 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 microscope, is cut, the vessels are seen 

 to dilate. On stimulation of the peripheral end of the cut 

 nerve they dilate still more, 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 

 (Ga^kell). The dilatation on section of the nerve has been 

 held to indicate the existence in it of vaso-constrictor fibres, 

 and the dilatation on stimulation of the nerve, the existence 

 of a larger number of vaso-dilators, which overcome the 

 constrictors when both are excited. And it has been argued 

 that this is of use to the contracting muscle, which requires 

 a free flow of blood to supply it with food materials and to 

 carry off its waste products. The average flow of blood 

 through a mammalian muscle is also increased during con- 

 traction, apart from the initial increase due to the com- 

 pression of the muscular veins. The outflow of blood from 

 the main vein of one of the muscles used in mastication in 

 the horse was found to be three times as great during 

 voluntary work with it (in chewing) as in rest. And although 

 no increase in the blood-flow through the skeletal muscles of 

 a completely curarized mammal has ever been satisfactorily 

 demonstrated, we can hardly doubt that they are provided 

 with vaso-dilator fibres, and more scantily with vaso-con- 

 strictors. The existence in the vagus of vaso-constrictor, 

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

 coronary arteries of the heart has also been asserted. It 



