316 YASO-MOTOR FIBRES OF MUSCLES. [BOOK i. 



pathetic) fibres, but the former, like the vaso-constrictor fibres in 

 the sciatic, are predominant, and special means are required to 

 shew the presence of the latter. 



In the splanchnic nerve (abdominal splanchnic) which supplies 

 fibres to the blood vessels of so large a part of the abdominal 

 viscera, there is abundant evidence of the presence of vaso- 

 constrictor fibres, but the presence of vaso-dilator fibres has not 

 yet been shewn. Division of this nerve leads to a widening of 

 the blood vessels of the abdominal viscera, stimulation of the 

 nerve to a constriction; and as we shall see, since the amount 

 of blood vessels thus governed by this nerve is very large indeed, 

 interference either in the one direction or the other with its vaso- 

 motor functions produces very marked results, not only on the 

 circulation in the abdomen but on the whole vascular system. 



In nerves going to muscles vaso-dilator fibres predominate, 

 indeed in these the presence of any vaso-constrictor fibres at all 

 has not at present been satisfactorily established. When a muscle 

 contracts there is always an increased flow of blood through the 

 muscle ; this may be in part a mere mechanical result of the 

 change of form, the shortening and thickening of the fibres 

 opening out the minute blood vessels, but is not wholly, and 

 probably not even largely, thus produced. A notable feature of 

 vaso-motor fibres is that, in very many cases at all events, their 

 action is not affected by small or moderate doses of urari such as 

 render the motor nerves of striated muscle powerless. Thus in a 

 frog placed under the influence of a moderate amount of urari, 

 stimulation of a nerve going to a muscle will produce vaso-motor 

 effects unaccompanied and unobscured by any contraction of the 

 striated fibres. By placing a thin muscle of a frog, such as the 

 mylo-hyoid, under the microscope, and watching the calibre of the 

 small arteries and the circulation of the blood through them while 

 the nerve is being stimulated, the widening of the blood vessels, as 

 the result of the stimulation, may be actually observed. This 

 experiment appears not to succeed in a mammal ; and it has been 

 suggested that when a muscle contracts, some of the chemical 

 products of the metabolism of the muscle may, by direct action 

 on the minute blood vessels apart from any nervous agency, lead 

 to a widening of those blood vessels ; this however is doubtful. 

 With regard to vaso-constrictor fibres the only evidence that they 

 exist in muscles is that when the nerve of a muscle is divided 

 the blood vessels of the muscle widen, somewhat like the blood 

 vessels of the ear after division of the cervical sympathetic. This 

 suggests the presence of vaso-constrictor fibres carrying the kind 

 of influence which we called tonic, leading to an habitual moderate 

 constriction ; it cannot however be regarded, by itself, as conclusive 

 evidence ; but we must not discuss the matter here. 



Speaking generally then, most, if not all the arteries of the 

 body are supplied with vaso-motor fibres running in this or that 



