VASOMOTOR ACTIONS. 219 



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 bloodvessels as the result of the stimulation may be ac- 

 tually 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 mi- 

 nute bloodvessels apart from any nervous agency, lead to a widening of those 

 bloodvessels ; this, however, is doubtful. With regard to the vaso-constrictor 

 fibres, the only evidence that they exist in muscles is that when the nerve of 

 a muscle is divided the bloodvessels of the muscle widen, somewhat like 

 bloodvessels of the ear after division of the cervical sympathetic. This sug- 

 gests the presence of vaso-constrictor fibres carrying the kind of influence 

 which we called tonic, leading to an habitual moderate constriction ; it can- 

 not, 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 sup- 

 plied with vasomotor fibres running in this or that nerve, the fibres being 

 either vaso-constrictor or vaso-dilator, and some nerves containing one kind 

 of fibres only, some both in varying proportion. Almost every nerve in the 

 body, therefore, may be looked upon as influencing a certain set of blood- 

 vessels, as governing a vascular area, the area being large or small, and the 

 government being exclusively constrictor or exclusively dilator, or mixed. 



The Course of Vaso-constrictor and Vaso-dilator Fibres. 



155. Both the vaso-constrictor and the vaso-dilator fibres have their 

 origin in the central nervous system, the spinal cord, or the brain, but the 

 course of the two sets appears to be very different. 



In the mammal, as far as we know at present, all the vaso-constrictor 

 fibres for the whole body take their origin in the middle region of the spinal 

 cord, or rather leave the spinal cord by the nerves belonging to this middle 

 region. Thus in the dog the vaso-constrictor fibres, not only for the trunk 

 but for the limbs, head, face, and tail, leave the spinal cord by the anterior 

 roots of the spinal nerves reaching from about the second dorsal to the fourth 

 lumbar nerve, both inclusive. Running in the case of each nerve root to 

 the mixed nerve trunk they pass along the visceral branch, white ramus 

 communicans, to the chain of splanchnic ganglia lying in the thorax and 

 abdomen the so-called thoracic and abdominal sympathetic chain (Fig. 77). 

 From these ganglia they reach their destination in various ways. Thus 

 those going to the head and neck pass chiefly through the second and third 

 dorsal and partly through the fourth, fifth and first dorsal nerves, thence 

 upward through the annulus of Vieussens to the lower cervical ganglion, 

 and thence, as we have seen, up the cervical sympathetic. Those for the 

 abdominal viscera pass off in a similar way to the abdominal splanchnic 

 nerves (Fig. 77, abd. spl.). Those destined for the fore limbs pass along in the 

 fourth to the ninth dorsal nerves, both inclusive, chiefly in the seventh, and 

 sometimes a few in the tenth, and so reach the brachial plexus ; while those 

 for the leg pass through the eleventh dorsal to the third lumbar, both inclusive, 

 a few passing through the tenth dorsal and the fourth lumbar, and finally 

 to the sciatic plexus. Those for the tail pass through in the first to 

 third lumbar inclusive. The constrictor fibres of the skin of the trunk 

 probably reach the spinal nerves in which they ultimately run in a similar 

 manner. All the vaso-constrictor fibres, whatever their destination, leave 

 the spinal cord by the anterior roots of spinal nerves, and then, passing 

 through the appropriate visceral branches, join the thoracic or abdominal 



