VASOMOTOR ACTIONS. 223 



brought about by stimulation of the sentient surfaces. And other instances 

 might be quoted in which vaso-dilator fibres appear to be connected with a 

 " centre " soon after their entrance into the nervous system. 



If, as seems probable ( 153), the bloodvessels of a'muscle dilate by vaso- 

 motor action whenever the muscle is thrown into contraction, either in a 

 reflex or voluntary movement, the vaso-dilator fibres of the muscle would 

 seem to be thrown into action by impulses arising in the spinal cord not far 

 from the origin of the ordinary motor impulses and accompanying those 

 motor impulses along the motor nerve. 



159. The case of the vaso-constrictor fibres is somewhat more com- 

 plicated on account of the existence of tonic influences ; since the same fibres 

 may, on the one hand, by an increase in the impulses passing along them, 

 be the means of constriction, and, on the other hand, by the removal or 

 diminution of the tonic influences passing along them, be the means of dila- 

 tation. We have already traced all the vaso-constrictor fibres from the 

 middle region of the spinal cord to the splanchnic system in the thorax and 

 abdomen, from whence they pass (1) by the abdominal splanchnic and by the 

 hypogastric nerves to the viscera of the abdomen and pelvis (concerning the 

 vasomotor nerves of the thoracic viscera we know at present very little) ; 

 (2) by the cervical sympathetic or cervical splanchnic, as it might be called, 

 to the skin of the head and neck, the salivary glands and mouth, the eyes 

 and other parts, and probably the brain, including its membranes ; (3) by 

 the brachial and sciatic plexuses to the skin of the fore and hind limbs, and 

 by various other nerves to the skin of the trunk. The chief parts of the 

 body supplied by vaso-constrictor fibres appear to be the skin, with its ap- 

 pendages, and the alimentary canal, with its appendages, glandular and 

 other ; the great mass of skeletal muscles appears to receive an insignificant 

 supply of vaso-constrictor fibres. 



If, now, in an animal, the spinal cord be divided in the lower dorsal 

 region, the skin of the legs becomes flushed, their temperature frequently 

 rises, and there is a certain amount of fall in the general blood-pressure as 

 measured, for instance, in the carotid ; and this state of things may last for 

 some considerable time. Obviously, the section of the spinal cord has cut 

 off the usual tonic influences descending to the lower limbs ; in consequence 

 the bloodvessels have become dilated, thus causing the general peripheral 

 resistance to become proportionately diminished, and the general blood- 

 pressure to fall. The tonic vaso-constrictor impulses for the lower limbs, 

 therefore, have their origin in the central nervous system higher up than 

 the lower dorsal region of the spinal cord. 



If the spinal cord be divided between the roots of the fifth and sixth 

 dorsal nerves (that is to say, at the level where the path of the splanchnic 

 fibres from the cord seems to divide (see Fig. 77) those issuing above pass- 

 ing upward to the fore limbs and head, and those issuing below passing to 

 the abdomen and lower limbs), the cutaneous bloodvessels of the lower limbs 

 dilate, as in the former case, and on examination it will be found that the 

 bloodvessels of the abdomen are also largely dilated ; at the same time the 

 blood-pressure undergoes a very marked fall ; it may, indeed, be reduced to 

 a very few millimetres of mercury. Obviously, the tonic vaso-constrictor 

 impulses passing to the abdomen and to the lower limbs take origin in the 

 central nervous system higher up than the level of the fifth dorsal nerve. 



If the section of the spinal cord be made above the level of the second 

 dorsal nerve, in addition to the above-mentioned results the vessels of the 

 head and face also become dilated ; but, in consequence of the fall of general 

 blood-pressure just mentioned, these vessels never become so full of blood, 

 the loss of tone is not so obvious in them as after simple division of the 



