322 COURSE OF VASO-CONSTRICTOR IMPULSES. [BOOK i. 



the ordinary motor impulses, and accompanying those motor 

 impulses along the motor nerve. 



173. The. case of the vaso-constrictor fibres is somewhat 

 more complicated 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 dilation. 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 vaso-motor 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 appendages, 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 any at all. 



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

 dorsal region, the skin of the legs becomes flushed, their tempera- 

 ture 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 

 blood vessels have become dilated, in consequence the general 

 peripheral resistance has become proportionately diminished, and 

 in consequence the general blood-pressure has fallen. 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 5th and 

 6th dorsal nerves, (that is to say, at the level where the path of 

 the splanchnic fibres from the cord seems to divide, see Fig. 58, 

 those issuing above passing upwards to the fore-limbs and head, 

 and those issuing below passing to the abdomen and lower limbs), 

 the cutaneous blood vessels of the lower limbs dilate, as in the 

 former case, and on examination it will be found that the blood 

 vessels 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 



