CHAP, iv.] THE VASCULAR MECHANISM. 279 



membranes, though the presence of vaso-motor fibres in the 

 brain itself is much disputed, (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, 

 as we have seen, to receive a relatively small supply of vaso-con- 

 strictor fibres. 



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

 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 

 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 

 thoracic region of the spinal cord. 



If the spinal cord be divided higher up, say above the roots of 

 the fifth or sixth thoracic nerves, 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 milli- 

 meters 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 

 thoracic nerve. 



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

 the second thoracic nerve, in addition to the abovementioned 

 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 cervical sym- 

 pathetic, since the latter operation produces little or no effect on 

 the general blood pressure. 



Obviously then the tonic vaso-constrictor impulses, which 

 passing to the skin and viscera of the body maintain that tonic 

 narrowing of so many small arteries by which the general peri- 

 pheral resistance, and so the general blood pressure, is maintained, 

 proceed from some part of the central nervous system higher up 

 than the upper thoracic region of the spinal cord. And, since 

 exactly the same results follow upon section of the spinal cord in 

 the cervical region right up to the lower limit of the spinal 



