236 CIRCULATION OF THE BLOOD 



Generally speaking the result of these localized reflexes is a vasodilation, 

 but under certain circumstances vasoconstriction may be the result. The 

 reflex may spread to the corresponding part of the body on the opposite side, 

 and not infrequently parts far removed from the region innervated by the 

 afferent nerve show a reflex constriction or dilation of their vessels. Thus 

 the vast region innervated by the splanchnics is very easily constricted by 

 stimulation of all possible kinds of sensory nerves. It may also be dilated 

 as the result of a sensory excitation. The vessels of the skeletal muscles as a 

 rule appear to be dilated by sensory impulses. The dilation appears first in 

 the muscles which stand in close functional relationship with the nerves stimu- 

 lated ; but it may be called out also by excitation of distant afferent nerves. 



If these reflex effects are not confined within too small a vascular field., 

 they influence the general blood pressure. 



Since in almost every afferent stimulation, vessels somewhere are con- 

 stricted and others dilated, it is evident that changes of pressure may take 

 place in both the positive and the negative direction. 



As a rule, afferent excitation produces a reflex rise of pressure (Fig. 97). 

 Under certain circumstances a fall is obtained instead, for example : when 



FIG. 97. Reflex rise of blood pressure in the rabbit (to be read from right to left). At a the 



skin was stimulated. 



the afferent nerve stimulated is subjected to the cold ; when after having been 

 sectioned it is allowed to regenerate to a certain stage; if the stimulus is 

 weak. 



At present it is impossible to decide whether these different reflexes are 

 caused by two kinds of efferent nerves or by the difference in behavior of the 

 vasomotor center to stimuli of different strength. There are nerves however 

 which, so far as our present information goes, mediate only a fall of pressure, 

 whatever the strength of stimulus. Such a nerve is the depressor, already 



