AND BLOOD PRESSURE 



175 



in a condition of shock from injury raises the arterial pressure from 

 its low level by an amount which is no less than in a normal animal. 

 The centre, he says, is able to act quite efficiently. The fault 

 may lie, however, in the sensory synapses. The centre fails to 

 be excited by the usual weak excitation streaming into it, or the 

 usual effect may be reversed pressor turned into depressor reflexes, 

 and so the arterial tone flags. 



Bayliss has brought forward evidence that there exists a 



FIG. 22. Inhibition of constrictors in Love"n reflex. Upper curves, volume of 

 hind-limb ; lower curves, arterial pressure. First excitation, median nerve ; second, 

 sixth lumbar posterior root (Bayliss). 



reciprocal innervation in the case of vaso-motor reflexes as in 

 those affecting skeletal muscle. Under normal conditions the 

 arterioles are in a state of moderate contraction or tone, which 

 may continue even when the vessels are separated from connection 

 with the nervous system. Such tone is a normal property of smooth 

 muscle, and in the case of the vessels it seems to be kept up by 

 the contractile reaction of the arterial wall to the distending force 

 of the pulse, as well as, it is thought, by the internal secretion of 

 the adrenal glands acting on the sympathetic nerve plexus in the 



