176 



THE VASCULAR SYSTEM 



c.c. 



wall (Elliot). This tone can be increased by vaso-constrictor 

 and diminished by vaso-dilator impulses. These impulses may 

 be continuous, resulting from a state of tonic excitation of the 

 respective centres, which is partly of reflex origin and partly de- 

 pends on the quality of the blood circulating through the centres, 

 e.g. by the percentage of 2 and C0 2 in the blood. Bayliss finds 



that in depressor reflexes there 

 is, along with inhibition of tone 

 in the vaso-constrictor centres, 

 an excitation of the vaso- 

 dilator centres, and that in 

 pressor reflexes along with ex- 

 citation of constrictors there 

 is under appropriate conditions 

 inhibition of dilator tone. 



When an afferent nerve 

 from any particular organ is 

 excited there is produced along 

 with the usual pressor reflex on 

 the general blood pressure a 

 vaso-dilatation in the organ 

 itself. By this means the 

 maximal supply of blood is 

 sent to an active organ. In 

 these local or Love"n reflexes 

 as they are called, Bayliss 

 finds evidence of both excita- 



FIG. 23. F, depressor ; R, pressor 

 afferent nerves affecting arteriole muscle 

 through C.C., constrictor, and D.C., 

 dilator centres, as shown by + and - signs 

 (Bayliss). 



tion of dilators and inhibition 

 of constrictors. Chloral and 

 chloroform convert pressor into 

 depressor reflexes, acting not 

 on the afferent neurone, but on some point in the reflex arc, 

 probably the synapse. 



It is not possible to cause a great fall of arterial pressure merely 

 by severe sensory excitation in an etherised and morphinised 

 animal. The pressure falls lower the longer the animal is kept 

 anaesthetised on the table exposed to cold, to chloral or chloroform, 

 or to operations which expose large surfaces and lead to actual loss 

 of blood fluid or to obstruction of venous return, vaso-dilatation, 

 congestion and transudation. Opening the abdomen and exerting 



