340 Dr. W. M. Bayliss. On Reciprocal [Jan. 20, 
tions have been brought forward at various times tending to confirm the 
above hypothesis. These will best be discussed under the separate headings. 
of the next section. 
Before proceeding to the experimental results, it will be advisable to make 
clear the meanings attached to some of the words used. By “ vaso-motor” 
I intend to include not only vaso-constrictor effects, as is sometimes done, 
but also those of a vaso-dilator nature. Both actions are, in fact, vaso-motor, 
one producing movement in the sense of narrowing of vessels, the other 
widening. Similarly, by vaso-motor centre, or centres, I mean the centres 
from which are given off both the vaso-dilator and vaso-constrictor fibres, 
whatever may be the situation of these centres. 
It is also necessary to remember that under normal conditions the 
arterioles are in a state of moderate contraction or tone, which may continue 
even when these vessels are separated from connection with the central 
nervous system. This tone is probably a normal characteristic of smooth 
muscular tissue; but, apart from this, it is kept up by the contractile 
reaction of the arterial muscle to the distending force of the heart-beat,* as. 
well as by the effect of the internal secretion of the adrenals, in the case of 
those arterioles supplied with sympathetic nerve-fibres.+ 
This inherent muscular tone of the arterioles can be affected in two 
directions by impulses from the central nervous system, at all events in the 
case of most organs of the body. It can be increased by impulses down 
vaso-constrictor fibres or diminished (inhibited) by impulses down vaso- 
dilator fibres. Moreover, these impulses may be continuous, resulting from a 
state of tonic excitation of their respective centres, in all probability of reflex 
origin. 
It is then obvious that, when we observe a vaso-dilatation in a particular 
organ in response to excitation of an afferent nerve, this result may be due to 
two things, either an excitation of vaso-dilator fibres or, supposing that the 
vaso-constrictor centre is in a condition of tonic excitation, to an inhibition of 
this centre, resulting in a cessation of the vaso-constrictor impulses previously 
producing increased contraction of the arterial muscle. Both results may be 
simultaneously brought about. 
Conversely, when what is conveniently called a “ pressor” reflex is brought. 
about, that is, one associated with increased contraction of arterioles and 
consequent rise of arterial blood-pressure, we may have both excitation 
of vaso-constrictor fibres and inhibition of tone in the vaso-dilator centre. 
Now, in order to investigate experimentally the various possibilities referred. 
* W. M. Bayliss, ‘Journ. of Physiology,’ vol. 28, p. 220, 1902. 
+ T. R. Elliott, ‘Journ. of Physiology,’ vol. 32, p. 401, 1905. 
