1908. | Innervation in Vaso-motor Reflexes, ete. 349 
the abdominal sympathetic is sometimes absent, so that after section of the 
cord no reflex dilatation of the limb can be obtained.* 
2. Pressor Reflexes. 
(i) Excitation of Constrictors—When the arterial pressure rises in response 
to excitation of the central end of a sensory nerve, it is universally admitted 
that the effect is due to excitation of vaso- 
constrictor fibres. But, asin the last case, the 
possible co-operation of dilators has not been 
taken into account. I have, therefore, taken 
a tracing of the change of volume in the 
hind-leg of a cat, in which the cord has been 
transected in the middle of the lumbar 
region, in order to cut off the dilator supply 
of the part. Fig. 5 reproduces this experi- 
ment. At the place indicated by the signal, 
the central end of the median nerve was 
excited. The arterial pressure rises, and, at rca Janne 
the same time, the limb constricts. The effect Hanae 
is prolonged, as is usual in the cat. Further 
‘experiments were deemed unnecessary, as 
the phenomenon is undisputed. 
(i) Inhibition of Dilators—This case has 
proved to be the most difficult one of the 
four as regards experimental investigation. 
It is obvious that, in order to be able to show 
inhibition of tone in the vaso-dilator centre, 
Fie. 5.—Constriction by excitation 
of vaso - constrictors. Upper 
this tone must first be in existence. Just as curve, volume of leg. Lower 
in the experiments of Sherrington, where CUtVé arterial pressure. Zero, 
25 . below time-signal. 
extensor tone was usually produced by decere- eat ao ee 
bration, in my experiments it was necessary to induce tone in the dilator 
centre by keeping the animal as warm as possible, and, in the eviscerated 
animal, with renal vessels tied, by the intravenous injection of warm saline, in 
order to raise the arterial pressure as high as possible. Nevertheless, a 
* Note added to Proof.—I have recently observed another case of dilation by inhibition 
of constrictor tone in the absence of dilator fibres. The blood-flow through the sub- 
maxillary gland of the cat is increased in rate by exciting the central end of the vagus 
on the opposite side, even after section of the Chorda tympani nerve, if the cervical 
sympathetic is left intact. The effect, as far as it is possible to compare results on 
different animals, seems to be somewhat less than that obtained when the dilators are 
intact and the constrictors cut, as described in the previous section. 
