DR. MARSHALL HALL. 
621 
capable of transmission from the extreme branches and points of 
incident nerves along their trunks to their roots into the marrow 
or centre, and by a reflex operation back again along the motor 
nerves to their extreme ramifications : thus establishing a circle 
of action similar to the one already described as forming the 
modus operundi of the sentient and motor principle in the cere- 
bral system. The following experiment will illustrate this: — 
Dr. Marshall Hall took a turtle, and, having removed the 
head, laid bare the spinal marrow by sawing away a longitudinal 
portion of the shell. He then irritated the marrow by means of 
galvanic influence, the forceps, &c. At first, he had the pheno- 
mena of sudden motions in both the posterior extremities, and in 
the tail, according to the law of Haller. But he had also slower 
and more continual movements in the anterior extremities, esta- 
blishing a new law of action of the vis nervosa , upwards, in the 
course of the spinal marrow. Similar experiments to this the 
Doctor admits had been made both by Flourens and Muller be- 
fore, but then the animals experimented on had not been decapi- 
tated, and consequently the cerebral influence still remained ; — 
sensation and volition were operating with, or at least their 
influence was not isolated from, the vis nervosa. From similar 
experiments on decapitated animals, the Doctor concludes that 
the vis nervosa not only acts in directions from the branches of 
nerves towards their trunks and into the spinal marrow, but up- 
wards or forwards as well as downwards or backwards in the 
spinal marrow itself. He also discovered that it was not abso- 
lutely necessary to stimulate the nervous fibres themselves to 
demonstrate this retrograde action, but that it was equally mani- 
fested by irritation or titillation of the cutaneous or cutaneo-mu- 
cous surfaces of the body: on being irritated, the eyelids closed 
and the sphincter ani contracted. From all which the Doctor con- 
cludes that he has, 11 physiologically speaking,” demonstrated 
the existence of “ a new kind of nerve that is — 
“ I. An Incident Motor Action; and, 
II. An Incident Motor Nerve 
the two combining the excitor and motor — or, in one word, the 
excito-motory power. Moreover, the Doctor presents us with 
the following as the 
