IRRITABILITY. 
submits to confinement at the very season when 
‘every thing invites her abroad ; an animal de- 
lighting in motion, made for motion, all whose 
motions are so easy and so free,—hardly a mo- 
" ment at other times at rest,—is for many hours 
‘of many days together fixed to her nest as 
closely as if her limbs were tied down by pins 
‘and wires. For my part, I never see a bird in 
that position, but I recognize an invisible hand, 
‘detaining the contented prisoner from her fields 
and groves, for a purpose, as the event proves, 
_ the most worthy of the sacrifice.” 
> (W. P. Alison.) 
ss, 
_ INTESTINAL CANAL. See Sromacu 
AND InTesTINaL CaNaAL. 
4 
; 
IRRITABILITY; etym. irrito, to irritate, 
_ stimulate, excite; Syn. contractility, Dr. Bos- 
tock ; the vis insita, as distinguished from the 
vis nervosa, of Haller; Germ. Reizbarkeit ; 
that — vital power in the muscular fibre 
_ by which it contracts on being stimulated. 
The term irritability is certainly not the 
best which might have been devised to express 
this vital power, for it only expresses the suscep- 
tibility of being irritated ; the term contractility 
is equally inadequate, for it only expresses the 
result or effect of irritation in peculiar textures; 
the designation irrito-contractility, if not ob- 
jectionable by its length, would in my opinion 
Sayer the fulness of this property in the mus- 
_ cular fibre of animal bodies. 
The term irritability was employed by 
_ Glisson, and some of its phenomena were not 
unknown to Harvey, Peyer, Baglivi, and other 
early physiologists; but it is to Haller that we 
owe the accurate distinction of this principle 
from other principles in the animal economy, 
its full development, and its application to 
| Physiology. Many were the disputes in his 
own time as to the degree of his originality 
; and merit in this mattter, and Whytt proved a 
" steady and persevering opponent to his claims ; 
_ but posterity has done him the justice which 
“ contemporaries pertinaciously withheld ; 
and now whenever there is a doubt as to the 
meaning or acceptation of the term irritability, 
that doubt is at once dispelled by adding the 
epithet Hallerian. 
The best test of the Hallerian irritability 
is the electric influence. It is by means of this 
agent that we detect the presence and the per- 
sistence of this vital power. Generally the 
parts which are originally most irritable pre- 
serve their irritability longest; but we are not 
prepared to say that this is an invariable rule. 
galvanism is the best test of irritability, 
soa muscle, endowed with a high degree of 
pesabilicy, becomes in its turn an excellent - 
test of electricity ; and it was by the irritability 
in the muscles of the frog that Galvani first 
‘detected that form of electricity which has 
since borne his name, or that of galvanism. 
It is an important question, whether the 
property of irritability belongs to the pure and 
isolated muscular fibre, or whether it belongs to 
this, combined with the nerves—the nervo-mus- 
cular fibre. The two textures cannot be separa- 
29 
ted, the muscular fibres cannot be isolated from 
the fine fibrille of the muscular nerves, and 
therefore the question cannot be determined by 
distinct experiment. But many facts, anatomi- 
cal and analogical, would lead us to attach 
the term irrito-contractility, at least, to the 
compound texture ; the nervous portion receiving 
the stimulus, the muscular undergoing the 
contraction. 
Why are the muscles which perform 
involuntary functions so richly endowed with 
nerves? Some of the disciples of Haller, and 
especially Behrens, contended, indeed, that the 
muscular structure of the heart, for example, 
was not supplied with nerves. The anatomist 
whom I have just quoted wrote a treatise 
entitled, “ Dissertatio qué demonstratur Cor 
Nervis carere,” in which he asserted that the 
cardiac nerves were distributed entirely to the 
bloodvessels; to this the celebrated Scarpa 
triumphantly replied in his “ Tabulz Neurolo- 
gice Cardiacorum Nervorum,” &c. 
Dr. A. P. W. Philip ‘has placed himself 
at the head of the Hallerian school of the 
present day: Legallois had asserted that the 
spinal marrow was the constant and essential 
source of the action of the heart, which accord- 
ingly ceased when the influence of the spinal 
marrow was removed. But Dr. Philip de- 
tected a source of fallacy in Legallois’ experi- 
ments, and discovered that although to crush 
the spinal marrow suddenly, as in those 
experiments, suspended the action of the heart, 
yet that the spinal marrow might be slowly and 
gradually destroyed, and the action of the 
heart still remain uninterrupted. Similar ex- 
periments were afterwards made with similar 
results by M. Flourens, and published in his 
admirable “ Recherches sur le Systeme Ner- 
veux,” p.18. But though Dr. Philip has the 
merit of detecting the error of Legallois and of 
establishing the fact that the circulation may 
continue after the destruction of the spinal 
marrow, he has totally failed in proving that 
the action of the heart is independent of the 
nervous system, and that the irritability of 
Haller is exclusively a muscular power. It 
should be remembered that, after the removal of 
the brain and spinal marrow, the grand centres of 
the nervous system, the ganglionic or subsidiary 
nervous centres, remain, and that even after the 
removal of the heart from the animal body 
altogether,—in which case I have proved that its 
power of maintaining the circulation remains,—* 
there are still probably as many nervous as" 
muscular fibres ; and we know that the nerves 
themselves possess, independently of the ner- 
vous centres, the vis nervosa, or power of 
exciting under the influence of stimuli, the 
muscular fibre to contraction. 
I have also positively ascertained that 
after the destruction of the brain and spinal 
marrow in the eel, the heart is susceptible of 
being impressed through the medium of the 
ganglionic system. “ In an eel, in which the 
brain had been carefully removed, and the 
nue my Essay on the Circulation of the Blood, 
Pp» : : 
