IRRITABILITY. 



29 



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.) 



INTESTINAL CANAL. 

 AND INTESTINAL CANAL. 



See STOMACH 



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 peculiar 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 

 express 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 oeconomy, 

 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 

 his 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 text 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. 



As galvanism is the best test of irritability, 

 so a muscle, endowed with a high degree of 

 irritability, 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- 



ted, the muscular fibres cannot be isolated from 

 the fine fibrillse 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 qua demonstrator 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 " Tabulae Neurolo- 

 gicse Cardiacorum Nervorum," &c. 



Dr. A. P. VV. 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 

 powerof maintainingthe 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 



* See my Essay on the Circulation of the Blood, 

 p. 121. 



