IRRITABILITY; etym. irrito, to irritate, stimulate, excite ; Syn. contractility, Dr. Bos tock ; the vas 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 ceconomy, 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 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.
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 thenerves—the nervo-mus cular fibre. The two textures cannot be separa ted, the muscular fibres cannot be isolated from the fine fibrillae 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 quk 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 " Tabula Neurolo gicae 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.