It was shown by Hook, in the early days of the Royal Society,f that if, the respiration being suspended, an animal appeared to be dying, the beat of the heart and the signs of life were speedily restored, on performing arti ficial respiration, or even by forcing air through the trachea, bronchia, and pulmonary air-cells and allowing it to escape through incisions made through the pleura.
It was, in the next place, clearly shown by Goodwyn, in one of the most beautiful spe cimens of physiological inquiry in any lan guage,t that in suspended respiration, it is the left side of the heart which first ceases to contract, the right side still continuing its function for several minutes, until the supply of blood may be supposed to fail.
The facts detailed by Harvey had shown that the left side of the heart was endued with less irritability than the right ; the experiment of Hook, that respiration restored the action of the heart, if it had previously ceased ; that of Goodwyn, that this cessation and restoration of functions were observed in the left side of the heart. It was obvious, on the other hand, that the respiration belongs, as it were, to the left side of the heart.
It appears plainly deducible from these facts. that in circumstances and structures the most similar, the respiration is accurately inversely as the irritability.
For the sake of a comparison with the hy bernating animal, the object of which will be explained hereafter, I thought it right to repeat this experiment.
Before I proceed to detail the result, I may just describe an easy method of performing that part of it which consists of artificial respi ration. A quill is firmly fixed in the divided trachea : a small hole is then cut into that part of the quill which is external; Read's syringe is then adapted to the other end of the quill. At each motion" of the piston downwards, the lungs are distended; whilst the piston is raised, the air escapes through the opening in the quill, producing expiration. The experiment, there fore, only requires the common action of the syringe.
The experiment itself answered my expecta tion. During the cessation of respiration, the left ventricle ceased to beat, the right ventricle retaining its function ; on renewing its respira tion, the left ventricle resumed its beat. It
appears from this experiment, that from want of a degree of irritability equal to that of the right ventricle, and its own proper stimulus of arterial blood, the left ventricle ceased its con tractions. The function of the right ventricle must soon cease in consequence, from want of a supply of blood.
These facts prove that arterial blood is the necessary stimulus of the left side of the heart, its irritability being low ; but that venous blood is a sufficient stimulus of the right, from its higher irritability : the phenomena plainly flow from the law, that the quantity of respiration and the degree of irritability observe an in verse ratio to each other, and from the facts on which that law is founded. In this double sense, besides that of distinct cavities, the mammalia have, therefore, two hearts ; and as the highly aerated blood of the left is the pecu liar property of birds and the mammalia, so the highly irritable fibre of the right may be compared to that of the heart of reptiles and the fishes.
Except for the objection to new terms, the left side of the heart might be termed arterio contractile, and the right veno-contractile ; the first being stimulated by arterial, the second by venous blood.
It is quite obvious that the heart will bear a suspended respiration better, the more nearly its irritability approaches to that which may be designated veno-contractile. The power of bearing a suspended respiration thus becomes a measure of the irritability. It is expressed, numerically indeed, by the length of time during which the animal can support a sus pended respiration ; a conclusion of the highest degree of importance in the present inquiry.
Birds die almost instantly on being sub merged in water ; the mammalia survive about three minutes, the reptiles and the batrachia a much greater length of time.
The unborn fwtus, the young animal born with the foramen ovale open, the reptile, the mollusca, having all a state of the heart ap proaching to the veno-contractile, bear a long continued suspension of the respiration, com pared with the mature animal of the higher classes.