But the most remarkable fact deducible from this reasoning is the following : if such a case existed as that of the left side of the heart being nearly or absolutely veno-contractile, such an animal would bear the indefinite suspension of respiration ; such an animal would not drown though immersed in water. Now there is pre cisely such a case. It is that of the hyberna ting animal. It may be shown that in the state of perfect hybernation the respiration is nearly suspended ; the blood must, there fore, be venous. See HYBERNATION. Yet the heart continues to contract, although with a reptile slowness. The left ventricle is, there fore, veno-contractile, and in this sense, in fact, sub-reptile. The case forms a solitary excepL tion to the law pointed out by Harvey, that the left ventricle ceases to contract sooner than the right. If in the hybernating animal the left ventricle does cease to beat sooner than the right, it is only in so slight a degree as to be referred to the greater thickness of its parietes, and the slight degree in which respiration still remains. It is obvious that the foregoing state ment must be taken with its due limitations.
Venous blood is unfit for the other animal pur poses, even though it should stimulate the heart to contraction.
Another mode of determining the degree of irritability, is the application of stimuli, as galvanism. A muscular fibre endued with high irritability, as that of the frog, and the galvanic agency are mutually tests of each other.* A third criterion and measure of the irrita bility is afforded by the influence of water at temperatures more or less elevated, in in ducing permanent contraction of the muscular fibre.
There are two other properties of animals which depend upon the varied forms of the inverse ratio which exists between the respira tion and the irritability. The first is activity, the second, tenacity of life.
The activity, which, I believe, M. Cuvier has confounded with the irritability, is generally directly proportionate to the respiration, and intimately depends upon the condition of the nervous system resulting from the impression of a highly arterial blood upon its masses, and not upon the degree of irritability of the muscu lar fibre. It is the pure effect of high stimulus.
To show that M. Cuvier has blended the idea of the irritability of the muscular fibre with that of the activity of the animal, it is only necessary to recur to the passages already quoted from that author, and to adduce the observations with which they are connected. " On vient de voir a quel point les animaux vertebres se ressemblent entre eux ; ils offrent cependant quatre grandes subdivisions ou classes, caracterisees par l'espbce ou la force de leurs mouvements, qui dependent elles memes de la quantite de leur respiration, at tendu que c'est de la respiration que les fibres musculaires tirent l'energie de leur irritabilite."t
" Comme c'est la respiration qui donne an sang sa chaleur, et a la fibre la susceptibilite pour l'irritation nerveuse, les reptiles ont he sang froid, et les forces musculaires moindres en totalite que les quadruOdes, et a plus forte raison que les oiseaux ; aussi n'exercent-ils gubre que les mouvements du Tamper et du linger; et, quoique plusieurs sautent et courent fort vite en certains moments, leurs habitudes sont generalement paresseuses, leur digestion excessivement lente, leurs sensations obtuses, et dans les pays froids ou temperas, ils passent presque tons l'hiver en lethargie."I It is extraordinary that M. Cuvier should have associated the elevated temperature of the blood with a high irritability of the muscular fibre, when they are uniformly separated in nature, and are, indeed, absolutely incompa tible in themselves. The muscular fibre of the frog is so irritable, that it would instantly pass into a state of rigid contraction, if bathed with a fluid of the temperature of the blood of birds.§ The same confusion of ideas on the subject of the activity of the animal and the irritability of the muscular fibre prevails, I believe, amongst our own physiologists ; at least, in conversation with two, who may rank amongst the first, I found that they had uniformly con sidered the respiration and the irritability to be directly, instead of inversely, proportionate to each other.
That singular and interesting property of the lower orders of animals termed tenacity of life is, on the other hand, distinctly associated with a high degree of irritability of the mus cular fibre. The property may be defined as consisting of the power of sustaining the pri vation of respiration, the privation of food, various mutilations, divisions, &c. It is greater as we descend in the zoological scale. As activity depends upon the presence and condi tion of the spino-cerebral masses acted upon by arterial blood, tenacity of life depends upon the diminution or absence of these masses and of this highly arterialized blood, being greatest of all in those animals which approach a mere muscular structure. Almost the sole vital pro perty then remaining is the irritability; and this property does not immediately suffer from division.