IRRITABILITY. 



35 



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 foetus, 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. 



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 hibernation the respiration 

 is nearly suspended ; the blood must, there- 

 fore, be venous. See HIBERNATION. 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 excep- 

 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 oflrent 

 cependant quatre grandes subdivisions ou 

 classes, caracterisees par 1'espece 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 1'energie de leur irritabilite."f 

 " Comme c'est la respiration qui donne au 

 sang sa chaleur, ^et a la fibre la susceptibilite 

 pour l'irritation nerveuse, les reptiles ont le 

 sang froid, et les forces musculaires moindres 

 en totalite que les quadrupedes, et a plus forte 

 raison que les oiseaux ; aussi n'exercent-ils 

 guere que les mouvements du ram per et du 

 nager; 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 tempercs, ils passent 

 presque tous 1'hiver en lethargie."J 



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. 



* Bostock on Galvanism, pp. 4, 14. 



t Le Regne Animal, tome i. pp. 56, 57. 2de 

 edit. 



Ibid, tome ii. pp. 1, 2. 2de edit. 



See an Essay on the Circulation, chap. vii. 

 pp. 180, 181. 



D 2 



