TOLERANCE OP DEPRIVATION OF AIR. 265 



system. But a very slight irritation is sufficient to produce 

 respiratory movements ; the heart's action is quickened ; 

 and the animal for a time shows an increase of its general 

 activity. 



310. Animals will in general bear deprivation of air well 

 or badly, according as the respiration is more or less active. 

 Thus a warm-blooded animal usually dies, if kept beneath 

 water for more than a few minutes ; though there are some 

 which are enabled, by peculiar means, to sustain life much 

 longer ( 265). In cold-blooded animals, however, whose 

 demand for oxygen is much less energetic, this treatment may 

 be continued for a much longer time without the loss of life. 

 Thus the common Water-Newt naturally passes a quarter of 

 an hour or more beneath the surface, without coming up to 

 breathe j and it may be kept down for many times that 

 period without serious injury. And, as we might expect from 

 their peculiar condition, warm-blooded animals, when hyber- 

 nating, may be kept beneath water for an hour or more, 

 without apparent suffering; although the same animals, in 

 their active state, would not survive above three minutes. 

 There is reason to believe that a similar condition may be 

 produced in Man, by the influence of mental emotion, or of a 

 blow on the head, at the moment of falling into the water ; 

 so that recovery is by no means hopeless, even though the 

 individual may have been more than half an hour beneath the 

 surface, 



Structure and Actions of the Respiratory Apparatus. 



311. In animals whose organization is most simple, the act 

 of respiration is not performed by any organ expressly set 

 apart for it ; but it is effected by all the parts of the body that 

 are in contact with the element in which the animal lives, 

 and from which they derive their necessary supply of oxygen. 

 This is the case, for example, in the lower class of Animal- 

 cules, in the Polypes, Jelly-fish, Entozoa, and many other 

 animals. Even in the higher classes, a considerable amount 

 of respiratory action takes place through the skin, especially 

 when it is soft and but little covered with hair, scales, &c., as 

 in Man, and in the Frog tribe ; but we almost invariably find 

 in them a prolongation of this membrane, specially designed to 

 enable the blood and the air to act upon each other, and having 



