B A T R A C H I A. 



311 



consequently there is little or no heat produced in the 

 system. 



When the gills are absorbed, the arterial branches 

 which carried the blood to them also disappear, only 

 two remaining, which go to the two lungs. These 

 two remaining branches carry to the lungs only a 

 portion of the blood which the heart propels, the 

 remainder going to the systematic circulation without 

 being acted on by the air. The action of the air (or 

 the water, according to the habit) upon the blood 

 appears to be the grand or primary action which 

 regulates all the rest, and the energy of the whole 

 system of the animal is in proportion to the quantity 

 of the blood thus acted on, and the rapidity with 

 which the action takes place. In birds, it takes place 

 not only by the whole current of the blood which 

 passes through the heart passing through the lungs 

 ilso, but along almost the whole course of the arteries, 

 so that the bird is " lungs all over." In mammalia, 

 the whole passes through the lungs, and in reptiles a 

 part only, and in the case of batrachian reptiles only 

 a small part ; hence their coldness, and capability of 

 enduring hunger and other privations. 



The change which takes place in the batrachia 

 from breathers of water to breathers of air, by a mere 

 struHurnl alteration, and without any change of 

 substance, proves that, in principle, these two means 

 of breathing are the same, and that the difference is 

 merely one of mode. The animal which the one 

 moment sends its blood to the gills to be acted on by 

 water, and the next moment to lungs to be acted on by 

 air, is one and the same animal, and the blood so sent 

 is one and the same blood. Therefore, the action which 

 it undergoes must be one and the same action ; only 

 the mode is different. As stated in the article Am, 

 it is really air which is breathed in both cases, only, 

 in the case of the gills, that air is mixed with water, 

 but there is no decomposition of that water, and no 

 change of it farther than that a certain volume of the 

 air, which it contained in the state of oxygen, is 

 converted into an equal volume of carbonic acid gas. 

 Thus, that which is otherwise only a rational theory, 

 is demonstrated to be truth in the case of the 

 batrachia, and from them the application to all other 

 animals becomes obvious, and even necessary. 



There are some other very important points which 

 receive elucidation from the study of the batrachia. 

 Breathing by gills is a much less efficient process 

 than breathing by lungs. There are in the tadpole of 

 the frog tribe eight gills, four on each side, and there 

 is an epual number of arterial branches which carry 

 the blood to them ; but there are only two which go 

 to the lungs of the same animal, and they carry only 

 a portion of the blood ; consequently, a smaller degree 

 of the action of lungs in breathing air suffices for the 

 purposes of life in the very same animal. But the 

 particular forms of the two kinds of breathing appa- 

 ratus shows the different modes in which the two 

 fluids act. Lungs consist of cells, into which the air 

 is received, and over the walls of which the small 

 vessels containing the blood are ramified. Gill 

 consist of small fibres or filaments, over the surfaces oi 

 which the vessels are ramified, and these filaments 

 are surrounded by the current of water. The air i 

 thus received into the lungs ; and the gills are immersed 

 in, or surrounded by, the water. In the former, the 

 action is the resistance of the walls of the cells, which, 

 in animals of warm temperament, is increased by the 

 expansion of the air inconsequence of the heat, and 



that, by the way, is one of the reasons why the lungs 

 pant and labour when the body is much heated. The 

 action upon gills is the pressure of the water, which, 

 of course, varies with the depth, and explains why 

 animals which inhabit at great depths breathe much 

 more slowly, and are much more tenacious of life, 

 than those which inhabit near the surface. Attention 

 to these points enables us to understand many of the 

 differences of structure, and explain many of the 

 habits of different races of animals, \\ ithout the labour 

 of individual observation ; thus greatly abridging the 

 time and trouble necessary for acquiring that general 

 knowledge of natural history which should be pos- 

 sessed by all who claim the character of being even 

 moderately well educated, and rendering the science 

 a source of pleasure to a much greater number of 

 persons. 



Even when the batrachia arrive at that stage in 

 which they breathe air with lungs, their respiration is 

 feeble, as well as limited to a portion only of the 

 blood. They want the proper structure of respiring 

 animals; for though their gills are (with the exceptions 

 already mentioned) replaced by lungs, they have not 

 the apparatus for working those lungs which are pos- 

 sessed by the mammalia, and for which birds have 

 substitutes in their air-cells and air-tubes. They have 

 no ribs nor breast-bone, and therefore the thorax does 

 not act in breathing. 



The frogs, as the types of the order, will best serve 

 for illustration upon this, as upon all other points. 

 The frog receives the air which is to be conveyed to 

 the lungs by the nostrils, but there is no vacuum or 

 cavity formed by the expansion of the thorax, as 

 in the mammalia, so that the mere pressure of the 

 atmosphere, following the expansion, does not inflate 

 the lungs as in these. An effort is required after the 

 air is taken into the body, and that is performed 

 chiefly by the tongue. When a frog is breathing 

 with its head to the observer, it may be seen raising 

 and lowering the skin between the bones of the under 

 jaw, but with its mouth firmly shut all the time. The 

 depressions of the skin of the lower jaw last much 

 longer than the elevations, because, during them, 

 there is a double operation to be performed the 

 expulsion of the air already in the lungs, and the re- 

 admission of fresh air into the body. When that 

 skin first descends, there is a contractile action of the 

 abdomen, by which the air in the lungs is driven out ; 

 and when that is all expired, the abdomen returns to 

 the neuter or natural state ; but being without bones, 

 it cannot form a vacuum, and thus the lungs could not 

 be inflated but by some other action capable of over- 

 coming the resistance of their cells. It is for this 

 purpose that the skin under the jaw is raised, but the 

 motion of raising it is in the tongue. That first shuts 

 the nostrils by pressure near the anterior part of the 

 roof of the mouth, and then rising gradually back- 

 wards, it fills all the cavity of the mouth, and 

 forces the air which it contained into the cells of the 

 lungs, which constitutes the respiration. The de- 

 pression of the tongue opens a passage by the nostrils, 

 and the muscles of the abdomen contracting, force 

 the air out by that passage after it has performed its 

 office. So far is the frog from deriving any aid from 

 the mouth in breathing, that it cannot breathe with 

 the mouth open, as there is then no means of inflating 

 the lungs ; and if the mouth is forcibly kept open, 

 the animal as certainly dies of suffocation as the 

 mammalia when both mouth and nostrils are kept 



