NATURE OF THE FUNCTION. 391 



back, communicating with the air, and having a beautifully-reticulated plexus 

 of blood-vessels on its walls. In none of the Invertebrata, however, does the 

 respiratory apparatus communicate with the mouth ; which is an organ solely 

 appropriated, in them, to the ingestion of food. In the Mollusca, indeed, the 

 channel through which the water, that has passed over the aerating surface, 

 leaves the chamber (formed by a fold of the mantle or general envelop) that 

 contains the gills, is the same as that through which the excrementitious 

 matter is discharged from the intestine ; and the gills themselves are very 

 commonly situated in the neighbourhood of the anal orifice. This fact is inte- 

 resting, in regard to the character of the temporary respiratory apparatus of 

 the Human embryo. In Fishes and the larvae of Batrachia, which are the 

 highest animals that breathe by gills, these organs are so disposed in connection 

 with the cavity of the mouth, that fresh currents of water are continually being 

 forced over them by its muscles ; and thus the energy of their action is greatly 

 increased. Moreover, the whole blood which is propelled from the heart, 

 proceeds first to the respiratory organs, instead of passing through them on its 

 return from the systemic circulation, as in most of the aquatic Invertebrata. 

 Still, as the quantity of oxygen which the blood can obtain in this manner is 

 very small, being limited to that contained in the atmospheric air dissolved in 

 the water, the amount of aeration must be considered as low. 



524. In the lowest Vertebrata that possess any thing like a pulmonary 

 cavity, this has a structure as simple as that of the air-sac of the Snail. This 

 is the case in many Fishes, where it is known as the air-bladder ; it is fre- 

 quently single in this class, and communicates with the intestinal canal near 

 the stomach, or is altogether destitute of outlet. In others, however, it is 

 double, and its duct opens into the oesophagus near the mouth ; so that its 

 analogy to the lungs of higher animals is very evident. The Batrachia begin 

 life as fishes, breathing by gills during their tadpole state ; but at the time 

 that the legs are developed and the tail has decreased, the pulmonary organs 

 also are evolved, and the course of the blood is altered, so that it is no longer 

 transmitted through the gills, which speedily shrivel and disappear ( 42). 

 There are some species, however, whose metamorphosis is checked, so that 

 in their permanent condition both lungs and gills are present ; but the former 

 are then present in a very rudimentary form, not being more developed than 

 the air-sacs of many Fishes. The lungs of Reptiles are, almost universally, 

 simple sacs with little subdivision into cells. Where such subdivision exists, 

 it is usually at the upper part of each lung, the rest being one undivided bag, 

 on the walls of which the pulmonary vessels are distributed. They afford us, 

 therefore, a good opportunity of studying the distribution of these vessels ; and 

 the accompanying figures represent the course of the circulation as observed 

 in them. It will be seen that the trunk of the pulmonary artery runs along 

 one side of the sac, and that of the pulmonary vein along the other (Fig. 93) ; 

 and that numerous branches arise from the former, which subdivide into 

 capillaries that ramify over the whole surface, and then reunite into small 

 veins which terminate in the latter. The islets of parenchyma left between 

 the capillary vessels, are seen to be much smaller than those which are usually 

 to be observed in the systemic circulation (Figs. 94, 95) ; so that the mem- 

 brane is more copiously traversed by vessels than any other that is known. 

 The walls of the capillaries, moreover, are. much less distinct than those of the 

 systemic circulation. These two conditions are obviously favourable to the 

 exposure of the largest possible quantity of blood to the influence of the air ; 

 but as the surface is not an extensive one, the amount which can be thus 

 exposed at any one time is very limited ; and the pulmonary artery is in fact 

 one of the smaller branches of the aorta, which conveys a mixed fluid to the 

 system at large. 



