Of the Third Species of Respiration. Fishes.
— It may be said that the thorax of fishes usually presents four elastic cartilaginous arches, which approximate and separate, open or close the gills, at the same time increasing or diminishing the capacity of the so-formed thorax. These ribs, or branchial arches, sup port the gills, which are covered by a great flap (operculum) on each side of the base of the skull.
The respiratory current enters at the mouth, passes through the fissures on each side of the latices, and escapes through the branchial openings, placed laterally, covered by the moveable operculum. This stream is uniformly in one direction,— from before, backwards. It might be asked, why does not the water rush in by the branchial opening when the mouth "threatens" a vacuum ? It will be observed that the margin of the oper culum, or great lateral flap, is edged with a delicate membrane, which acts as a valve, this, by the pressure of the water, is forced close round the lateral openings : thus, the water, upon the expansion of the jaws, is prevented entering behind, and consequently rushes in towards the gills by the mouth ; the jaws now close, the operculum immediately opens by the force of the jaws contracting with the mouth full of water, which contraction, or expiration, forces the water through the branchial arches and ultimately out by the lateral openings.
Thus the respiration is of a mixed order. The first stage by atmospheric and hydraulic pressure ; the second stage by direct muscular force, similar to that of swallowing.
If we cut off the delicate fringe around the operculum the fish is suffocated, the opera tion being analogous to puncturing the human thorax.
Fishes also possess a power of regulating their respiration. We have watched fishes when in a quiescent state move their respi ratory organs so gently that the motion was nearly imperceptible, and at times quite so ; but if at such times you alarm the animal, respiration becomes vigorous, and a compa ratively vast body of water rushes past their respiratory organs. The same may be like wise observed when fishes have remained long in a small quantity of water, as if the respira tory movements became more and more vigo rous with the deterioration of their element, but give them a fresh supply of water and re spiration becomes quiescent again.
Of the Fourth Species of Respiration. Anzphi bia.—In this class there is a gradual develop ment of the animal formation from an aquatic to an aerial being ; so likewise is the aqueous gradually converted into aerial respiration.
The respiration of some of these animals is indeed most curious, — curious as to the very limited quantity of air necessary for their well being, and curious as to whether they have this limited quantity supplied regularly or otherwise. For instance, you may keep an aquatic turtle out of water for days, and it will keep constantly respiring air ; immerse it in water, and it will remain below the sur face for half an hour, or an hour, without any inconvenience, and some of these animals will breathe at the surface during the day, and sleep at the bottom all night without once rising for air, while during the day the same animal cannot remain below above half an hour without showing signs of discomfort.
And again, the common tortoise during by hernation breathes so small a quantity of air, that we have never been able to form any calculation of the quantity then respired.
In frogs there are no ribs by which the lungs may be moved ; consequently there is no vacuum formed by their thorax dur ing respiration ; they fill the lungs like the tortoise, the newt, the chameleon, &c., by the working of their jaws ; or, in other words, they swallow their air just as we swallow our food. In this respect their respiratory movements resemble those of fishes; the first process being through the agency of external pressure, by making a vacuum with the mouth ; the se cond, that of forcing, by the operation of the pharynx. They resemble mammalia in having an internal lung, retaining the air for some time, and in expelling it through the same channel by which it entered. The respira tion of the frog has gained attention, and is hence better understood than that of many other animals of this class. The following is the mechanism of its respiration, as described particularly by Townson*, though before no ticed by Swammerdam and Malpighi. When the broad lingual bone which forms the floor of the mouth is drawn down from the pa late by its muscles, the air of the mouth is rarefied, and an additional quantity enters by the nasal apertures, which admit of being closed by valves. The lingual bone is then raised, the nasal apertures are closed, and the air is now forced, or rather swallowed, through the riina glottidis into the pulmonary sacs, and can also fill the laryngeal pouches which open into the mouth. Expiration is produced partly by the pressure of the abdo minal muscles, and partly by the peculiar mus cular power of the pulmonary parietes. To the careless observer the frog does not appear to breathe : it is never seen to open its mouth ; there is no motion of its sides like breathing, and when it is provoked (or rather through fear), though it still keeps its mouth close shut, its sides and back rise, and it blows itself up ap parently by some internal power. Upon observ ing it more narrowly, that skinny and bag-like part of its mouth which is under the jaw, is seen to be in constant motion. While this bag is dilating and contracting, the mouth is never opened to take in new air, but it seems to live all the while on one mouthful of air, and seems to be playing it backwards and forwards between the mouth and lungs. If we now observe the nostrils, a twirling motion, which lets in air at each movement of the jaws, is apparent, corresponding to the quantity of air inspired. If we keep the mouth open we presently see the animal struggle for breath, for we by this means disable the forcing apparatus from pro pelling the required air into the lungs.