1020 



THORAX. 



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. Amphi- 

 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 hy- 

 bernation 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 rima 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. 



The newt breathes with the jaws and nos- 

 trils like the frog. It has, like the frog, a 

 constant motion, by short strokes of the bag 

 under the jaw. This bag is formed by 

 the membranes of the mouth, covered and 

 moved by the genio-hyoid and mylo-hyoid 

 muscles. Every minute, or less, it stops, 

 as if intending some particular motion ; then 

 gradually the bag swells out under the lower 

 jaw to a great size ; the contained air is then 

 pressed down into the lungs, .and in propor- 

 tion as the jaws are emptied, the sides of the 

 animal are swelled up. The toad, the chame- 

 leon, and the green lizard breathe in the same 

 way, propelling mouthfuls of air down into 

 the lungs. The chameleon can force down a 

 greater or smaller quantity of air, as its needs 

 or fears prompt it. At times it seems to fill 

 its body almost to bursting with air. The 

 tortoise, like the frog, holds its jaws close, 

 and swallows the air ; alternately depressing 

 and elevating the hyoid bone. The first of 

 these motions permits the air to enter the 

 nostrils, when, tbe'tongue immediately closing 

 their internal aperture, the second motion 

 forces the air into the lungs. It is not un- 



* Tracts and Obs. on Nat, Hist., &c, London, 1799. 



