1018 



THORAX. 



sternum in one mass, rigid and immoveable : 

 the crocodile and lizard have perfect ribs, but 

 their sternum is almost entirely cartilaginous ; 

 and, lastly, in man, the components of the 

 thoracic cavity may have a mobility to com- 

 mand or exceed a space equal to the whole 

 cavity allotted for the respiratory organs. The 

 relative quantity of air which he can respire 

 for the aeration of his blood is probably 

 greater than in any other animal, and his 

 movements are more under the control of his 

 will, and are greatly influenced by mental 

 emotions. 



CLASSIFICATION OF THE RESPIRATORY 

 MOVEMENTS IN ANIMALS. It is not easy 

 to name any particular part in the perfect 

 thorax of the higher vertebrata, which is 

 equally destined for respiratory motion 

 throughout the class. Commonly with ribs 

 and intercostal muscles, we connect the idea 

 of a thorax, or a breathing chamber for re- 

 spiration ; but a fish has ribs, and likewise 

 intercostal muscles, yet not for any of the 

 purposes of respiration, nor do we acknow- 

 ledge them to belong to its thorax. A frog 

 has a thorax for respiration with internal 

 lungs, but no ribs, nor, consequently, any 

 intercostal muscles. Nor is a diaphragm 

 necessary to thoracic respiration ; for it is 

 mostly absent in birds and reptiles, and quite 

 rudimentary in the few instances (such as the 

 ostrich, crocodile, and chelonia\ in which it is 

 met with. In the chelonia neither ribs, spine, 

 nor sternum are concerned in the respiratory 

 movements. 



The movements of respiration tend to bring 

 before some surface, air or air contained 

 in water ; or to bring a certain surface con- 

 tinually into a fresh medium, In whatever 

 way this may be accomplished, whether 

 by moving the whole body, or part of the 

 body to and fro, such movements, likewise, 

 are not uniformly for the mere purpose of re- 

 spiration, i. e. the mere purpose of aeration. 

 All reptiles take more air into their capacious 

 lungs than they require for oxygenating the 

 blood, particularly in the aquatic kinds (as in 

 the turtle), where the air serves to buoy up 

 their heavy and slow-moving bodies in the 

 dense element they inhabit. Serpents are 

 provided with numerous highly-moveable ribs 

 and powerful intercostal muscles, capable of 

 rapid and extensive inspiration and expiration. 

 They can perfectly distend their body with air. 

 The same may be observed in the chameleon. 

 These volumes of air cannot alone be subservient 

 to respiration, as it cannot all come into contact 

 w r ith the simple undivided respiratory sacs. 

 We see, therefore, there is no necessary rela- 

 tion between the quantity of air an animal may 

 inspire and the extent of respiratory surface. 

 The long hissing sound which serpents pro- 

 duce to alarm their prey, is effected by the 

 expulsion of this great volume of air, by their 

 ribs, through the narrow passages of the nos- 

 trils. 



In the higher mammiferous animals, we find 

 respiration more especially destined for the 

 chemical purpose of oxygenating the blood. 



Hence a more limited quantity is taken in, and 

 it is speedily thrown out again. Large animals 

 make fewer respirations than small ones. Ac- 

 cording to Scoresby *, the whale breathes 

 four or five times a minute ; the dog, the cat, 

 and rabbit, from twenty to thirty in the same 

 period ; and in small birds the respirations 

 are remarkably rapid. 



Whatever be the form of the aerating or- 

 gan, "breathing" is accomplished either by, 

 1st, the weight of the atmosphere rushing 

 into certain cavities, because certain parts of 

 these cavities dilate and threaten a vacuum ; 

 or, 2ndly, by the direct projectile or collapsing 

 force of an organ throwing the ambient ele- 

 ment onward. These two ways are generally 

 more or less combined in the same animal. 

 Nor does there appear to be any relation be- 

 tween the grade of the animal and the order 

 of respiratory movement obtained. We notice 

 in the respiration of man a regular inspiration 

 and expiration, two currents in different di- 

 rections ; and in the lowest animal, the 

 connecting link with the vegetable kingdom, 

 the porifera, or sponge tribe, there are like- 

 wise two respiratory currents by distinct 

 channels, which are as regular as the motion 

 of rivers from their source to the ocean, or 

 any other movement depending upon the 

 established order of things. In some spe- 

 cies of medusae, there are peculiar sacs on 

 the inferior surface of the body, which, 

 during the expansion of the body, admit 

 water through certain apertures, and again 

 expel it during the succeeding contraction, re- 

 presentiug a perfect inspiratory and expi- 

 ratory action. The complexity, therefore, of 

 respiratory movements does not correspond 

 with the increasing development of the 

 breathing organs. Those animals which have 

 an internal sacculated lung, always retain a 

 certain quantity of the breathing element in 

 " reserve " within their system ; whereas those 

 animals which have external lungs, or gills, 

 have no " reserve " respirable medium. They 

 need none, because where there is an exter- 

 nal lung, the ambient element answers the 

 purpose of the "reserve"; it is always in 

 contact with the breathing surface. This 

 " reserve," in mammalia, c., is not, pro- 

 bably, so necessary to aeration as for the 

 purpose of ejecting any matter which may 

 obstruct the air passages or, in more po- 

 pular language, for " coughing " up any matter 

 out of the throat. 



The different kinds of respiratory move- 

 ment may be arranged as follows ; 



1. Infusorial Animalcules By projectile force. 



2. Insects - Uncertain. 



3. Fishes - By vacuum & pro- 



jectile force. 



4. Amphibia - - do. do. 



5. Birds - do. do. 



6. Mammalia - - do. do. 



Of the First Kind of Respiratory Movement. 

 Infusoria. These animals breathe by a stream 

 propelled in one direction, produced by the ra- 



* El. Pliysiol. Wagner, 8vo. 1844, p. 670. 



