THORAX. 



1021 





common to notice tortoises yawn ; but how 

 different is their yawning from that of man, 

 who makes, at that time, a deep inspiration, 

 while, in the tortoise, respiration is impossible. 

 We are not prepared exactly to say how the 

 tortoise and turtle expire ; but probably the 

 expiration is performed by the contraction of 

 the abdominal muscles between the lower 

 shield or plastron and the posterior extremi- 

 ties ; for either of these animals* can at will, 

 when alarmed, forcibly expel air with a hiss- 

 ing sound, although its shell is unyielding. 



The most remarkable respiratory move- 

 ment we have noticed, has been in the com- 

 mon turtle. Sometimes this animal will swell 

 out his hard case, the sternum or plastron 

 yielding to some internal force ; but it is diffi- 

 cult to say by what means this is distended 

 and kept distended. 



It is clear this animal can gorge itself with 

 air until it cannot sink in water, and that at 

 pleasure it can disgorge itself and fall to the 

 bottom, where it lives upon only a fraction of 

 the quantity of air it had just previously ex- 

 pelled. 



Of the Fifth Species of Respiration. Birds. 

 Here we have a contracting and dilating 

 thorax, with ribs and sternum. The cavity of 

 the chest is not divided by a diaphragm, but 

 is common to the whole digestive organs as 

 well as the lungs ; or, as is said, they are 

 "all chest and no belly." They differ from all 

 other animals in this respect, that the lungs 

 do not hang in the cavity of the trunk as unat- 

 tached sacs, but are attached in the form of 

 flattened masses, of spongy, bright red, cellular 

 texture, to the posterior side of the thorax, 

 reaching to the pelvis. They have vesicles or 

 air bags extending through the whole body ; 

 and the cancellated structure of their bones is 

 connected with the true lungs ; so that if we 

 tie the trachea and amputate the wing, leaving 

 the stump of the bone exposed, the bird can 

 inspire and expire through the humerus. In 

 the same manner that the diffusion of air 

 through all parts of the body in insects makes 

 the highest extent of respiration in invertebrata, 

 so also is it with birds among the vertebrata. 



The sternum and ribs, together with the 

 immoveable range of dorsal vertebrae, all con- 

 tribute to dilate and narrow the thorax, after 

 the manner of a bellows movement. This di- 

 latation and contraction draws the air through 

 the true lungs, which never move, and imme- 

 diately the air cells are expanded. By this 

 means two conditions are obtained ; the air is 

 drawn through the lungs for aeration ; and 

 the air filling the cancellated structure, renders 

 the bird specifically lighter. 



The high flying rapacious bird can thus by 

 a respiratory movement attenuate the air in 

 his body, when soaring in the atmosphere, and 

 again at pleasure condense it in every inter- 

 stice of his frame, when he drops like a can- 

 non ball, to pounce on his prey ; but imme- 

 diately before seizing it, again he attenu- 

 ates the air within him to break his fall ; other- 

 wise he would be dashed to pieces upon the 

 pointed crag, and die along with his victim. 



This beautiful provision is wholly due to 

 his respiratory movement, at one time acting 

 as a condensing, and at another time as an 

 exhausting syringe. 



Of the Sijcth Species of Respiration. Mam- 

 malia. In this class we first meet with a 

 perfect muscular septum (diaphragm) forming 

 the two cavities of the trunk ; the one for the 

 lungs, and the other for the abdominal viscera. 

 All animals which have a diaphragm, maintain 

 respiration in a manner similar to each other ; 

 for, indeed, it appears that the ouly use of 

 this muscle is to maintain a movement of air 

 that unceasing pumping to and fro of inspi- 

 ration and expiration. Their respiration, or 

 at least their insjnration y is purely of the 

 vacuum order. 



The diaphragm is the chief muscle of ordi- 

 nary breathing. It can act with great power, 

 protruding the viscera, by its descent, at each 

 ordinary inspiration. This is strikingly seen 

 in animals recumbent and at rest, as in the 

 cow, horse, goat, dog, &c., when it ap- 

 pears as if the animal was breathing with its 

 abdomen. The ribs likewise in some degree 

 maintain respiration in the lower mammiferous 

 animals, particularly in disease. For instance, 

 the respiration of the horse or dog, when the 

 lung is emphysematous, or what is familiarly 

 termed " broken-winded," is costal , and at 

 such times the respiratory action of the ribs 

 may be beautifully seen. 



It is most probable that in mammalian re- 

 spiration we have the highest order of accom- 

 modation for peculiar respiration, according: to 

 the condition of the animal ; i. e. an instinc- 

 tive power to respire by different parts of the 

 thoracic cavity, according to the needs of the 

 animal, whether modified by health or disease. 



Nearly two hundred years ago, Lower 

 changed the respiratory movements of the dog 

 from diaphragmatic to costal, by paralysing the 

 diaphragm through the medium of the phrenic 

 nerve. (Phil. Tr. Abr., vol. i. p. 179.) 



The respiration of mammalia is the bellows 

 action inflation of the lungs by expansion of 

 the thorax, or inspiration by vacuum, and ex- 

 piration by propulsion. 



The projectile force in the respiration of 

 mammalia is nearly all due to mere elastic 

 contractility ; i. e. ordinary expirations are 

 produced by the elasticity of the lungs and 

 ribs, returning backwards, or collapsing, after 

 their distension by the inspiratory muscles. 

 This mere dead and involuntary force performs 

 one half of our respiration. 



Man is not distinguished either by the force, 

 extent, or complexity of his respiratory move- 

 ments ; he is exceeded in all these particulars 

 by inferior animals. The roar of the lion 

 gives the idea of an overwhelming expiratory 

 power ; nor are his lungs less complicated ; 

 and the vibration of thousands of cilia, pro- 

 moting currents around the monad, is more 

 complex than the simple respiratory thoracic 

 action of mammalia. The most striking dif- 

 ference is that produced by mental influence, 

 which appears to command the most delicate 

 modifications of this movement, so indicative 



