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 my lo-h3 oid 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, the tongue immediately closing their internal aperture, the second motion forces the air into the lungs. It is not un 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 vertebra, 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 Sixth Species of Respiration. Mani 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 only 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 inspiration, 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.1. 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, of 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 of the passions of his Mind, while in the lower animals we see none of these.