. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 100 AMPHIBIA. simple structure of these pulmonary sacs, it will readily be seen that the function of respiration could be only very ineffectively aided by the latter organs, even were there no other diffi- culty arising from the imperfect structure of the apparatus which in the air-breathing amphibia serves the office of conveying the air into the lungs. A short description of the means by which the act of inspiration is effected in the frog will enable us to judge how far it may be possible that the rudimentary lungs in
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 100 AMPHIBIA. simple structure of these pulmonary sacs, it will readily be seen that the function of respiration could be only very ineffectively aided by the latter organs, even were there no other diffi- culty arising from the imperfect structure of the apparatus which in the air-breathing amphibia serves the office of conveying the air into the lungs. A short description of the means by which the act of inspiration is effected in the frog will enable us to judge how far it may be possible that the rudimentary lungs in the pro- tens and siren are to be considered as performing any such function. In the adult frog, toad, salamander, and all others of the higher orders of amphibia, the reception of air into the lungs is effected not by the primary expansion of the pulmonic cavity and the consequent rush of air into it, but by the act of forcing air into the lungs, or in fact by a simple act of swallowing. This is effected in the following manner. The os hyoides and tongue are brought downwards to a considerable extent, and the cavity of the mouth being thus much enlarged, the air enters by the nostrils. The pharynx is then shut at the posterior part, so as to prevent the passage of air into the oeso- phagus, and the cavity being suddenly con- tracted by means of the muscles acting on the os hyoides, the air is necessarily forced through the glottis and trachea into the lungs, as the posterior nares are closed either by their mar- gins acting as a valve, or by the pressure of the tongue against them. This view of the mode of inspiration explains the cause of the well- known fact, that if the mouth of frogs be held open they perish from actual suffocation ; for the motions of the os hyoides being thus im- peded, and an external passage being also afforded for the air, respiration by the injection of air into the lungs is obviously impossible. Any other mode of inspiration, connected with the pr
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