29 S RESPIRATION. 



CHAPTER XVII. 

 RESPIRATION. 



SECTION I. PRELIMINARY STUDY OF THE EXTERNAL MOVEMENTS 

 OF RESPIRATION. 



86. Respiratory Movements of the Frog. To ob- 

 serve the respiratory movements in the frog, the animal must 

 be fixed on its back. It is seen that that part of the floor of 

 the pharyngeal cavity which corresponds to the submaxillary 

 space, i. e.. to the space which lies between the episternal car- 

 tilage, and the two branches of the lower jaw bone, alternately 

 rises and falls at intervals of about one or two seconds. On 

 more attentive examination, it is found that these movements 

 are due to the alternate retraction and advance of the body of 

 the hyoid bone, the general form of which can be readily dis- 

 tinguished under the skin. To study their nature, the skin 

 must be divided in the middle line from the mouth to the 

 sternum, and detached from the subjacent muscles as far out- 

 wards on either side as the jaw. In this way a view is ob- 

 tained of all the muscles attached to the hyoid bone, without 

 interfering with the mechanism of respiration (see fig. 246). 

 By its long and slender anterior horn, the hyoid bone is con- 

 nected with the skull (i. e., with the cartilaginous part of the 

 petrous bone) in such a manner that, although the two car- 

 tilages are not united by a joint, the hyoid works on the 

 petrous bones as if it were hinged to them. This being borne 

 in mind, it is easy to understand the action of the muscles 

 which are attached to it. Those which come from the sternum 

 and bones connected with it, in drawing the hyoid backwards, 

 cause it, at the same time, to descend in such a way as to in- 

 crease the space between its upper surface and the roof of the 

 mouth and pharynx, and to extend that part of the submax- 

 illary space which intervenes between the arch of the hyoid 

 and that of the lower jaw. On the other hand, those muscles 

 which stretch from the chin (the genio-hyoid), and from the 

 petrous bones (the petrohyoid muscles) to the body of the 

 bone, combine in drawing it upwards and forwards, to such a' 

 degree, indeed, that when the latter are in action, the sub- 

 maxillary space becomes concave. All this can be readily 

 seen in the living animal; for although the muscles above- 

 mentioned are covered by the submaxillary or mylohyoid 



