XXl] RESPIRATORY MUSCLES 527 



If the animal be laid on a board with the ventral side uppermost 

 and skinned, a thin sheet of muscles, the mylohyoid, will be seen 

 stretching between the two halves of the lower jaw. When this 

 muscle is relaxed the floor of the mouth is arched upwards and the 

 underside of the head consequently becomes concave. When the 

 muscle contracts and straightens, the cavity of the mouth enlarges 

 and air is drawn in. Above the mylohyoid (underneath from the 

 point of view of the dissection) are two longitudinal muscular 

 bands, and in these are embedded the reduced remains of the 

 visceral arches to which the gills of the larva were attached 

 (Fig. 256). These muscles are called geniohyoid in front of the 

 arches, stern ohyoid between them and the pectoral girdle, and 

 they are continued backwards along the belly as the straight muscles 

 of the abdomen, the recti abdominis. 



These sternohyoid muscles can draw the visceral arches 

 downwards and backwards and probably assist the mylohyoid in 

 depressing the floor of the mouth. The geniohyoid muscle on 

 the contrary pulls the arches forwards and helps to restore them 

 and the floor of the mouth with them to their old position. In this 

 action muscles called petrohyoid, which run from the arches to the 

 outer surface of the auditory capsule, also take part. These muscles 

 are representatives of the levatores arcuum of fish (see p. 457), 

 and they raise the arches and consequently the floor of the mouth. 



The glottis or opening into the lungs is stiffened at the sides by 

 a pair of cartilages, which it seems probable are the remains of a 

 hinder pair of visceral arches : and these cartilages have muscles 

 attached to their sides which drag them apart and which belong to 

 the same series as those which raise the arches. Hence the same 

 muscular action which lifts the floor of thej mouth opens the glottis 

 and admits air into the lungs. 



How these muscles co-operate to effect the regular pumping of 

 air in and out of the lungs has been thoroughly investigated only 

 in the case of the Frog, and will be described when we come to 

 deal with that animal. 



The remaining muscles of the body are not much altered from 

 those of the fish. In the tail and the ventral part of the trunk 

 there are V-shaped myotomes, but this arrangement is disturbed in 

 the neighbourhood of the limbs. 



Turning now to the skeleton we find that the vertebrae bear 

 stout transverse processes with which are articulated 

 short ribs (Fig. 252). The ribs borne by the sacral 



