XXI] RESPIRATORY MOVEMENTS 555 



motor nerves going to the myotomes. Now as these longitudinal 

 flaps were converted into paired fins, and these by a continual 

 narrowing of their bases acquired greater distinctness from the body, 

 tha^pyrtions of the myotomes supplying the musculature and the 

 nerves in connection therewith became so to speak bunched together 

 at the base of the limb. In adult Craniata all trace of the original 

 metameric arrangement of the limb muscles is lost ; but the meta- 

 merism of the nerves can still be seen, and the bundles of these 

 supplying the pectoral and the pelvic limbs are known as the 

 brachial and the sciatic plexus respectively. In the frog, 

 where the limbs are of far greater importance to the life of the 

 animal than are the fins to fish, the nerves forming the brachial 

 and the sciatic plexus are powerful trunks (Fig. 275, 2, 3, and 

 710). 



The lungs are shorter than in the Newt but much wider, and 

 their inner surface is qovered-with a network of low ridges which 

 much increases their area. 



The breathing movements of the frog have been analysed in 

 detail, and we may give the results of this analysis as an example 

 of respiratory movements in general amongst Amphibia, for there 

 is little doubt that all Amphibia breathe in much the same fashion. 

 The breathing movements of the frog have been classified into 

 (a) aspiration, (b) expiration, (c) inspiration. 



In aspiration air is drawn into the buccal cavity through the 

 nostrils, this cavity being enlarged (as described in the case of the 

 Newt) by the contraction of the mylohyoid muscle. In expiration 

 air is forced out of the lungs into the buccal cavity and mingles here 

 with the pure air drawn in from outside. This is, effected by the 

 dragging apart of the arytenoid cartilages guarding the opening of 

 the glottis whilst at the same time pressure is put on the lungs by 

 the contraction of the sternohyoid muscle which drags back the 

 basilingnal. plate towards the pectoral girdle. Finally in inspiration 

 the mixed air is pumped back into the lungs. 



The pressure on the lungs is relieved by the contraction of the 

 geniohyoid muscle which drags the basilingual plate forwards and 

 the buccal cavity is reduced in size by the elevation of the basi- 

 lingual by the action of the petroglossal muscles which attach it to 

 the skull. 



In the last phase of this movement some air must escape 

 through the nostrils otherwise the frog would be continually taking 

 in air and giving out none, which is an impossibility. 



