578 



REPTILIA 



[CH. 



layer of fibres. The fibres of the external layer slope upwards and 

 forwards and, in contracting, cause the ribs to rotate forwards ; the 

 fibres of the inner layer slope upwards and backwards and have the 



reverse effect. Respiration 



13 



II 



is effected by the pulling 

 forwards and backwards of 

 the ribs by these inter- 

 costal muscles. In their 

 relaxed condition the ribs 

 slant strongly backwards. 

 When they are pulled for- 

 ward by muscles attaching 

 them to the anterior verte- 

 brae and by the external 

 intercostals, they rotate for- 

 wards so as to stand out 

 at right angles to the ver- 

 tebral column and thus 

 enlarge the cavity of the 

 chest, that is, the coelom. 

 The diminution of pressure 

 in this air-tight cavity at 

 once causes an inrush of 

 air through the glottis, the 

 elastic lungs are expanded 

 and their walls closely fol- 

 low the chest wall. It 

 will be noticed that the 

 mechanism of inspiration is 

 very different from that of 

 Amphibians (pp. 527, 555). 

 The network of low ridges 

 which is found already on 

 the inner side of the Frog's 

 lung has in the Reptile 

 greatly increased in com- 

 plexity. The primary ridges 

 are much higher, and be- 

 tween them are lower 



secondary and even tertiary ridges: the cavity of the lung is as 

 it were partly filled up by a spongy mass. In all Saurians however 



FIG. 285. Diagram of arterial arches of 

 Chamaeleo viewed from the ventral aspect. 



i, IT, ni, iv, v, vi. First to sixth arterial 

 arches. 12. Tracheolingual (ventral 



carotid). 13. Common carotid (dorsal 

 carotid). 15. Eight systemic arch. 



16. Left systemic arch. 17. Dorsal 



aorta. 19. Pulmonary. 20. In- 



nominate. 21. Scapular (equivalent 



of subclavian of ventral type). 22. Sub- 

 clavian (dorsal type). 24. Coeliac. 



