XII. II 



VENTRAL MUSCLES 



321 



factors that control the arrangement of muscle-fibres into 'muscles' 

 that discussion of homologies is difficult. 



The hypaxial musculature, formed from the more ventral portions 

 of the myotomes, is more developed than in fishes and differentiated 

 into several parts, for the purpose of slinging the viscera, which of 

 course need support in air in a way that is unnecessary in water. 



sub. max. 



dep. mand. 



d. rorrad 

 deU. 



h.. 



Fig. 191. Dissection of muscles of frog from ventral surface. 



cor.br. coraco-brachialis; cor.rad. coraco-radialis; dep. mand. depressor mandibulae; h. head of 



humerus; s. sternum; other letters as Fig. 190. (Partly after Gaupp.) 



These muscles are differentiated into layers whose fibres run in 

 different directions. The plan found, with modifications, in all 

 tetrapods is seen in amphibian larvae and includes four sets of fibres. 

 The external obliques run caudally and ventrally; inside this 

 layer is the internal oblique, running in the opposite direction, and 

 within this again the transversus abdominis running approximately 

 dorso-ventrally (Fig. 189). The rectus abdominis consists of fibres in 

 the midline running antero-posteriorly. 



In the adult frog three of these sets of fibres can be recognized. In 

 the mid-ventral region (Fig. 191) are the longitudinally arranged 

 fibres of the rectus abdominis, making a sling between the sternum 

 and the pubis. These fibres are interrupted at intervals by transverse 

 fibrous tendinous inscriptions, giving an appearance of segmentation. 

 In the mid-ventral line is the tendinous linea alba. The sling formed 



