Y 

 ppa 



/><! 



pa 



ML 



/ 





o& 



ac. 



St. 







s 





p rose. 



xii. 13 LIMB MUSCLES 325 



anterior and posterior groups is much modified. The more con- 

 spicuous muscles are the tibialis anterior and peroneus running from 

 the femur to the tarsus so as to flex the ankle joint. Long and short 

 flexors move the toes, as in the fore-limb. At the back of the tibio- 

 fibula the gastrocnemius (plantaris longus) runs from the femur to 

 be attached by the tendo Achillis to the tarsus. Its main action is to 

 extend the ankle in the movements of jumping and swimming. Tibialis 

 posterior runs from the tibia 

 to the tarsus. Within the foot 

 there is an elaborate system of n — i~~P m - 



small muscles for bending and 

 stretching the toes and abduct- 

 ing them away from each other, 

 so as to expand the web for 

 swimming. 



The whole system is de- 

 signed to produce the charac- 

 teristic sudden simultaneous 

 extension movement of all the 

 joints of both hind limbs, by 

 which the frog moves both in 

 water and on land. The hind 

 limbs can also be used for 

 alternate walking movements, 

 especially in toads (Fig. 177). 



13. 



V 



ttPi 



pa 



t 



hm 



Fig. 193. Diagram of skull bones and other 

 structures, a, an osteolepid ; B, astegocephalian. 



Letters for this and Fig. 194: ac. auditory capsule; 

 ex. extrascapular; jr. frontal; hm. hyomandibula ; 

 it. intertemporal; j. jugal; /. lachrimal; na. nasal; 

 ?nx. maxilla; p. pineal; pa. parietal; pi. pituitary; 

 pm. pre-maxilla; po. post-orbital; pof. post-frontal; 

 ppa. post-parietal; prf. pre-frontal; p.rost. post- 

 rostrals; qj. quadratojugal; sq. squamosal; st. stapes; 

 sut. supratemporal; t. tabular. 

 (After Westoll.) 



The skull of Stenoce- 

 phalia 



The skull of the Devonian 

 and Carboniferous amphibia 



was essentially like that of the osteolepid fishes in the arrangement of 

 the bones, but the proportions had been altered so that the pre-optic 

 region was relatively large and the more posterior 'table' of the skull 

 short (Fig. 193). 



The nasals and frontals, which were small in crossopterygians, 

 were quite long in stegocephalians, whereas the parietals were shorter 

 and the post-parietals absent altogether in the later forms. The differ- 

 ence is so marked that for a long time people were deceived in identifi- 

 cation of the bones and it was said that the pineal opening lay between 

 the frontal bones in fishes but between the parietal bones in tetrapods. 

 The bones identified as 'frontal' in the fish types were, of course, 



