362 ELEMENTARY ANATOMY. [LESS. 



In the arm, the extensor muscles cross each other; in the 

 leg they do not. 



The triceps is the great extensor of the arm, the quad- 

 riceps of the leg. 



The peroneus longiis and flexor accessorius are leg muscles 

 which resemble nothing in the arm of any animal. An 

 interlacing, like that which takes place between the flexor 

 tendons of man's foot, is absent in his hand, but is present 

 in the hand of some animals, e.g. Nycticebus. 



An opponens present in the pollex is wanting in the hallux 

 in man, but the hallux of the Orang is furnished with an 

 opponens. 



30. The muscular system is that in which the plastic power 

 which co-adapts structure and function is pre-eminently con- 

 spicuous, as is well shown in the Frog's foot and the wing- 

 edge muscle of Bats and Birds. Thus homologies become 

 difficult to determine, being disguised by such an abundance 

 of adaptive modifications. 



Though there is a general correspondence between the 

 development of the skeleton and of the muscles which clothe 

 it, yet sometimes skeletal parts may be greatly increased in 

 size, while at the same time there is a simultaneous decrease 

 in the relative development of the muscles annexed ,*as in the 

 hand of Bats and the thorax of Chelonians. 



The endo-skeletal muscles may be divided into (i) axial, 

 and (2) appendicular. 



The axial muscles, like the skeleton (as we saw in' the 

 Sixth Lesson), may be subdivided into three groups : (i) ep- 

 axial, (2), paraxial, and (3), hypaxial. They have a primitive 

 relation to vertebral segments, but this relationship is lost, 

 and the segments coalesce antero posteriorly, in non -gill- 

 bearing Vertebrates. 



The epaxial group includes the inner part of the erector 

 spinae and its continuations, attaining perhaps its maximum 

 of development in the Flat Fishes, e.g. the Sole, and its 

 minimum in Fishes like Ostracion and in the Tortoises. 



The paraxial group includes the outer part of the erector 

 spina? and its continuations, also the scaleni, levatores 

 costarum, intercostals, abdominal muscles, rectus, and outer 

 lower tail muscles of Tailed-Batrachians and at least many 

 Fishes. This group is at its maximum of differentiation in 

 Serpents. 



The hypaxial group includes the recti antici, longus colli, 

 sub-vertebral muscles of Birds, Serpents, and Tailed-Batra- 



