282 ELEMENTARY ANATOMY. [LESS. 



such bones acute are termed flexors. Those, on the contrary, 

 which tend to open out such an angle are termed extensors. 



Some muscles attached to a long bone which is relatively 

 fixed at one end, tend to make it describe the superficies of a 

 cone, or a movement of circumduction. Such muscles are 

 termed rotators. 



Some muscles move a bone away from a given axis, and 

 are therefore termed abductors. Others tend to bring it 

 towards such an axis, and such are called adductors. The 

 epithets "protractors" "retractors" "elevators" and " de- 

 pressors" (terms which require no explanation), are also some- 

 times made use of. 



There cannot, however, be any rational classification of 

 muscles according to the functions they execute, because 

 such functions often vary in different animals with regard to 

 the very same muscle. 



4. A sound CLASSIFICATION of muscles must be morpho- 

 logical, and may be made to follow that classification of parts 

 which has been already given with respect to the skeleton, or 

 may be constructed independently. An independent general 

 consideration of the muscular system, however (whether ac- 

 cording to its simplest or most complex condition), will come 

 most fitly at the end of this lesson. 



Arranging the muscles (in the first place) according to the 

 skeleton, we have 



(a) Muscles of the exo-skeleton, and 



(&} Muscles of the endo-skeleton. 



To this it will be convenient to add a third category, 

 namely 



(c] Muscles of the viscera. 



The exo-skeletal system of muscles may consist of smooth 

 or of striped fibres. 1 



Such are the small muscles which go from the deep layer 

 of the skin to the hair-sacs, and by their contraction make 

 the hair " stand on end." These muscles become large and 

 important in Birds, and are in them striped instead of un- 

 striped as in Man. 



Other muscles, to be hereafter noted, belong to this cate- 

 gory, such as the Platysma Myoides, and certain muscles 

 of the face of man. 



The endo-skeletal system is naturally divisible, like the 

 skeleton itself, into an axial and appendicular portion. These 



1 For an account of this difference of structure in muscular fibres, see Lesson 

 XII. of " Elementary Physiology/' 15. 



