66 ANATOMY OF THE RABBIT 



hinge-joint confines motion to one plane, while in the cases of the 

 shoulder and hip, a joint of the ball-and-socket type allows motion 

 on points at various angles to a plane according to which muscle 

 or group of muscles may be brought into action. 



The recognition of origin and insertion depends on usual but 

 not invariable relations. The exact effect of muscle contraction 

 depends as a rule on the relative positions of the parts and on the 

 synchronous action of other muscles. A muscle like that forming 

 the diaphragm does not possess an insertion after the fashion of 

 ordinary muscles; and in some cases, as in the intrinsic muscle of 

 the tongue or the so-called orbicular or sphincter muscles, both 

 origin and insertion may be absent. 



In the study of the skeletal muscles, moreover, it should be 

 borne in mind that the identification of "origin" and "insertion" 

 is largely a matter of convention. Actually "fixed" and "movable" 

 points depend upon the movement being effected at the moment. 

 An excellent example of the necessity of convention in this respect 

 is afforded by the human arm in which, as opposed to the ordinary 

 use of the muscles, most of the relations would be reversed if the 

 body is considered suspended by the hands, that is in the "bra- 

 chiate" position commonly assumed by arboreal primates. Also 

 the action commonly attributed to any muscle is usually an artificial 

 abstraction, for in life muscles act in groups, not singly, and the 

 precise effect of any given contraction will be modified by the other 

 muscles acting at the same time. Finally, it should be noted that a 

 muscle does not always act as a unit, but sometimes one part may 

 contract independently of the remainder. 



Embryonic Derivation 



Voluntary muscle arises chiefly from the segmented areas or 

 myotomes of the embryo. The extent to which segmentation is 

 shown in the adult, however, depends for the most part on how far 

 the definitive muscle is removed from the vertebral column or seg- 

 mented portions of the skeleton. The vertebral muscles themselves 

 show throughout their attachments to successive vertebrae the 

 marks of segmental origin and the segmental character is obvious 

 in the intercostal muscles and in the division of the rectus abdominis 

 by "tendinous inscriptions." Many others, however, such as those 

 of the abdomen, to a certain extent those of the limbs, and those 



