GENERAL CONSIDERATION OF THE VOLUNTARY MUSCLES. 469 



and the lumbricales, — while others may pass between portions of the skeleton and 

 special organs upon which they act, as is exemplified by the muscles of the eyeball. 

 Whatever may be the nature of the structure to which the attachment is made, 

 it is convenient for purposes of description to regard one of the points of attachment 

 of each muscle as the fixed point from which it acts in contraction, and to speak of 

 this as its origin, and to regard the other as the point upon which it acts, speaking 

 of it as the iyiscrtion. It must be understood, however,- that this distinction between 

 the two attachments is somewhat arbitrary, since what is usually the fixed point may 

 under certain circumstances become the movable one. For instance, in the case of 

 a muscle passing from the pelvis to a leg bone, if the body be erect, the contraction 

 of the muscle will cause an inclination of the trunk on the hip-joint, the attachment 

 to the leg bone being then the fixed point and that to the pelvis the movable one. 

 In other positions of the body, however, the contraction of the muscle will produce 

 a movement of the leg, the fixed and movable points being exactly reversed. Since, 

 however, the movement of the leg may be regarded as the more usual result of the 

 contraction of the muscle, the pelvic attachment is arbitrarily regarded as the origin 

 and the attachment to the femur or tibia the insertion of the muscle in question. 



Fig. 491. 



Tendinous 

 septa of 

 origin 



Lateral portioi 



Tendinous 



Central portion 



^1 



septa of insertion 



Tendon of insertion 



Diagrams showing semi-pinnate {A) and pinnate {B) arrangement of muscle-fibres, which pass from tendon ol 

 origin above to that of insertion below. C, compound pinnate arrangement, as in central division of deltoid muscle. 

 (After Poirier.) 



Form. — The muscles assume various forms, dependent to some extent upon 

 the structures to which they are attached. Some are thin sheets with almost parallel 

 fibres, others are more or less band-like, while others may have considerable thick- 

 ness, and be quadrate, triangular, or spindle-shaped. Surrounding certain of the 

 orifices of the body are what are termed orbicular or sphincter muscles (Figs. 495, 

 499), consisting of a muscular sheet whose fibres have a crescentic course around 

 either side of the orifice, the lips of which will tend to be drawn together by the con- 

 traction of the muscle. 



Where the surfaces for attachment are considerable, the fibres composing a 

 muscle have a more or less parallel course ; but where a comparatively small area is 

 all that is available for the attachment of a strong muscle, as is the case with many 

 of the limb muscles, it is clear that such an arrangement cannot obtain. The muscle- 

 fibres then converge from either one or both sides to be inserted one above the other 

 into the tendon, forming what is termed a semipinnatc (e. g. , many of the muscles 

 of the leg, Fig. 609;, ox pinnate muscle (e. g. , interossei dorsales. Fig. 590.} This 

 convergence may take place towards either one or both tendons of attachment, and 

 occasionally these may spread out over opposite surfaces of the muscle to form apo- 

 neurotic sheets w^hich overlap, so that the muscle-fibres pass obliquely from the "Sur- 

 face of one tendon to that of the other {e.g. , gastrocnemius, semitendinosus, Fig. 

 635). Finally, in some of the broader muscles {e.g., deltoid and subscapularis) the 

 muscle-fibres may arise from and converge to a series of tendinous bands which 



