56 



ANATOMY AND PHYSIOLOGY. 



when healthy blood is not sent to it, it loses its contractile power. 

 Each muscle is composed of small bundles of fibres, connected to- 

 gether by fine cellular membrane, running parallel to each other, 

 and enveloped in a common sheath. Every muscle has at least 

 two attachments ; the one which is most fixed is called its origin, 

 the one which is most ordinarily moved, is its insertion. The 

 action of a muscle consists in shortening itself so as to bring its 

 insertion nearer to its origin. In so doing, its fibres assume a zig- 

 zag form, and the whole muscle becomes thicker, as may be easily 

 proved by grasping the arm when straight with the other hand, and 

 then bending it, when the muscles on the inside of the arm will be 

 felt swelling as they shorten themselves. Or, let the fingers be laid 

 on the side of the cheek, and the jaws firmly closed, and then the 

 strong muscles whose action it is to close the jaws, will be felt be- 

 coming hard under the finger. A muscle may have its origin con- 

 verted into its insertion by having the latter fixed. For instance, 

 the strong muscles which bring down the arms in striking, may be 

 reversed in their action ; if you catch a beam above your head, 

 their insertion into the arms will now be fixed, and they will swing 

 up the body, so as to enable you to grasp the beam with the feet. 



Most muscles have a tendon or sinew at one end, sometimes at 

 both, by which they are fixed to their points of attachment in the 

 movable parts of the skeleton. The forms of the muscles are very 

 various, but are principally the following: 1. A single-bellied 

 muscle, small at each end, thick and round in the middle ; the 

 middle swelled part is called its belly. A tendon is represented at 

 one end. Of this kind are most of the muscles of the limbs. (Fig. 



1.) 2. A double-bellied muscle, when there are two bellies meeting 

 and becoming inserted by one tendon. (Fig. 2.) 3. A strap-shaped 



Fig. 2. 



muscle when the fibres run parallel to one another, forming a thin 

 strap, with or without a tendon. (Fig. 3.) 4. A fan-shaped muscle 



Fig. 3. 



broad and thin at its origin, narrow and thick at its tendinous in- 



