362 MYOLOGY 



Rhomboideus; (5) from the number of their divisions, as the Biceps and Triceps; 

 (6) from their points of attachment, as the Sternocleidomastoideus, Sternohyoideus, 

 Sternothyreoideus. 



In the description of a muscle, the term origin is meant to imply its more fixed 

 or central attachment; and the term insertion the movable point on which the force 

 of the muscle is applied; but the origin is absolutely fixed in only a small number 

 of muscles, such as those of the face which are attached by one extremity to immov- 

 able bones, and by the other to the movable integument; in the greater number, 

 the muscle can be made to act from either extremity. 



In the dissection of the muscles, attention should be directed to the exact origin, 

 insertion, and actions of each, and to its more important relations with surrounding 

 parts. While accurate knowledge of the points of attachment of the muscles is 

 of great importance in the determination of their actions, it is not to be regarded 

 as conclusive. The action of the muscle deduced from its attachments, or even 

 by pulling on it in the dead subject, is not necessarily its action in the living. By 

 pulling, for example, on the Brachioradialis in the cadaver the hand may be slightly 

 supinated when in the prone position and slightly pronated when in the supine 

 position, but there is no evidence that these actions are performed by the muscle 

 during life. It is impossible for an individual to throw into action any one muscle; 

 in other words, movements, not muscles, are represented in the central nervous 

 system. To carry out a movement a definite combination of muscles is called into 

 play, and the individual has no power either to leave out a muscle from this com- 

 bination or to add one to it. One (or more) muscle of the combination is the chief 

 moving force; when this muscle passes over more than one joint other muscles 

 (synergic muscles) come into play to inhibit the movements not required; a third 

 set of muscles (fixation muscles) fix the limb i. e., in the case of the limb-movements 

 and also prevent disturbances of the equilibrium of the body generally. As an 

 example, the movement of the closing of the fist may be considered: (1) the prime 

 movers are the Flexores digitorum, Flexor pollicis longus, and the small muscles 

 of the thumb; (2) the synergic muscles are the Extensores carpi, which prevent 

 flexion of the wrist; while (3) the fixation muscles are the Biceps and Triceps 

 brachii, which steady the elbow and shoulder. A further point which must be 

 borne in mind in considering the actions of muscles is that in certain positions 

 a movement can be effected by gravity, and in such a case the muscles acting are 

 the antagonists of those which might be supposed to be in action. Thus in flexing 

 the trunk when no resistance is interposed the Sacrospinales contract to regulate 

 the action of gravity, and the Recti abdominis are relaxed. 1 



By a consideration of the action of the muscles, the surgeon is able to explain 

 the causes of displacement in various forms of fracture, and the causes which pro- 

 duce distortion in various deformities, and, consequently, to adopt appropriate 

 treatment in each case. The relations, also, of some of the muscles, especially 

 those in immediate apposition with the larger bloodvessels, and the surface mark- 

 ings they produce, should be remembered, as they form useful guides in the 

 application of ligatures to those vessels. 



MECHANICS OF MUSCLE. 2 



In studying the mechanical action of muscles the individual muscle cannot 

 always be treated as a single unit, since different parts of the same muscle may 

 have entirely different actions, as with the Pectoralis major, the Deltoid, and the 

 Trapezius where the nerve impulses control and stimulate different portions of the 



1 Consult in this connection the Croonian Lectures (1903) on "Muscular Movements and Their Representation in 

 the Central Nervous System." by Charles E. Beevor, M.D. 



2 R. Fick. Bd. ii, in Bardeleben's Handbuch der Anatomie des Menschen. 



