320 THE MUSCULATURE 



musculature of the limbs is differentiated. In the mammals this appears not to be the case, and 

 the muscles are differentiated from the non-segmental tissue of the limb-buds. 



Where mammalian musculature is primitively segmental, each segment becomes associated 

 with a corresponding spinal nerve or, in the head, with a nerve which corresponds in series with a 

 spinal nerve. Even when subsequent differentiation brings about marked alterations in the 

 axial musculature, the nerves maintain to a considerable degree a segmental distribution. 



Into each of the hmbs, where the intrinsic musculature is at no time segmental, there extends 

 during embrj-onic development a series of segmental spinal nerves, so that in them, as in the 

 region of the body axis, a certain segmentation in the nerve-supply can be made out in the adult. 

 That part of the limb nearest the head in early embryonic development has its muscles supplied 

 by the most cranial, that part nearest the caudal extremity of the body by the most caudal, 

 of the nerves which supply the limb musculature. There is here, however, considerable over- 

 lapping of the segmental areas. 



Variation. — In man some variation in the arrangement of the muscles is met 

 with in every individual, and often marked deviations from the normal conditions 

 are found. The muscles vary in their mode of origin or insertion, and in the ex- 

 tent to which muscles of a given group are fused with one another or to which the 

 chief parts of a complex muscle are isolated from one another. Some muscles, 

 like the palmaris longus and the plantaris, are frequently entirely absent, and 

 other muscles generally absent are frequently present. 



In addition to these frequent variations there are others so rare that many authors prefer 

 to speak of them as anomalies rather than variations. Sometimes muscles may be found 

 doubled by longitudinal division, or two or more muscles normally present may be fused into 

 a single indivisible muscle. Occasionally there occur muscles constantly present in some of 

 the lower animals, but normally not met with in the human body (anomalies of reversion or of 

 convergence). In such instances the muscle may be normally represented by a tendon or fascia. 

 At times the anomalies are supposed to be not a reversion to an ancestral condition, but a dis- 

 tinct step in advance. This, however, is difficult to prove. At other times no phylogenetic 

 relation is apparent, and the anomaly is looked upon as a monstrous sport or as the result of 

 some pathological condition. 



The nerve-supply of the muscles is of value in the study of muscle variations. 

 There is, however, not infrequent variation in the nerves with relation to the 

 supply of the muscles. 



Physiology. — From the standpoint of morphology the muscles are grouped 

 according to their intimate relations to one another and to the peripheral nerves, 

 relations, as noted above, that are made more clear by a study of comparative 

 anatomy and embryology. From the physiological aspect a different grouping of 

 the muscles is required, because muscles belonging morphologically in one group 

 maj^ have different physiological functions in the animal body. The chief features 

 of the mechanical action of muscles may be briefly considered here. 



Most muscles act on the bones as levers. In physics three types of levers 

 are recognised In levers of the first type (fig. 339, I) the fulcrum (F) lies be- 

 tween the place where power (P) is exerted on the lever and the point of resist- 

 ance or load (L) . Levers of this kind are frequently met with in the body. 

 A good example is seen in the attachment of the skull to the vertebral column. 

 The fulcrum lies at the region of attachment; the weight of the skull tends to 

 bend the head forward, while the force exerted by the dorsal muscles of the 

 neck serve to keep the head upright or to bend it back. 



In levers of the second class (fig. 339 II) the point on which power is exerted 

 moves through a greater distance than the point of resistance. Speed of move- 

 ment is thus sacrificed to power. Levers of this type are exceedingly rare in 

 the animal body. An example in the human body is the foot when the body is 

 raised on the toes. 



In levers of the third class (fig. 339, III) the point on which force is exerted 

 moves a less distance than the point of resistance. Power is thus sacrificed to 

 speed. This is the common form of leverage found in the body. A good ex- 

 ample is found in the action of the muscles which flex the forearm on the arm. 

 The region in which the biceps and brachialis are attached is but a short dis- 

 tance from the elbow-joint or fulcrum, while the hand may be looked upon as 

 the region of resistance to the force exerted. A movement of the point P 

 through a short distance will cause L to move through a great distance. 



The more the angle between a mu.scle or its tendon and the bone on which it acts approaches 

 a right angle, the greater is the power of movement exerted by the muscle. The arm in fig. 339, 

 III, i.s in the position of greatest advantage for the action of the biceps on the forearm. AH 

 boys know that it is easier to 'chin' oneself after the arm is partly bent than when hanging 



