2c MORPHOLOGY OF THE LIMB MUSCLES. 



ann and the palm of the hand, the foot being in a position of over-extension, it- 

 follows that the muscles on the anterior aspect of the leg- are really extensors, 

 and that their action in moving the foot so as to approximate the toes to tlio 

 front of the leg merely increases the over-extension ; while, on the other hand, 

 the muscles on the posterior aspect of the leg are really flexors, and their action 

 in straightening the ankle-joint is to reduce merely this over-extension by an 

 opposite movement of flexion. Nevertheless, it has been customary to apply the 

 name of flexion to the action of raising the foot on the front of the leg, and that 

 of extension to its depression. In the following description the usual terms are 

 employed. 



The (jti.'<frocncmini^ muscle acts both as a flexor of the knee and an extensor of 

 the ankle-joint. "\Mien the anterior muscles of the leg fix the ankle-joint, it i» 

 fitted to act as a flexor of the knee ; when the knee is fixed either by being 

 placed in complete extension or by the sustained action of the extensor muscles, 

 the gastrocnemius acts entu-ely on the foot, and combines with the sulcus in 

 lifting the heel from the ground, and in raising the body on the toes. 



The tibialis autlciis and 2't'''0Ji('u.f tcrfiiia are flexors of the foot ; the tihialiK 

 pnsticiiK and jx-roiinig limr/us and hrrri.s are extensors. The two tibial muscles acting- 

 together, raise the inner border of the foot, and draw' it inwards. The three 

 peronei muscles raise the outer border of the foot and draw it outwards. The 

 peroneus longus, in crossing the foot inferiorly, strengthens the transverse arch 

 ionned by the metatarsal bones and anterior range of tarsal bones ; especially 

 when that arch is pressed upon by the weight of the body falling on the balls of 

 the toes, as in stooping with bended knees. 



The fi-ror and i-.i-tmrnr musch:s of the toes, including the himhrirales and 

 inf/ro.f.iri. act like the coiTesponding muscles in the hand. The direction of the 

 flexor longus digitorum is, howe^'cr. modified by the Jfr.ror /trcr.s-.soriiis, so as to 

 bring its line of action into the direction of the middle of the foot and of the 

 short flexor. The flexor accessorius and tendon of the long flexor muscles, 

 likewise assist the short flexor and the plantar aponeurosis in maintaining the arch 

 of the foot. 



MORPHOLOGY OF THE LIMB-MUSCLES. 



It has already been mentioned (p. 18')) that the muscles of the limbs and limb 

 girdles are primarily derived mainly from the ventro-lateral division of the 

 epaxial set of muscles. In the case of the upper limbs there are no muscles of 

 hypaxiai origin, Ijut in the lower limb there are at least two muscles which may 

 be regarded as prolongations outwards from the hjisaxial set, one passing to its 

 inner and anterior (preaxial) aspect (psoas), the other to its posterior and outer 

 (postaxial) aspect (pyiiformis). These two muscles are consequently without 

 homologues in the upper limbs. As each limb begins bud-like to project from, 

 the side of the emliryo, it may be supposed to receive an enveloping sheath of 

 muscle from the side of the body (Humphry). This sheath is at first a simple 

 infundibulum, liut it gi-adually extends in length with the growth of the limli, 

 and becomes segmented, 7;/7?v' ^w.y.y?/, with the segmentation of the hard parts. 

 The deeper fibres of the sheath are restricted to passing from one segment to the 

 next, while the superficial fibres are frequently found to pass over one segment 

 altogether, to the more distal parts. Humphry holds that of the three layers 

 into which the trunk portion of the ventro-lateral muscle divides, two layers 

 may be traced into its ap^iendicular segments, corresponding respectively to the 

 external oblique and superficial portion of the internal oblique layers. In the 

 upper limb, from the greater freedom of movement of the girdle, and from its 

 more superficial jiosition, the first segment of the muscidar sheath is longer, and 

 its constituent parts more easily recognised, than in the case of the trunk-girdle 

 muscles of the lower limb, which are not differentiated to the same extent from 

 the trunk miiscles. Traced to the shoulder-girdle, the superficial laj'er of tmnk- 

 linib muscles is found to divide into three principal masses. Anteriorly are the 

 pectorals ; superiorly and posteriorly are the cleido-mastoid and trapezius, the 

 latter being continued to the bone of the foreann as the deltoid : and inferiorly 

 and posteriorly the latissimus dorsi. Beneath the trapezius are the rhomboids. 



