274 THE MUSCLES OF THE LOWER LIMB. 



peroneus longus and Irevls are extensors. The tibialis posticus produces the movement of 

 inversion of the foot, in which action it is aided by the flexors of the toes. The three peronei 

 and the extensors of the toes draw the foot upwards and outwards, and thus produce the 

 movement of eversion. The peroneus longus, in crossing- the foot inferiorly, strengthens the 

 transverse arch formed 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 bent knees ; it also comes into play in walking, lifting the outer border 

 of the foot, and throwing the weight onto the first metatarsal bone as the hinder foot is- 

 leaving the ground (p. 198). 



The flexor and extensor muscles of the toes, including the lumlricales and interosseous 

 muscles, act like the corresponding muscles in the hand. The direction of the flexor longua 

 digitorum is, however, modified by the flexor accessor ius, so as to bring its line of action into- 

 the direction of the middle of the foot and of the short flexor. The tibialis posticus and the 

 flexors of the toes give important assistance to the ligaments and the plantar fascia in sup- 

 porting the longitudinal arch of the foot. 



MORPHOLOGY OP THE LIMB-MUSCLES. 



It has already been mentioned (p. 201) 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 limb there are no muscles of hypaxial origin, but in the lower limb the 

 psoas may be regarded as a prolongation outwards from the hypaxial set. As each limb 

 begins bud-like to project from the side of the embryo, it may be supposed to receive an 

 enveloping sheath of muscle from the side of the body. This sheath appears to be derived 

 solely from the external of the three layers into which the trunk-portion of the ventro-lateral 

 muscle divides (see later the morphology of the trunk-muscles). Forming at first a simple 

 infundibulum, it gradually extends in length with the growth of the limb, and becomes 

 segmented pari passu 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 more distal parts. In the upper 

 limb, from the greater freedom of movement of the girdle, and from its more superficial 

 position, the proximal portion of the muscular sheath is longer, and its constituent parts are 

 more easily recognized than in the case of the corresponding parts of the lower limb, which 

 are not differentiated to the same extent from the trunk-muscles. The muscles passing from 

 the trunk to the shoulder-girdle and the humerus fall into two primary divisions. Anteriorly 

 are the pectorals and the subclavius, supplied by the ventral branches of the limb-nerves. 

 Posteriorly are the more numerous muscles supplied by the dorsal divisions of the limb- nerves, 

 and separated again into two masses, a superior and an inferior. The superior mass includes 

 a superficial sheet consisting of the cleido-mastoid and trapezius (in part at least), the latter 

 of which is continued to the humerus by the deltoid, and a deeper sheet constituted by the 

 rhomboidei and the serratus magnus with its cervical prolongation the levator anguli scapulse. 

 The inferior mass is composed of the latissimus dorsi and a deeper part, the teres major, 

 which has become segmented off and acquired an attachment to the scapula : the occasional 

 origin of the latissimus dorsi from the lower angle of the scapula is an indication of the 

 primitive connection of these muscles. As the muscles of this series have been formed by a 

 process of differentiation occurring in a continuous sheath, so they are often found to some 

 extent united together, either as varieties in man, or as regular structures in some of the 

 lower animals. Thus in Lepidosiren the pectoralis major and latissimus dorsi are blended 

 together at their margins, so as to form one continuous muscle ; and in man the frequently 

 recurring axillary arches or muscular slips which pass from one muscle to the other across 

 the axillary space may point to an imperfectly segmented condition. Again, the pectoralis 

 major is always united iin man with the superficial fibres of the external oblique, but in 

 Cryptobranchus the two are continuous in their entire thickness. The cleido-mastoid and the 

 clavicular part of the trapezius are sometimes connected, and in those animals in which the 

 clavicle is deficient they are continued into the representatives of the clavicular portions of 

 the pectoralis major and deltoid to form a cephalo-humeral muscle. 



In the case of the lower limb, the superficial layer of the anterior muscles, owing to the 

 absence of the ventral extensions of the trunk-skeleton and the greater development of the 

 limb-girdle, arises entirely from the latter, and thus the pectoralis major is probably repre- 

 sented by the superficial adductors, longus and brevis, while the deep layer is suppressed in 

 association with the loss of mobility in the hip-girdle. The gracilis also belongs to this 

 group, corresponding to the occasional chondro-epitrochlearis prolonged from the lower fibres 

 of the pectoralis major. The superior mass of the posterior division is not differentiated from 

 the primitive sheet, so that the trapezius, rhomboidei and serratus magnus are represented 

 only by a part of the external oblique ; and the inferior mass forms the gluteus maximus, 



