The external group comprises the peroneus longus and brevis, rising from the outer surface of the fibula and inserted into the tarsus (Pl. 11.-3), the longus tendon passing across the sole to the base of the first metatarsal bone, the brevis to the base of the fifth metatarsal. These are supplied by the musculo-cutaneous nerve.
The posterior group is divided into a superficial and a deep set. The superficial is composed of the gastrocnemius, the two heads of which rise from the two condyles of the femur, the soleus, which rises from the upper parts of the back of the tibia and fibula, the plantaris, which comes from just above the external condyle of the femur, and the pop/iteus which, although on a deeper plane, really belongs to this group and rises by a tendon from the outer con dyle while its fleshy part is inserted into the upper part of the back of the tibia. The gastrocnemius and soleus unite to form the tendo Achillis, which is attached to the posterior part of the cal caneum, while the plantaris runs separately as a very thin tendon to the same place. These muscles are supplied by the internal popliteal nerve. The deep set is formed by three muscles which rise from the posterior surfaces of the tibia and fibula, the flexor longus digitorum, the tibialis posticus, and the flexor longus hal lucis from within outward. Their tendons all pass into the sole, that of the flexor longus digitorum being inserted into the terminal phalanges of the four outer toes, the flexor longus hallucis into the terminal phalanx of the big toe, while the tibialis posticus sends expansions to most of the tarsal bones. The nerve supply of this group is the posterior tibial. On the dorsum of the foot is the extensor brevis digitorum, which helps to extend the four inner toes, while in the sole are four layers of short muscles, the most superficial of which consists of the adductor hallucis, the flexor brevis digitorum, and the adductor minimi digiti, the names of which indicate their attachments. The second layer is formed by muscles which are attached to the flexor longus digitorum ten don ; they are the accessorius, running forward to the tendon from the lower surface of the calcaneum, and the four lumbricales, which rise from the tendon after it has split for the four toes and pass between the toes to be inserted into the tendons of the exten sor longus digitorum on the dorsum. The third layer comprises the flexor brevis hallucis, adductor obliques and adductor transversus hallucis and the flexor brevis minimi digiti. The fourth layer con tains the three plantar and four dorsal interosseous muscles, rising from the metatarsal bones and inserted into the proximal pha langes and extensor tendons in such a way that the plantar muscles draw the toes towards the line of the second toe while the dorsal draw them away from that line. Of these sole muscles the flexor brevis digitorum, flexor brevis hallucis, adductor hallucis and the innermost lumbrical are supplied by the internal plantar nerve, while all the rest are supplied by the external plantar.
The development of the muscular system is partly known from the results of direct observation, and partly inferred from the study of the part of the nervous system whence the innervation is derived. The unstriped muscle is formed from the mesenchyme cells of the somatic and splanchnic layers of the mesoderm (see VERTEBRATE EMBRYOLOGY), but never, as far as we know, from the mesodermal somites. The heart muscle is also developed from mesenchymal cells, though the changes producing its feebly striped fibres are more complicated. The skeletal or real striped muscles are derived either from the mesodermic somites or from the branchial arches. As the mesodermal somites are placed on each side of the neural canal in the early embryo, it is obvious that the greater part of the trunk musculature spreads gradually round the body from the dorsal to the ventral side and consists of a series of plates called myotomes (fig. 3). The muscle fibres in
these plates run in the long axis of the embryo, and are at first separated from those of the two neighbouring plates by thin fibrous intervals called myocommata. In some cases these myocommata persist and even become ossified, as in the ribs, but more usually they disappear early, and the myotomes then unite with one another to form a great muscular sheet. In the whole length of the trunk a longitudinal cleavage at right angles to the surface occurs, splitting the musculature into a dorsal and ventral part, supplied respectively by the dorsal and ventral primary divisions of the spinal nerves. From the dorsal part the various muscles of the erector spinae series are derived by further longi tudinal cleavages either tangential or at right angles to the surface, while the ventral part is again longitudinally split into mesial and lateral portions. A transverse section of the trunk at this stage, theref ore, would show the cut ends of three longitudinal strips of muscle : (I) a mesial ventral, from which the rectus, pyramidalis sterno-hyoid, omo-hyoid and sterno-thyroid muscles are derived; (2) a lateral ventral, forming the flat muscles of the abdomen, intercostals and part of the sternomastoid and trapezius; and (3) the dorsal portion already noticed. The mesial ventral part is remarkable for the persistence of remnants of myocommata in it, forming the linae transversae of the rectus and the central ten don of the omo-hyoid. The lateral part in the abdominal region splits tangentially into three layers, the external and internal oblique and the transversalis, the fibres of which become differ ently directed. In the thoracic region the intercostals probably indicate a further tangential splitting of the middle or internal oblique layer, because the external oblique is continued headward superficially to the ribs and the transversalis deeply to them. The more cephalic part of the external oblique layer probably disap pears by a process of pressure or crowding out owing to the en croachment of the serratus magnus, a muscle which, as its nerve supply indicates, is derived from the lower cervical myotomes. The deeper parts of the lateral mass of muscles spread to the ven tral surface of the bodies of the vertebrae, and form the hypaxial muscles—such as the psoas, longus colli and recti capitis antici. The nerve supply indicates that the lowest myotomes taking part in the formation of the abdominal walls are those supplied by the first and second lumbar nerves, and are represented by the cremaster muscle in the scrotum. In the perineum, however, the third and fourth sacral myotomes are represented, and these muscles are differentiated largely from the primitive sphincter which surrounds the cloacal orifice, though partly from vestigial tail muscles. In the head no distinct myotomes have been demon strated in the mammalian embryo, but as they are present in more lowly vertebrates, it is probable that their development has been slurred over, a process often found in the embryology of the higher forms. Probably nine cephalic myotomes originally existed, of which the first gives rise to the eye muscle supplied by the third nerve, the second to the superior oblique muscle sup plied by the fourth nerve, and the third to the external rectus supplied by the sixth nerve. The fourth, fifth and sixth myotomes are suppressed, but the seventh, eighth and ninth possibly form the muscles of the tongue supplied by the twelfth cranial nerve.