and the flexor longus hallucis 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 (tig. I 1). which helps to extend Embryology. 4

The development of the muscular system is partl 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 EMBRYOLOGY). but never, as far as we know, from the mesodermic somites.- The heart muscle is also developed from mesenchymal cells, though the “Xi changes producing itslfeebly striped fibres are more complicated. 3, ,., . ' j The skeletal or real striped muscles are derived either from the meso-§ § , , 3 ~ ~ ' ', dermic somites or from the- branchlal arches. As theimesoderrnic ZZ somites are placed on each side of the neural canal in the early il, =, 'is ii embr o, it is obvious that the eater art of the trunk musculature

. s (Y, , Y sr P .

W- . ¢' f»! f s reads raduall round the body from the dorsal to the ventral 1 ~ ., t » . P g . Y .

• " ~.-', 'Q/"1 side and consists of a series of plates called myotomes (fig. 12). The

I" f /V muscle fibres in these 'plates run in the long axis of the embryo, and "1§ ' / / are at first se arated rom those of the two ner hbouring plates by ri - /f P, 3-

¢- , thin fibrous intervals called myocommota. In some, cases these

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I (fl ' W gil 'iii From A. M. Paterson, Cunningham's T ext Book of Analomy. V ll ' 'y xi , FIG. 12.-Scheme to Illustrate the Disposition of the Myotomes " ' i 7- in the Embr o in Relation to the Head, Trunk and Limbs.

 Y

vpj:, i , la A, B, C, First three cephalic myotomes. - v ', , I, 2, 3, ,' as persis ing ce ia ic myo omes. Vai J, ' gg N 4 L t tv pl l t

ff ' ' ~ C, T, L, S, Co., The m otomes of the cervical, thoracic, lumbar, s, . Y

From A. M. Paterson, Cunningham's Text Book of Anatomy. FIG. II.*MUSCl€S of the Front of the Right Leg and Dorsum of the Foot.

the four inner toes, while in the sole are four layers of short muscles, the most superficial of which consists of the abductor hallucis, the flexor brevis digitorum, and the abductor minimi digiti, the names of which indicate their attachments. The second layer is formed by muscles which are attached to the flexor longus digitorum tendon; they are the accessories, running forward to the tendon frornlthe 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 extensor longus digitorum on the dorsum. The third layer comprises the flexor brevis hallucfis, adducfor obliquus and adductor trans versus hallucis and the jiexor brevis minimi digfti. The fourth layer contains the three plantar and four do/sal interosseous muscles, rising from the metatarsal bones and inserted into the proximal phalanges and extensor tendons in such away 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, fiexor brevis hallucis, abductor hallucis and the innermost lumbrical are supplied by the internal plantar nerve, while all the rest are supplied by the external plantar. sacral and caudal regions.

I., II., III., IV., V., VI., VII., VIII., IX., X., XI., XII., Refer to the cranial nerves and the structures with which they may be embryo logically associated.

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 rnusculature 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 longitudinal 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, therefore, would show the cut ends of three longitudinal strips of muscle: (1) a mesial ventral, from which the rectus, pyrarnidalis sternoghyoid, omohyoid and sterno-thyroid muscles are derived; (2) a lateral ventral, forming the Hat muscles of the abdomen, intercostals and part of the sternomastoid and trapeziums; and (3) the dorsal portion already noticed. The mesial ventral part is remarkable for the persistence of remnants of myocommata in it, forming the lineae transverse of the rectus and the central tendon of the omo-hyoid. The lateral

part in the abdominal region splits tangentially into three layers