THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 



267 



largely disappears. The muscle mass then becomes divided longitudinally 

 into two parts, (i) a dorsal and (2) a ventro-lateral (Figs. 224, 225 and 226). 



1. The dorsal part is destined to give rise to those dorsal muscles of the 

 trunk that are not associated with the extremities, and is innervated by the 

 dorsal rami of the spinal nerves (Fig. 225). 



2. The ventro-lateral part again divides longitudinally into (a) a lateral 



" External oblique 

 - * External intercostal 



}&'rS-' ' >^ '.*"-... v Internal intercostal 

 '*... "* Internal oblique 



. .-, , ** Transversalis 



ft '. , / "" Rectus 



Ventro-lateral 

 musculature 



FIG. 227. Diagrammatic cross section through the 6th~7th thoracic segments of a human embryo 

 of 17 rnm. (5^ weeks). Bardeen and Lewis. 



and (b) 

 between 

 (a) 





a ventral part, although the line of division is not so distinct as 

 the original (i) dorsal and (2) ventro-lateral parts (Fig. 227). 

 The lateral part subdivides tangentially and gives rise in the cervical 

 region to the longus capitis, longus colli, rectus capitis anterior, to the 

 scaleni, and to parts of the trapezius and sternomastoideus (Figs. 228 

 and 229). In the thoracic region it gives rise to the intercostaks 

 and to the transversus thoracis (Figs. 227 and 230); in the abdominal 

 region to the psoas, quadratus lumborum, and to the obliqui and 

 transversus abdominis (Figs. 229 and 230). 



