298 



TEXT-BOOK OF EMBRYOLOGY. 



largely disappears. The muscle mass then becomes divided longitudinally 

 into two parts, (i) a dorsal and (2) a ventro-lateral (Figs. 262, 263 and 264). 



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. 263). 



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



External oblique 



External inteicostal 



Internal intercostal I Ventro-lateral 



Internal oblique | musculature 



Transversalis 



Rectus 



FIG. 265. Diagrammatic cross section through the 6th-yth thoracic segments of a human embryo 

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



and (b) a ventral part, although the line of division is not so distinct as 

 between the original (i) dorsal and (2) ventro-lateral parts (Fig. 265). 



(a) 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. 266 

 and 267). In the thoracic region it gives rise to the intercostales 

 and to the transversus thoracis (Figs. 265 and 268) ; in the abdominal 

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

 transversus abdominis (Figs. 267 and 268). 



