284 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



tissue remains unchanged and undifferentiated. In the ventral 

 band between the left somatopleure and the right are formed 

 the sternum and the linea alba (Fig. 227). In lower vertebrates, 

 in fishes, and to a less marked extent in amphibians and reptiles, 

 the somatopleure becomes segmented from end to end of the trunk, 

 each muscle segment being distinctly recognizable in the adult. 



ventral median line 

 axial somatopleure / 



~~ F ■'.:[■ :. - ■■". ■"-'■.•"' ~- u ;-..;■. ;.-. ~ ~mter segment 



if ^-IllJIllllIi inter-segment 

 VrA;; c /y-: ; C-. : '.'~y (basis of rib.) 



jJL myotome 

 '-•*" (inter-cost, muse) 



vertebra- 



vertebra — £355^ 



T 



J:^ 



ventral median line,(basis of 



axial myotome somato-pl. myotome sternum) 



Fici. 229.— Scheme of the Manner in which the Somatopleure is segmented. 



Formation of Ribs. — Ribs, like all true skeletal bones, pass 

 through three stages : 1. They are represented by a membranous 

 basis in the fibrous tissue (septa) between the muscular seg- 

 ments of the somatopleure (Fig. 229). 2. The fibrous basis 

 of the rib becomes cartilaginous. 3. Ossification of the cartilage 

 begins in the 7th week, but the process of ossification leaves the 

 ventral parts of the costal segments untouched ; they form the 

 costal cartilages ; in lower forms they may become ossified and 

 form sternal ribs. In lower vertebrates, such as reptiles, each rib 

 articulates with the neural arch of a vertebra by two heads, 

 dorsal and ventral (Fig. 123, p. 153). In man and mammals 

 only the first and the lowest two ribs retain the vertebral 

 attachment. The heads of the upper ribs (2nd to 10th) 

 migrate forwards and gain attachments to the disc and vertebra 

 in front of their own segments. The dorsal head of the rib in 

 man is represented by the tuberosity (see also p. 152). 



The Sternum. — In man and anthropoids the sternum has 

 become flat and highly modified with the alterations in the shape 

 of the thorax (Fig. 208, p. 254). With the adaptation to the upright 

 posture the thorax becomes flattened from back to front ; its 

 transverse diameter is as great, or greater, than the antero- 



