THE SEGMENTATION OF THE BODY. 157 



marked in the thoracic region ; the ribs and their cartilages 

 are developed in them. In the neck the septa are almost lost ; the 

 intermediate tendon of the omo-hyoid and the septa occasionally 

 found in the sterno-hyoid and -thyroid, complexus and trachelo- 

 mastoid muscles are the only representatives of them in the cervical 

 region. 



IT. The Muscles of the 11th Dorsal Septum. — All the muscles of 

 this segment are developed from the muscle plate of the primitive 

 segment (see Figs. 125 and 126). There is a cavity, which pro- 

 bably arises as a diverticulum of the coelom, in each primitive 

 segment (Fig. 6 9, p. 90). The cells of the mesoblast on the inner 

 side of the segmental cavity become columnar and form the 

 muscle plate (Fig. 126). Each segment has its own muscle plate. 

 The cells of each plate increase rapidly in number ; they spread 

 into the somatopleure, and form the muscles of the body-wall 

 and limbs. Each cell becomes elongated and directed across its 

 segment from septum to septum. The intercostal muscles retain 

 this arrangement, but in the abdominal region the fibres fuse 

 with those of neighbouring segments to form muscular sheets — 

 the external oblique, internal oblique, transversalis and rectus. 

 In fishes the embryonic segmental arrangement of the muscu- 

 lature persists. The manner in which the final groups of 

 muscles are derived from the muscle plates is not known, but 

 in the typical segment with which we are at present dealing it 

 will be seen that the musculature falls into two groups (see 

 Fig. 125): (1) axial (acting on the spine — the erector spinae, 

 etc.), and (2) ventro-lateral or body-wall muscles (intercostals, 

 rectus, oblique muscles, etc.). The musculature of the limbs is 

 derived from the ventro-lateral group. 



Many of the ventro-lateral muscles (trapezius, rhomboids, 

 and latissimus dorsi), migrate dorsalwards over the axial muscles 

 and take origin from the spines of the vertebrae (Fig. 125). 



Each muscle fibre is a cell derived from the endothelial cells 

 which make up the muscle plate. The protoplasm of each cell 

 is converted into a living contractile substance (myosin), which 

 reacts to nerve stimuli. 



III. The Arteries of the 11th Segment (Fig. 127).— The 11th 

 intercostal is the artery of the segment. It gives off a dorsal branch 

 to supply the axial muscles, the spinal column, spinal cord and 



