CHAPTER XII. 



THE SEGMENTATION OF THE BODY. 



Segmentation of the Body. — The human body or trunk 

 consists of 33 or 34 segments. Each segment is fundamentally 

 of the same type, but the resemblance is obscured owing to ex- 

 tensive modifications which they undergo to form the cervical, 

 dorsal (thoracic), lumbar (abdominal), sacral (pelvic) and caudal 

 regions of the body. The outgrowth of the limbs also renders it 

 difficult to recognise in the adult the simple system of segments 

 which can be seen in the embryo at the end of the third week 

 (Fig. 233, p. 289). 



Until lately the segmentation of the human body was a matter 

 of only speculative importance, but recent advances in our 

 knowledge of the distribution of nerves, has shown that it has a 

 direct bearing on diagnosis and treatment. 



Constitution of a Typical Segment (11th Dorsal). — It is 

 better to study the development of one typical body segment and 

 from that the student will be able to note for himself the 

 modifications which have taken place in the more highly differ- 

 entiated segments of the body. By the end of the third week,, 

 the process of segmentation, which began in the occipital 

 region a few days previously, has spread backwards and 

 separated the 18th body segment (11th dorsal) from the one in 

 front and behind. As already explained, the process of segmen- 

 tation affects only the paraxial block of mesoblast which lies on 

 each side of the neural canal and notochord. In Figs. 125 and 

 126 a segment is represented in the adult and in the embryonic 

 condition. 



The following elements make up the 11th dorsal segment: (1) 

 Its skeletal basis; (2) Muscular element; (3) Eenal element; 



