THE HUMAN MECHANISM 



as is the intestine; and their blood and nerve supplies run 



to them in similar folds. 



8. The axial skeleton. The bones and cartilages of which 



the skeleton is composed may be classified into an axial 



skeleton (of the head, 

 fKidney neck, and trunk) and 

 an appendicular skele- 

 ton (of the arms and 

 legs). The axial skele- 

 ton comprises (1) the 

 backbone, or vertebral 

 column, (2) the ribs 

 and breastbone, and 

 (3) the skull. 



9. The backbone, 

 or vertebral (spinal) 

 column. This is com- 



FIG. 6. Diagrammatic cross section of the 

 abdominal cavity 



Showing the relation of the kidneys and great 

 blood vessels to the peritoneum. The intestine has 



been removed, the cut border of the mesentery posed of Separate ir- 



regular ringlike bones, 



being shown 



or vertebrae, placed one above another and bound together by 

 bands of strong connective tissue known as ligaments. It is 

 customary to divide the backbone into the following regions : 



Cervical, 7 vertebrae of the neck. 



Thoracic, 12 vertebrae of the chest, to which ribs are attached. 



Lumbar, 5 vertebrae of the " small of the back." 



Sacral, 5 vertebrae (fused together) to which the large hip bones 

 are attached. 



Coccygeal, 4 or 5 very small, simple vertebrae (constituting the skele- 

 ton of a rudimentary tail and corresponding to the tail of lower 

 animals). 



When one looks at the spinal column from behind, the 

 vertebras are seen to be placed one upon another, but all in 

 the median dorsoventral plane of the body (see Fig. 7). Seen 

 from the side, however, several curves come into view, as 

 shown in Fig. 10. On the ventral side, in the cervical and 



