I THE BUILD OF THE BODY 9 



thirty-three bones, joined together by a very strong and 

 tough substance into a long column, which lies much 

 nearer the dorsal (or back) than the ventral (or front) 

 aspect of the body. The bones thus cut through are 

 called the bodies of the vertebrae. They sepai-ate a 

 long, narrow canal, called the spinal canal, which is 

 placed upon their dorsal side, from the spacious chamber 

 of the chest and abdomen, which lies upon their ventral 

 side. There is no direct communication between the 

 dorsal canal and the ventral cavity. 



The sjjinal canal contains a long white cord — the spinal 

 cord — which is an important part of the nervous system. 

 The ventral chamber is divided into the two subordinate 

 cavities of the thorax and abdomen by a remarkable, 

 partly fleshy and partly membranous, partition, the dia- 

 phragm (Fig. 1, D), which is concave towards the abdo- 

 men, and convex towards the thorax. The alimentary 

 canal (Fig. 1, Al.) traverses these cavities from one end 

 to the other, piercing the diaphragm. So does a long 

 double sei'ies of distinct masses of nervous substance, 

 which are called ganglia, are connected together by 

 nervous cords, and constitute the so-called sympathetic 

 system (Fig. 1, Sy.). The abdomen contains, in 

 addition to these parts, the two kidneys, one placed 

 against each side of the vertebral column and connected 

 each by a tube, the ureter, to a muscular bag, the 

 bladder lying at the bottom of the abdomen ; the liver, 

 the pancreas or "sweetbread" and the spleen. The 

 thorax incloses, besides its segment of the alimentary' 

 canal and of the sympathetic, the heart and the two 

 lungs. The latter are placed one on each side of the heart, 

 which lies nearly in the middle of the thorax. 



Where the body is succeeded by the head, the upper- 

 most of the thirty-three vertebral bodies is followed by a 

 continuous mass of bone, which extends through the whole 

 length of the head, and, like the spinal column, separates 

 a dorsal chamber from a ventral one. The dorsal chamber, 



