4 THE HUMAN BODY. 



ones, insects, or oysters, but agrees in many points with 

 the groups of fishes, amphibians, reptiles, and birds. 

 These four are therefore placed with man and all other 

 Mammals in one great division of the animal kingdom 

 known as the Vertebrata. The main anatomical character of 

 all vertebrate animals is the presence in the trunk of the 

 body of two cavities, a dorsal and a ventral, separated by a 

 solid partition, and in the adults of nearly all vertebrate 

 animals a hard axis, the vertebral column (backbone or spine), 

 develops in this partition and forms a central support for 

 the rest of the body (Fig. 2,ee). The dorsal cavity is con- 

 tinued through the neck, when there is one, into the head, 

 and there widens out. The bony axis is also continued 

 through the neck and extends into the head in a modified 

 form. The ventral cavity, on the other hand, is confined 

 to the trunk. It contains the main organs connected with 

 the blood-flow and is thus often called the haemal cavity. 



Upon the ventral side of the head is the mouth open- 

 ing leading into a tube, the alimentary canal, f, which 

 passes back through the neck and trunk and opens again 

 on the outside at the posterior part of the latter. In its 

 passage through the trunk region this canal lies in the- 

 ventral cavity. 



The Mammalia. In many vertebrate animals the ven- 

 tral cavity is not subdivided, but in the Mammalia it is; a 

 membranous transverse partition, the midriff or diaphragm 

 (Fig. 1, z), separating it into an anterior chest or thoracic 

 cavity, and a posterior or abdominal cavity. The alimen- 

 tary canal and whatever else passes from one of these cavi- 

 ties to the other must therefore perforate the diaphragm. 



In the chest, besides part of the alimentary canal, lie 

 important organs, the heart, h, and lungs, lu ; the heart 

 being on the ventral side of the alimentary canal. The 

 abdominal cavity is mainly occupied by the alimentary 

 canal and organs connected with it and concerned in the 

 digestion of food, as the stomach, ma, the liver, le, the 

 pancreas and the intestines. Among the other more prom- 

 nent organs in it are the kidneys and the spleen. 



In the dorsal or neural cavity lie the brain and spinal 



