8 A COURSE ON ZOOLOGY. 



the parts of the body. The muscles form the red flesh ; 

 they completely cover the bones, and, by their shapes, 

 their more or less pronounced development, and their 

 projection from the general surface, they give to the 

 body its special outlines. 



Three large cavities and several smaller ones are con- 

 tained within bony structures. First in order is the 

 cranial cavity in the head, in which are contained and 

 protected the brain and its immediate connections; in 

 the upper part of the trunk is a large cavity bounded on 

 the sides by the ribs, which form a sort of cage sur- 

 rounding the space called thorax, breast, or chest, that 

 contains the important organs, the heart and lungs. 

 Lastly, the great abdominal cavity occupies the remain- 

 ing space in the trunk, and in this are placed the stomach, 

 liver, intestines, kidneys, and bladder. A horizontal 

 partition separates the abdominal and thoracic cavities. 



Besides the three large cavities that we have men- 

 tioned, there are several smaller ones, most of them 

 situated in that part of the skeleton that forms the face. 

 Among these are the orbital cavities, or orbits, in which 

 are placed the eyes ; the nasal cavity, for the organs of 

 smell ; the mouth, or buccal cavity, which contains the 

 tongue or organ of taste. It will be observed that all 

 these cavities of the face serve for the protection of 

 certain organs of sense; they are therefore near the 

 cranial cavity, and consequently close to the brain and 

 its connections, with which, as we shall presently see, the 

 organs of sense are directly joined. 



We might also mention the long, narrow cavity con- 

 tained within the whole length of the spinal column, 

 which is a sort of sheath enclosing the spinal marrow. 



The skeleton not only protects certain cavities, it in- 



