254 PRACTICAL ZOOLOGY 



thyroid and thymus situated in the neck region, and the adrenal 

 bodies extending along the ventral surface of the kidneys. The 

 removal of any of these glands, as a rule, is followed by irregu- 

 larities in the work done by the various organs of the body and 

 sometimes ends fatally. On the whole, internal secretions are 

 extremely important for the proper functioning of the various 

 parts of the body. They seem to act as regulative agents, and 

 thus secure the coordination of the different functions. All 

 glands are not for secretive purposes only, but, like the kidney, 

 may excrete waste products. 



The Skeleton and its Functions. The skeleton of an animal 

 supports the softer parts, furnishes points of attachment for 

 muscles, and protects various organs. Most of the invertebrates 

 possess exoskeletons such as that of the insect or crayfish. The 

 vertebrates, on the other hand, are provided with an internal 

 framework or endoskeleton. This consists of bone and cartilage. 

 That bone contains cartilage can easily be determined if a piece 

 is placed in hydrochloric acid. The acid dissolves out the 

 mineral constituents which give the bone rigidity, leaving the 

 cartilage, which furnishes pliancy and elasticity. 



The skull and vertebral column are often spoken of as the 

 axial skeleton, and the bones which support the appendages 

 (arms and legs) as the appendicular skeleton. The accompany- 

 ing figure (Fig. 145) shows the bones in the skeleton of a frog. 



The bones of the skull form a brain case or cranium which 

 protects the brain, and auditory and olfactory capsules which 

 protect the sense organs of hearing and smell respectively. 

 Besides these, the bones of the face (jaw bones, etc.) and of the 

 throat (hyoids) are included in the skull. 



The backbone or vertebral column serves as a central axis. It 

 consists of a series of bones called vertebra which are held 

 together by ligaments, but move upon one another so that the 

 body can be bent. Each vertebra bears a dorsal arch which 

 surrounds and protects the spinal cord. 



The bones which unite the fore limbs to the body constitute 



