TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS 39 



remove the waste materials which are formed in connection with 

 the chemical activities of the cells and which would interfere with 

 their proper working if allowed to accumulate. 



The Circulatory System consists of the heart and blood-vessels. 

 It serves to distribute to all the parts of the Body supplies of 

 oxidizable material, of repair material, and of oxygen, and to re- 

 move therefrom the accumulated waste products. These func- 

 tions are accomplished through the agency of a circulating me- 

 dium, the blood. 



The Respiratory System consists of the lungs, the bronchial 

 tubes, and the trachea, together with the respiratory muscles. Its 

 function is to bring the outside air to a region where the circulating 

 medium can take up abundant supplies of oxygen, and where it 

 can get rid of those waste products which are in gaseous form. 



The Digestive System consists of the alimentary canal and cer- 

 tain associated glands (salivary, liver, pancreas). It serves to bring 

 the various materials that are taken as food into the forms best 

 adapted for use as repair materials or as oxidizable substance; 

 when it has so prepared them it turns them over to the circulating 

 medium for distribution. 



The Excretory System consists of the kidney and bladder with 

 their connecting tubes, the liver, and the skin. It serves to with- 

 draw from the circulating medium and to eliminate from the body 

 those waste products which are in liquid form. 



Chemical Co-ordination is secured, as previously stated, by 

 specific hormones which govern those bodily activities that are 

 either not readily susceptible to nervous control or in which the 

 best results are secured by supplementing nervous control with 

 chemical. There are special organs, or parts of organs, which 

 manufacture hormones. These are often called ductless glands, 

 since they pour their secretions into the blood stream and not by 

 ducts to the surface. They might be grouped together as a sys- 

 tem, although nothing would be gained by so doing. Chemical 

 co-ordination plays a part in nearly all forms of bodily activity, 

 and the different hormones will be studied in connection with the 

 activities over which they exert influence. 



Through these systems provision is made for the activities of the 

 individual cells. These activities are many and complex. They 



