ORGANS AND FUNCTIONS OF HIGHER ANIMALS 111 



the iiuids secreted by the glands change the foods from a solid form 

 (usually insoluble) to that of a fluid. Such fluids may then pass, 

 by diffusion, through the walls of the food tube into the blood. 



(4) The organs of circulation: the tubes through which the 

 blood, bearing its organic foods and oxygen, reaches the tissues of 

 the body. In simple animals, as the hydra, no such organs are 

 needed, the fluid food passing from cell 

 to cell by diffusion. 



(5) The organs of respiration: gills or 

 lungs, the organs in which the blood re- 

 ceives oxygen and gives up carbon di- 

 oxide. The outer layer of the body serves 

 this purpose in very simple animals. 



(6) The organs of excretion: such as 

 the kidneys and skin, which pass off 

 nitrogenous and other waste matters from 

 the body. 



(7) The organs of locomotion : muscles 

 and their attachments and connectives ; 

 namely, tendons, lig'aments, and bones. 



(8) The organs of nervous control : the 

 central nervous system, which has con- 

 trol of coordinated movement. There are 

 scattered nerve cells in low forms of life ; 

 such cells are collected into groups and con- 

 nected with each other in higher animals. 



(9) The organs of sense : collections of 

 cells having to do with the reception and 

 transmission of sight, hearing, smell, 

 taste, touch, pressure, and temperature 

 stimulations. 



(10) The organs of reproduction: the 

 sperm-forming and egg-forming organs. 



The Human Body a Machine. — In all animals, and the hu- 

 man organism is no exception, the body has been likened to a 

 machine in that it turns over the latent or potential energy stored 

 up in food into kinetic energy (mechanical work and heat), which 

 is manifested when we perform work. One great difference exists 



The human body seen from 

 the side in longitudinal sec- 

 tion, tr, trachea ; g, lungs ; 

 d, diaphragm ; n, pancreas ; 

 s, spleen ; k, kidney ; ap, ap- 

 pendix. 



