BIOLOGICAL INTRODUCTION 7 



tho division of labour there is elaborated (a) an alimentary system, 

 by which the necessary foodstuffs are taken into the organism and 

 reduced to a proper state for absorption; (6) a respiratory system, 

 by which the oxygen necessary for tihe cell processes is introduced, 

 and the carbon dioxide produced by these processes eliminated; 

 (c) a transport and circulatory system, by which these necessaries 

 are conveyed to all the body cells to supply their needs, and the 

 products of cell activity conveyed away for excretion either by the 

 respiratory mechanism or by (d) a specially developed excretory 

 system. 



Finally there remain two special systems the nervous and the 

 reproductive. The nervous system serves to put the organism into 

 communication and correlation with outer chemical, physical, and 

 mechanical conditions. It does this by its receptor, conductor, and 

 effector functions. It is irritable, and receives a stimulus either 

 directly or indirectly, conducts the stimulus as a nerve impulse, dis- 

 charges it on some other cell or cells, and the.e produces its effect. 

 The system is primarily intended for communication between parts 

 of the body more or less widely separated; in the higher animals it 

 becomes an extremely complicated system, and according to its degree 

 of complexity and the manifold functions it performs, so is the 

 organism classed by man in the ladder of life. Man, placed by himself 

 at the top of the ladder, has the most complicated and most highly 

 developed nervous system. To the reproductive system is assigned 

 the highly important function^of maintaining the particular species 

 of the organism. In the multicellular organism, the cells other than 

 the reproductive cells perish after a longer or shorter period of exist- 

 ence. But the reproductive cells, under appropriate conditions, 

 give rise to fresh individuals, thereby perpetuating an unbroken chain 

 of living cells. 



Physiology is the science which treats of the normal functions 

 of these various systems. 



