12 THE FUNCTIONS OF ANIMALS. 



in addition to the other two, and this middle layer becomes, 

 for instance, predominantly muscular. Moreover, the units 

 or cells are not only arranged in strands or tissues, each 

 with a predominant function, but they become compacted 

 into well defined parts or organs. 



There is no idea more important in physiology than that 

 of the division of labour, therefore let me state it once 

 more. The Amoebae, and other single-celled organisms, 

 discharge all the functions (a, b, c, d, e,f) within a single 

 cell. Structurally these animals are very simple, but the 

 physiology of a completely functional cell must be very 

 complex. In a comparatively simple many-celled organism, 

 like a Hydra, the outer cells discharge several functions (say 

 a, b, c, d), and the inner cells discharge several functions, 

 partly the same and partly different (say c, d, e,f). The 

 more complex the animal becomes, the more restricted are 

 the functions of its individual cells ; different sets retain 

 the activities represented by a, b, or c, d, or e,f. Thus a 

 muscle-cell is predominantly contractile, a sense-cell pre- 

 dominantly sensitive, a ganglion-cell receives, regulates, and 

 originates nervous impulses, a red blood-cell carries oxygen 

 from the surface to the innermost recesses of the tissues. 

 None the less should we remember that each cell remains a 

 living unit, and that, in addition to its principal activity, it 

 usually retains others of a subsidiary character. It is a certain 

 conclusion, alike of common observation and the most 

 complete physiological analysis, that one living structure 

 may have a plurality of functions. 



History. — Physiologists, or those who study the activities 

 of organisms and of their parts, were at first content to 

 speak of these as the result of " animal and vital spirits," 

 of moods and temperaments. We ought not to do so now 

 until we have done our utmost to explain the activities in 

 lower terms. 



Stimulated, however, by the anatomists' disclosure of 

 organs, the physiologists soon began to explain the organism 

 as a complex engine of many parts. The muscles were 

 recognised ^s the mechanisms which produced movement 

 the heart pumped the blood through the body, the brain 

 was the seat of thought, and so on. This was an ex- 

 ceedingly necessary and natural step in analysis. Nor has 



