i6 THE FUNCTIONS OF ANIMALS. 



the inner layer absorb and digest the food, and are also 

 muscular. 



In worms and higher organisms, there is a middle layer 

 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 become compacted into 

 well-defined parts or organs. 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. 



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. 



■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 as the mechanism which 

 I produced movement, the heart pumped the blood through the body, the 

 I brain was the seat of thought, and so on. This was an exceedingly 

 necessary and natural step in analysis. Nor has it yet been thoroughly 

 taken in every case, for there are many organs, especially in backbone- 

 less animals, about whose predominant use we are uncertain. But the 

 physiologists of this school sometimes finished their work too quickly. 

 I That the liver was an organ for secreting bile was deemed a completely 

 \ satisfactory statement, until it began to be seen that this organ is the 

 \seat of many other activities. Moreover, some thought that it was 

 possible to deduce the function of an organ from its visible structure, as 

 one might infer the use of a piston from its shape. To a certain extent 

 this is true, as when we show how .an image is formed on the retina of 

 the eye. But we cannot, in terms of visible structure, explain another 

 1 function of the eye — that of distinguishing the " colours " of things. In 

 fact, it must be clearly understood that each organ is far more than a 

 I piece of mechanism in a living engine, — that it is a complicated factory 



of living units, each with subtle and manifold powers. 



/ In iSoi, Bichat analysed the animal body into its component tissues 



/ — muscular, nervous, glandular, &c., and being a physiologist as well as 



j an anatomist, sought to explain the activities of the organism in terms 



of the contractile, irritable, secretory, or other properties of its tissues. 



\ This was a further step in the analysis, and one of great importance. 



About forty years later, however, it began to be recognised that the 

 body was a great city of cells, each with a life of its own.' The functions 

 were not merely the activities of organs of various construction, or of 

 tissues with various properties, they were the results of the life of the 

 component units or cells. 



Finally, in those last days, the physiologists have touched the bottom 



