2 INTRODUCTION 



have become specially endowed with one or other of these 

 properties. In other words, there is a division of labour, 

 all the cells contributing their share for the good of the 

 whole, the cells which possess the same property being 

 grouped together into units known as tissues. The mus- 

 cular tissues are cells speciahsed in contractihty ; the 

 nervous tissues in irritabihty ; the digestive tissues in 

 assimilation. Special tissues exist also for excretion and 

 reproduction. 



Yet, though the one property has been exalted at the 

 expense of the others, these have not entirely disappeared. 

 All cells are assimilative ; muscle, though primarily con- 

 tractile, is irritable. Those properties other than the one 

 which is characteristic of the tissue have sunk to a 

 secondary position — they may be latent, but they are not 

 necessarily completely abolished. 



From the grouping together of a number of cells certain 

 consequences follow. The first is the need for binding the 

 cells together. A number of structures are developed to 

 play this passive role. Such are the connective-tissues — 

 bones, hgaments, and fibrous tissue. The second conse- 

 quence is that as the individual increases in size the 

 number of cells which are in direct contact with the sur- 

 rounding medium becomes smaller. In Amoeba, the cell 

 being completely surrounded with water, there is ample 

 opportunity for interchange of food and excretions. In 

 multi-cellular organisms, on the other hand, only the few 

 cells on the surface can be nourished and drained in this 

 way. As a means of overcoming this difficulty there is 

 developed a transport system — the blood. Each cell in our 

 bodies is bathed in a salt solution just as freely as though it 

 floated independently, and this salt solution brings it the 

 nourishment which it needs and removes the waste products 

 which it excretes. 



In the animal economy a factor of supreme importance 

 is the rapidity of the circulation. This is what we are 

 most apt to forget — possibly because we are unconscious 



