180 The Study of Animal Life part hi 



the function of an organ might be expressed in terms of the 

 properties of its tissues. 



If we pass to the next step of analysis, and think of the 

 body as a complex city of cells, we are better able to 

 understand what tissues are. Each cell corresponds to a 

 house, a tissue corresponds to a street of similar houses. 

 In a city like Leipzig many streets are homogeneous, formed 

 by houses or shops in which the predominant activity is the 

 same throughout. A street is devoted to the making of 

 clothes, or of bread, or of books. So in the animal body 

 aggregates of contractile cells form muscular tissue, of 

 supporting cells skeletal tissue, of secreting cells glandular 

 tissue, and so on. 



It is enough to state the general idea that a tissue is an 

 aggregate of more or less similar cells, and to note that the 

 different kinds may be grouped as follows : — 



I. Nervous tissue, consisting of cells which receive, 



transmit, or originate nerve-stimuli. 

 II. Muscular tissue, consisting of contractile cells. 

 III. Epithelial tissue, consisting of lining and covering 

 cells, which often become glandular, exuding the 

 products of their activity as secretions. 

 IV. Connective tissue, including cells which bind, 

 support, and store. 



Cells. — To the discovery and perfecting of the micro- 

 scope we owe the analysis of the body into its unit masses 

 of living matter or cells. From 1838-39, when Schwann 

 and Schleiden stated in their "cell doctrine" that all 

 organisms — plants and animals alike — were built up of 

 cells, cellular biology may be said to date. It was soon 

 shown as a corollary that every organism which reproduced 

 in the ordinary fashion arose from a single egg-cell or 

 ovum which had been fertilised by union with a male-cell 

 or spermatozoon. Moreover, the position of the simplest 

 animals and plants was more clearly appreciated ; they are 

 single cells, the higher organisms are multicellular. 



Now the cells of the animal body are necessarily varied, 

 for the existence of a body involves division of labour 



