88 MANUAL OF ELEMENTARY ZOOLOGY 



td regard protoplasm as a single complex mixture of 

 substances in which there go on the chemical changes 

 which are the basis of the process we know as life. 



Every tissue belongs to one of four classes: it is either 

 . epithelial, skeletal, muscular, or nervous. The 



Tissues: epithelial tissues are those which cover surfaces, 



Trssues. 8 ' internal or external. They consist of cells of 

 simple shape arranged to form a layer, with 

 little, or no ground-substance between them. When the 

 cells are one layer deep the epithelium is said to be 

 simple ; when there is more than one layer it is stratified. 

 Perhaps the least specialised example of this class of tissue 

 is the kind known as columnar epithelium, found, for 

 instance, lining the intestine of the frog. This is a simple 

 epithelium, consisting of a single layer 

 of tall cells standing side by side like 

 columns. Between the cells exist 

 exceedingly fine crevices which com- 

 municate below with lymph-spaces, 

 and across the crevices the protoplasm 

 of the cells' is continuous as fine 

 w threads. A modification of this kind 



FlG ; 47.-Isolated cells of ep i t helium, known as ciliated epi- 

 of ciliated epithelium , . , . r . r , . , _ f 



from the roof of the thelium, is found on the root of the 

 mouth of a frog. mouth of the frog. Here the outer 



border of the cell is set with very fine 

 protoplasmic hairs known as cilia, which are in constant 

 lashing motion in one direction. As they bend sharply 

 and recover . slowly, the effect of their combined action 

 is to drive the fluid which covers the epithelium in 

 the direction of their lashing. From each cilium a fine 

 thread runs down into the cytoplasm of the cell. A 

 third modification of columnar epithelium is the kind 

 known as sensory. In this some or all of the cells bear at the 

 outer end one or more stiff processes, the size and shape 

 of which vary greatly in different cases. Each such cell is 

 connected with a sensory nerve, either by being itself pro- 

 longed internally into a fibre, which runs in the nerve, or by 

 such a fibre ending against it. Cells of this kind are found, 

 for instance, in the olfactory epithelium, where each bears 

 a tuft of stiff bristles, and in the retina, where each ends 



