64 



ANIMAL BIOLOGY 



All protoplasm is irritable — it responds to stimuli — but ob- 

 viously the larger and more complex the body becomes, the more 

 necessary it is that the actions and reactions of its component cells 

 be instantly coordinated so that the parts act as a whole — an 

 organism. This function is performed by nervous tissue com- 

 posed of nerve cells, or neurons, whose long fibers are bound to- 

 gether by connective tissue into cables, or nerves. (Fig. 138.) 



Epithelial lining 



Connective tissue 



Blood vessel -t^ 



Circular muscle- 



Longitudinal muscle 

 Peritoneal membrane" 



Blood vessel - 



Fig. 34. — Portion of a transverse section of the small intestine of the Frog, 

 highly magnified, to show cellular differentiation, and tissues combined to 

 form an organ. 



One seldom thinks of blood, lymph, and other fluids that trans- 

 port materials in the body, as tissues, but in truth they must be 

 regarded as circulating tissue in which the living cells, or 

 corpuscles, are suspended in a fluid matrix, the plasma. Cir- 

 culating tissues minister to, and are in intimate contact with, 

 every tissue and cell of the organism. 



