424 



MAMMALIA— PHYSIOLOGY 



A cell is a minute portion of living substance or protoplasm, 

 sometimes inclosed by a cell membrane, and at some stage of its 

 development containing a nucleus. It is a physiological unit. 

 (See page 423.) 



When cells, similar in structure and originating from the same 

 germ layer of the embryo, are grouped together to perform some 

 special function, they are called tissues. 



Fig. 237. Four nerve cells. J and C, from the cerebellum; B, from the gray- 

 matter of the spinal cord; D, from the cerebrum; a, the axon. The cells J and D are 

 stained so that the main body and the dendrites (p. 76) are a uniform black; B and C, 

 so as to show the nucleus and cytoplasm. (Hough and Sedgwick, The Human Mecha- 

 nism. Courtesy of Ginn and Co.) 



We may classify tissues as Epithelial or bounding tissues, and 

 Connective or supporting tissues. E-pithelium is a tissue which is 

 distinctively cellular with its cells usually similar in size and form, 

 and united by a small quantity of intercellular substance. (Figure 



237-) 



Epithelium is found covering all outer surfaces and all inner 



free surfaces and is protective and secretory. It is the first tissue 



found in the embryo. Pavement epithelium is found lining the 



mouth, on the inside of the lungs, and covering the body. It is 



