XIV 



Specialized Cells: Muscles, Nerves and Sense 



Organs 



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It is difficult to convey any idea of the extraordinary variety of cells 

 in the animal organism. Besides those already mentioned there are, 

 for example, cells which lay down the bones of the skeleton in the 

 young animal; the cells which produce hair and nails, which in many 

 cases continue to function for the whole of life; and the mammary 

 cells of the female which, after the birth of young, produce large 

 quantities of milk. The latter are a veritable factory of proteins, fats 

 and other constituents. Here is a description by Professor H. D. Kay 

 of the operation of the milk-producing cells of the cow. 'The day's 

 work of one of these cubical cells entails the following cycle of opera- 

 tions. It begins as a rather squat cell with the nucleus in the middle. 

 Granules or globules, some of which stain with fat soluble dyes, then 

 begin to appear in the part of the cell nearest the alveolar space. The 

 cell begins to increase in length and size, the nucleus remaining close 

 to the basement membrane. The secretory products soon fill the 

 whole of one end of the tall distended cell. These products, and pos- 

 sibly a small part of the cytoplasm of the cell itself, are now extruded 

 into the lumen as milk, following which discharge the cell returns to 

 its original squat shape. This whole process is repeated several 

 times. . . . During the twenty-four hours in an actively lactating cow 

 there may be up to four or five, or even more cycles of operation. . . .' 

 However, perhaps the most interesting cells are those which give 

 the animal the power of movement and those which enable him to 

 become aware of the nature of his environment. These cells, which 

 form muscles and sense organs and nerves are so important in the 

 higher manifestations of life, that we must look at them in some de- 

 tail. They have to be taken together because, as a rule, the operation 

 of one involves the functioning of at least one of the other two. 



