210 THE PRINCIPLES OF AGRICULTURE 



outside themselves. As each cell becomes im- 

 prisoned in a minute cavity in such solid 

 structure, it is robbed of those common powers 

 or functions which belong to the single -celled 

 animal, and is specialized for the performance 

 of one constant, unchanging round of work. 

 Each cell has its own work to do. 



361. Cells may carry on processes of nutri- 

 tion. Some cells lie in the microscopic spaces 

 left in the hard bone, and conduct the nutrition 

 and changes in its substance. Other cells lie 

 in the substance of muscle or sinew, or of 

 brain, or of some other tissue, and no one 

 of these can construct bone nor any other 

 structure than that in which it lies. All such 

 cells are engaged in carrying on the nutrition 

 and growth of their respective tissues, and 

 are reserved for this work only. 



362. Cells may carry on nervous processes, 

 being set apart for vital work of a kind not 

 directly connected with nutrition. Nerve cells, 

 found in the brain, spinal-marrow, and some 

 other parts, receive impressions brought over 

 the nerve cords from distant parts of the body. 

 They generate and send out nerve force to 

 other parts. Some of these cells are set in mo- 

 tion by mental acts. 



363. Certain other cells, which line microsco- 

 pic sacs in organs known as glands, select from 



