36 MANUAL OF PHYSIOLOGY. 



for receiving, conducting, controlling, and distributing 

 impressions. 



3. Muscles or Contractile Tissues. In close relation to both the 

 previous and the next groups. 



4. Connective tissues formed only from the middle germ layer. 

 They are much modified in different parts, so as to give 

 shape to the body, and to support and hold the various 

 organs and parts firmly together. They are in fact the 

 materials used in the general body architecture. 



Epithelial Tissue, although the oldest kind of tissue both in 

 the animal series and in the germinal layers, retains the embry- 

 onic character of being entirely composed of cells placed in close 

 relationship to each other on the internal and external surfaces 

 of the body. The individual cells, moreover, retain the embry- 

 onic character in form and function, being soft rounded masses of 

 protoplasm, only altered in shape by the pressure of their neigh- 

 bors. The cells which lie next the nutrient vessels of the meso- 

 blast, are endowed with energetic powers of growth and reproduc- 

 tion. As the young cells are produced they take the place of the 

 parent cell, whose future life-history determines the special char- 

 acters of the different kinds of tissues. 



Sometimes the cells are retained, as in the skin, and are arranged 

 in several layers, one over the other. As the cells are conveyed 

 from the deeper layer, where they take their origin, towards the 

 surface, the efforts of the waning nutritive power of the protoplasm 

 are devoted to the manufacture of a tough insoluble substance. 

 The cells thus gradually lose their vital activities, and are con- 

 verted into horny scales, which form the external protecting skin, 

 and its many modifications that give rise to the different dermal 

 appendages, such as hair, feathers, etc. Instead of a horny sub- 

 stance, the protoplasm may manufacture fat in the bodies of the 

 cells, and the adult cells, being moved on by the young cells aris- 

 ing beneath them, are heaped together as an indefinite mass which 

 passes off as a fatty mixture. In other cases the reproductive 

 activity of the cell is in abeyance, and its remaining nutritive en- 

 ergy is devoted to the manufacture of a material which is poured 

 out of the cell at certain periods. Thus \ve have another great 



