62 ANIMAL PHYSIOLOGY. 



bladder, afford instances of stratified squamous epithelium. 

 The microscopic examination of the more delicate simple 

 squamous epithelia, is facilitated by treating the surface to 

 be examined with a solution consisting of one grain of nitrate 

 of silver in an ounce of distilled water, and, after a few 

 minutes, washing it and exposing to the light. Oxide of 

 silver is deposited, first in the lines of contact of the edges 

 of the cells, and, on a little further exposure, also in their 

 substance, particularly in the nuclei. 



Columnar epithelium has the cells elongated in a direction 

 vertical to the surface, and lying together like rods or prisms. 

 It is found in the whole length of the alimentary canal, from 

 the entrance into the stomach and onwards, and in the 

 majority of ducts of glands. In glands and their ducts, 

 there are also various transitional forms between columnar 

 and squamous, as, for example, the cubical ; and the terms 

 spheroidal epithelium and glandular are used for the various 

 irregular polygonal forms of corpuscle engaged in secretion 

 in the salivary and other glands. 



Ciliated epithelium is neither the mere mechanical protec- 

 tion which squamous epithelium is, nor the secreting struc- 

 ture which columnar and others often are, but has the 

 property of keeping the moisture on its surface in a perpetual 

 current. This it does by means of minute hair-like processes, 

 termed cilia, projecting from the free aspect of its cells, and 

 perpetually in motion. In a suitable fluid these cilia will 

 continue to move for hours under the microscope, after the 

 cells to which -they belong have been detached from all other 

 texture ; and therefore both the power of movement or con- 

 tractility, and the stimulus thereto, must be inherent in 

 themselves. The character of the movement is always the 

 same ; it is so rapid that it cannot be observed in detail till 

 it begins to get slower, but the general effect is something 

 like the undulation of a field of corn swept by the wind, or 

 still liker the vibration of hot air over a furnace seen against 

 the light. When the rapidity abates, each cilium is seen to 

 be slightly flattened, and to bend over to one side, and 

 recover with a feathering curve. The movement is at all 

 times in one direction, and that direction is in every case 

 toward the orifice of the passage, when there is one. Ciliated 



