ESSENTIALS OF A GLAND. 



155 



The sweat tube is the first example we have had of a Gland. 

 The essential features of a gland are : - 



1. Cells lining a cavity, the cells having the power of 

 taking something from the blood (or lymph). 



2. Blood supply or lymph supply. 



3. A tube or Duct to pour out on some surface the liquid 

 taken from the lymph. 



4. Nerves to the cells by which their action is controlled. 



Epithelium, or Epidermis. 



Compound Glanc 

 Fig. 52. Evolution of Glands. (After Landois & Stirling.) 



5. (Probably) Special nerve centers controlling the various 

 glands. The cells of the glands in many cases so alter the 

 substances taken from the blood that what is produced by the 

 gland differs from anything found in the blood. The gland 

 may be said to manufacture the liquid it gives off. 



The sweat glands, like all glands, are largely dependent on 



