The Basis of Life 39 



two functions. In general, it is considered that an excretion is a ma- 

 terial found in the blood, which is eliminated as a waste material. Usu- 

 ally these are metabolic wastes. A secretion, on the other hand, is 

 actually elaborated by the gland. 



Fig. 5. — Types of glands. A, Simple tubular; B, coiled tubular; C, branched 

 tubular; D, simple alveolar; E, simple branched alveolar; F, compound branched 

 alveolar. Stippled or darkened areas are secreting portions of glands. 



In its simplest form, a gland is but a single cell located on some 

 epithelial tissue. The goblet cells found in the intestinal lining are 

 examples of these. If the gland is depressed below the secreting surface, 

 a greater area for secretion is made available. This is a simple saccular 

 gland.. When the gland has a special duct extending from it to the 

 surface, it may become either saccular or tubular at the base. The saccu- 

 lar type gland is known as an alveolar gland, the other as a tubular gland. 

 Both of these can become much branched ; then they are known as com- 

 pound alveolar or cofnpoiind tubular glands. 



A simple type of alveolar gland is common in the skin of the frog, 

 while the oil glands and pancreas of man are examples of compound 

 alveolar ones. The sweat gland of the skin is an excellent example of a 

 simple tubular gland, and the liver is a complicated tubular one. 



