other "glands", as we shall see later (Chap. 16), are characterized by having 



no ducts. 



Absorption of Digested Food Tiny projections into the cavity of the 

 small intestine increase the absorbing surface of the lining several hundred 

 times (see illustration opposite). These projections are called villi (singular, 

 mllus), from a Latin word meaning "shaggy hair" which gives us also 

 velvet. The villi act both as absorbing and as transforming organs. That is, 

 the materials they absorb become chemically changed before being passed 

 on into the lymph, the colorless fluid which surrounds all the living cells in 

 the body. They thus behave like glands, only, so to say, in reverse. For 

 glands normally absorb materials from the lymph, transform them chemi- 

 cally, and then pass out new substances. 



Gland cells in 

 surface layer 



Simple tubular 



Simple alveolar 



:m II \:\^^^J 



Complex tubular 



Complex alveolar 



TYPES OF GLANDS 



Glands consist essentially of secreting cells arranged in a layer, which tends to fold 

 into depressions, or pockets. A gland may thus consist of one or a few cells secret- 

 ing on the surface, or it may consist of a simple tube, more or less enlarged toward 

 the bottom into an "alveolus", or pit. in some glands the tubes branch and subdi- 

 vide extensively, so that a great deal of secreting surface supplies one opening or 

 tube. The liver, the largest gland in the body, is a compound tubular gland. Alveolar 

 glands may also branch and become complex— the pancreas, for example 



170 



