94 



ELEMENTARY PHYSIOLOGY 



and again, it often enlarges still more, till its outline consists of a number of 

 little pouches, all communicating with one common cavity. This last form of 

 gland is generally known as the lobulated gland. 



Some glands develop to such an 



extent that they consist of a number of 



bodies resembling the simple glands 

 just described, but all communicat- 

 ing with one common duct. These 

 are called compound glands. If they 

 consist of a number of branching 

 tubes they are called compound tubu- 

 lar glands ; but if composed of a 

 group of sacs or lobules with one 

 common outlet they are termed race- 

 mose or compound lobulated glands. 

 But although the glands vary so con- 

 Fig. 93. Compound Tubular Gland, siderably in form, yet they are all 



constructed on one general principle 



all consist of a secreting membrane which is closely surrounded by a net- 

 work of minute blood-vessels. 



The whole interior of the alimentary canal, from the mouth to 

 the rectum, is lined with a soft kind of skin called the mucous 

 membrane. At the margins of the lips this membrane is con- 

 tinuous with the skin which covers the exterior surface of the body. 

 The mucous membrane is richly supplied with minute simple or 

 compound glands called the mucous glands. These separate from 

 the blood a watery fluid called mucus which serves to keep the 

 membrane moist. 



In addition to the mucous glands the structures surrounding 

 the cavity of the mouth are provided with numerous others called 

 the salivary glands. These are all compound lobulated glands, 



Fig. 94. Compound Lobulated Glands. 



A, a complete gland with its duct, d ; B, a single lobule enlarged, with d, its duct. 



and secrete a fluid called the saliva which aids considerably in 

 the process of digestion, as will be presently shown. 



The most important of the salivary glands are the three pairs 

 known as \hefiarotid, the submaxillary, and the sublingual. 



The parotid glands (Gr. para, near : and ous, the ear) are 



