82 ZOOLOGY. 



as on the skin and mucous membranes ; but they are chiefly 

 found in connexion with certain bodies termed glandular, 

 whose essential structure seems to consist in small cavities, 

 extremely minute, or pouches or canals of extreme tenuity ; 

 these receive bloodvessels and nerves, the latter no doubt 

 playing an important part in the phenomena of secretion. 

 The glands have been divided into perfect and imperfect, 

 according as they are provided or not with a tube or duct 

 intended to carry away the product of the secretion. 



154. Glands, properly so called, may all be arranged, as 

 regards their intimate structure, under two heads, or types ; 

 as being composed either of small sacs, with orifices more or 

 less contracted, or of tubes of extreme minuteness ; and the 

 differences they present have a reference mainly to the mode 

 in which these, as it were elementary, structures are grouped. 



155. The small secreting sacs are called follicles, when 

 ow they are called crypts, and many such may be seen 

 on the surface of mucous membranes. When each has a 

 separate or distinct oritice opening on the surface, they are 



Fig. 60. Intimate Structure of a Composite Gland 

 (the Parotid) ; a Salivary. 



called simple follicles, and many such exist on the surface of 

 the mucous membranes ; if grouped close together, but still 

 maintaining distinct orifices, they are called aggregated; 

 such are the glands of Meibomius on the eyelids, certain 

 gastric glands in some animals, &c. ; when grouped, but 

 having their orifices leading into a small cavity common to 



