EPITHELIAL TISSUES. 89 



the classification of glands. The fuller consideration of these 

 structures will, however, be deferred to a later time. In the classi- 

 fication here given we have been guided by that presented by 

 Maziarski, in an observation on the structure and classification of 

 glands, based on a series of reconstructions with the Born wax- 

 plate method and comprising nearly all the important glandular 

 structures of the human body. In brief, it may be stated that the 

 variation in glandular types affects principally the secretory portions 

 of glands, while the excretory ducts are more or less uniform. 

 Glands are classified, according to their shape, into tubular and 

 alveolar glands; each of these types is further divided into simple 

 and branched tubular, and simple and branched alveolar glands. 

 In certain glands tubules and alveoli unite to form the secretory 



- Excretory 

 duct. 



Fig. 52. Excretory ducts Fig. 53. Lumina of the secreting portion 



and lumina of the secretory of a reticulated tubular gland ; from the human 



portion of a compound tubular liver. Chrome-silver preparation ; X ' 2O - 

 gland. Lingual gland of the 

 rabbit. Chrome-silver prepara- 

 tion ; X 2I 5- 



portion; such glands are known as tubulo -alveolar glands. They 

 may also be simple or branched. 



Tubular Glands. In a tubular gland the secreting portion 

 consists of a longer or shorter tubule, which may be relatively 

 straight or variously twisted or coiled, one end of which ends 

 blindly while the other end opens on a free surface or into a duct. 

 The blind ends of the tubules of tubular glands often present more 

 or less well-marked enlargements. Simple tubular glands consist 

 of a single tubule, which may be lined throughout by secretory 

 epithelium or may be differentiated into a portion lined by secretory 

 epithelium and a portion lined by a non-secretory epithelium form- 

 ing a duct. An increase of the secretory surface of tubular glands 

 is obtained in one of the following ways: 



