EPITHELIAL TISSUES. 91 



formed in them into the tubules with which they are connected. 

 We have thus, in addition to the several types of tubular glands 

 above mentioned : Simple tubulo-alveolar glands, simple branched 

 tubulo-alveolar glands, and compound branched tubulo-alveolar 

 glands. 



Alveolar Glands. In the alveolar glands the secreting com- 

 partments have the form of variously shaped vesicles or saccules, 

 known as alveoli, lined by secretory epithelium, which communi- 

 cate with narrow tubules of varying length and lined by nori- 

 secretory epithelium, which form the ducts. Alveolar glands are 

 classified as : 



1. Simple alveolar glands, consisting of a single alveolus which 

 communicates with the surface by means of a narrow duct. 



2. Simple Branched Alveolar Glands. In this type a varying 

 number of alveoli are united through their respective ducts to a 

 larger duct which reaches the surface. 



3. Compound Branched Alveolar Glands. Glands of this type 

 consist of a varying number of simple branched alveolar glands 

 united by a common duct. 



4. Follicnlar Glands. Glands of this type may be classed under 

 alveolar glands, since they consist of numerous closed alveoli or 

 follicles, of round, oval, or even irregular shape, which do not 

 communicate with a duct system. 



The main features of this classification of glands are portrayed 

 in the accompanying diagram (Fig. 54). 



According to the above description multicellular glands may 

 be classified as follows : 



(I. Simple tubular glands: crypts of Lieberkiihn, the majority of 

 the sweat glands. 

 2. Simple branched tubular glands : fundus glands of stomach, 

 Tubular Plands the ma J orit y of the Pylo"c glands, uterine glands. 



\ 3. Compound branched tubular gjands : kidneys, testis, lachry- 

 / mal glands, serous glands of mucous membranes. 



I 4. Reticulated tubular glands : Liver (fully developed in mam- 

 mals). 



(I. Simple tubulo-alveolar glands: certain of the pyloric glands. 

 2. Simple branched tubulo-alveolar glands : Littre's glands, cer- 

 tain of the sweat glands, and modified sweat glands 

 (circumanal and axillary glands, ceruminous glands, 

 j ciliary glands). 



/ 3. Compound branched tubulo-alveolar glands : many mucous 

 \ glands, Brunner's glands, prostate, lung. 



1. Simple alveolar glands : the smallest sebaceous glands, the 



skin glands of amphibia. 



2. Simple branched alveolar glands : sebaceous glands, Meibo- 



mian glands. 



3. Compound branched alveolar glands : pancreas, mammary 

 ... . , j gland, serous salivary glands in the latter, however, a 



portion of the duct system possesses secretory function 

 (Maziarski). 



4. Follicular glands: ovary, hypophysis, thyroid (according to 



Streiff, certain of the closed follicles of the thyroid have 

 a tubular form, others show secondary alveolar enlarge- 

 ments on the primary follicles). 



