222 THE ORGANS 



branches being lined with secreting cells and opening into a superlicial 

 portion, which serves as a duct. Examples of slightly forked glands 

 are seen in the cardiac end of the stomach, and in the uterus. Other 

 tubular glands show much more extensive branching, the main duct 

 giving rise to a number of secondary ducts, from which are given off 

 the terminal tubules. The mucous glands of the mouth, oesophagus, 

 trachea, and bronchi arc examples of these more elaborate simple 

 tubular glands (Fig. 132, 3). 



(b) Compound tubular glands consist of a number, often of a 

 large number, of distinct duct systems. These open into a common 

 or main excretory duct. The smaller ducts end in terminal tubules. 

 Many of the largest glands of the body are of this type, e.g., the sali- 

 vary glands, liver, kidney, and testis (Fig. 132, 4). 



In certain compound tubular glands, as, e.g., the liver, extensive 

 anastomoses of the terminal tubules occur. These are sometimes 

 called reticular glajids. 



2. Alveolar Glands. — (a) Simple Alveolar Glands. — The sim- 

 plest form of alveolar gland consists of a single sac connected with the 

 surface by a constricted portion, the neck, the whole being shaped like 

 a flask (Fig. 132, i a). Such glands are found in the skin of certain 

 amphibians; they do not occur in man. Simple alveolar glands, in 

 which there are several saccules (Fig. 132,2a), are represented by the 

 smaller sebaceous glands. Simple branched alveolar glands, in which 

 a common duct gives rise to a number of saccules (Fig. 132, 3 a), are 

 seen in the larger sebaceous glands, and in the Meibomian glands. 



(b) Compound Alveolar Glands. — These resemble the com- 

 pound tubular glands in general structure, consisting of a large 

 number of duct systems, all emptying into a common excretory duct. 

 The main duct of each system repeatedly branches, and the small 

 terminal ducts, instead of ending in tubules of uniform lumen, as in 

 a tubular gland, end in sac-like dilatations, the alveoli or acini 

 (Fig. 132, 4 a). The best example of a compound alveolar gland is 

 the mammary gland, although the lung is constructed on the principle 

 of a compound alveolar gland. 



Ductless Glands 



Certain structures remain to be considered which are properly 

 classified as glands, but in which during development the excretory 

 duct has disappeared. Such glands are known as ductless glands. 



