104 PSYCHOBIOLOGY 



Glands are classified, according to their morphology, first, and in gen- 

 eral, as duct-glands and ductless glands. The duct-glands are again 

 classified as simple and compound, and also as tubular, saccular (or 

 alveolar), and solid. 



Ductless glands discharge their products directly into the blood stream, 

 or into the lymph, so that the veins and lymphatic vessels draining these 

 glands may be considered as also their ducts. The products of these glands 

 are frequently designated as internal secretions, or more technically, 

 hormones. Ductless glands are of course secretory only, and the secre- 

 tion is clearly a process of manufacture, i. e., the blood leaving these 

 organs contains substances not contained in the entering blood. The prac- 

 tical use of these substances (hormones) is to excite or sensitize cells to 

 which the blood stream carries them. 



Duct-glands produce, or separate from the blood or lymph, substances 

 which are needed for specific purposes outside of the body tissues, or of 

 which the body needs to get rid. The ducts through which these glandular 

 products are delivered to the proper points are therefore essentially separ- 

 ate from the other connections of the gland. The products of the duct- 

 glands are commonly designated as external secretions. 



DUCT-GLANDS. 



The duct-glands are sometimes referred to as true glands; sometimes 

 they are designated simply as glands; the intention in this case being that 

 the term ' gland ' shall always signify the duct-gland when not qualified 

 by the adjective ' ductless '. These usages are due to the fact that the 

 duct-glands were known and studied before it was known that the ductless 

 glands are secretory organs also, and the term ' gland ' has accordingly 

 seemed to belong to the former alone. This terminology is needlessly con- 

 servative; the ductless glands are as truly secreting organs as the duct- 

 glands and have as good a claim to the designation. It is unfortunate 

 that we have not terms more easily distinguished than ' duct ' and ' duct- 

 less ' : cannulated glands is a perfectly logical term for duct-glands, but 

 it is not in use. 



The simplest duct-gland is a pit or pocket in an epithelial surface, lined 

 with secreting cells. 17 This pit may be tubular in shape, whether straight 

 or coiled, or may be saccular (alveolar: acinous), i. e., pouch-like, with 

 its orifice smaller than its internal cavity. The sweat glands in the skin, 

 and many of the gastric glands in the stomach, are tubular: the only 



17 Many of the simple glands, and the small compound glands, although their duct- 

 orifices open on epithelial surfaces, lie mainly in the connective-tissue layers below 

 the epithelium. 



