456 SECRETION. [CHAP.XXXII. 



certain elements adapted for their growth ; but in secretion, these 

 separate products, after having probably undergone some changes 

 in the cell, are destined soon to be cast off, and no longer form an 

 integral part of the organism ; while, in nutrition, these elements 

 serve to keep up the integrity of the structure by which they have 

 been appropriated. It may be remarked, that the period of ex- 

 istence of the secreting cell, or the length of its life, varies consider- 

 ably in different organs. Sometimes a very short time is sufficient 

 for its development, growth, and decay; while in other instances, it 

 may persist for a long period of time. Some of the most important 

 modifications of secreting cells will be described under the head of 

 the glands, into the composition of which they enter. 



Of the Ducts of Glands. Ducts are tubular, usually branched 

 passages, forming continuations between the secreting tissue of the 

 glands and the surface of skin, or mucous membrane, on which 

 the secretion is eventually poured. They are a contrivance by 

 which a large mass of secreting tissue, packed in a small solid space, 

 can discharge its product at a given point. They often present 

 dilatations, wherein the secretion may be delayed for a time, and 

 where, in many instances, it may undergo changes; and they very 

 commonly contain a layer of unstriped muscle in their walls, by 

 whose agency their contents are propelled in the proper direction, 

 often against gravity. This contractile tissue is sometimes de- 

 veloped at their orifice into a sphincter muscle. It is only the 

 minuter glands which open directly on a surface, (e. g., those of ! 

 Lieberkiihn or Grew, in the intestinal mucous membrane), which 

 are without a duct. 



The walls of ducts present, most internally, an epithelium and 

 basement tissue, continuous with those of the skin or mucous 

 membrane. Outside, there are (when present), the muscular coat 

 and the areolar tissue, which is prolonged upon their exterior in 

 a modified form. Bloodvessels and nerves ramify upon and within 

 the latter coats. We owe the best description of the anatomy of 

 ducts to Henle. 



The epithelium may vary from the scaly to the columnar or 

 glandular variety, and it is sometimes ciliated. The most common 

 is the columnar, particularly in large ducts. In immediate con- 

 nexion with the proper mucous tissue, comes the muscular element, 

 arranged in two sets of fibres, one longitudinal, the other circular, 

 of which, the latter can usually be seen to be the more internal. 

 The fibrous elements are commonly nucleated, and resemble those 

 of the muscular coats of bloodvessels; then follows areolar tissue, 



