892 THE EXTERNAL SECRETIONS 



of their product. The material required for the formation of the 

 secretion, is derived from an intricate system of capillaries situated 

 in the immediate vicinity of their walls. These cells are arranged in 

 groups forming the so-called acini. By combining many of these acini 

 we obtain a lobule. Several lobules constitute a lobe and several 

 lobes the gland as a whole. In general, it may be said that glands 

 are either tubular or racemose in character and may be either simple 

 or compound. As an example of a simple tubular gland, we might 

 mention the sweat glands of the skin or the crypts of Lieberkiihn 

 of the small intestine; and as an example of a compound tubular gland, 

 the glands of the pyloric end of the stomach or those of the tongue 

 or uterus. Simple racemose or alveolar glands are those furnishing 

 the sebaceous material for the skin, and compound racemose glands 

 those furnishing the saliva. Some glands, such as the pancreas, are 

 of a mixed type, combining some of the characteristics of the tubular 

 with those of the racemose variety. They are called tubulo-racemose 

 glands. 



The Factors Concerned in the Formation of a Secretion. — It was 

 formerly believed that secretions and excretions are the products of 

 a process of filtration. It was conceived that the different cells of the 

 alveoli form a passive membrane, through which the blood plasma 

 percolates from a place of high pressure to a place of low pressure. Ob- 

 viously, this mechanism may be represented in a plastic manner 

 by adjusting a glass funnel lined with filter paper above a beaker. 

 The solution poured upon this paper takes the place of the blood, because 

 some of its constituents are forced by pressure through the paper into 

 the receptacle underneath. In accordance with the pure filtration 

 theory, the differences in the character of secretions are the result of 

 variations in the structure and chemical properties of the dialyzing 

 membrane and not of an active metabolism of its cellular constituents. 

 Later on this theory was modified by the addition of the factors of 

 osmosis and diffusion, but even in this case, the epithelium remains 

 a passive membrane through which these osmotic interchanges between 

 the blood and the secretion are effected in accordance with ordinary 

 physical laws. 



These factors which have been combined by Ludwig and his pupils, 

 into the so-called mechanistic theory of secretion, were soon found to 

 be inadequate, because they failed to explain many of the phenomena 

 connected with this process. Thus, it was found that the histological 

 picture of the resting gland is widely different from that of the active 

 gland, and that in many cases the constituents of the cells could be 

 traced directly into the ducts. This was followed by the discovery 

 of distinct secretory nerves, and lastly, by the observation that glandu- 

 lar processes may also be markedly influenced by chemical means 

 and frequently furnish a product which is not present in the blood. 

 All these data were eventually combined into the so-called chemical or 

 vitalistic theory of secretion, the chief advocate of which was Heidenhain. 



