260 SECRETION. 



excretory duct. Anatomists formerly supposed that the ducts of 

 glands were, like veins, merely prolonged arteries, except, of 

 course, where, as in the case of the liver, the secretion takes 

 place by a vein. But Dr. Mueller, of Bonn, in a most elaborate 

 work e , in which he demonstrates this to be the structure of all 

 glands in all animals, contends that the arteries ramify on the 

 inner surface of the canals as upon any membranes, and, after 

 forming a network, terminate in veins ; so that the fluid secreted 

 passes into the secretory ducts, which are in truth all excreting, 

 just as we see it poured upon serous and synovial membranes. 

 The excretory ducts of glands, therefore, precisely resemble the 

 trachea, which divides and subdivides till it ends in blind twigs 

 of extreme minuteness. Indeed, the extremities of the twigs of 

 the ducts of the salivary glands of some animals are enlarged 

 into a globular form, so that the woodcut which illustrates the 

 air cells at the extremities of the bronchial twigs, might be 

 taken for a delineation of the extremities of the salivary ducts 

 of the sheep, for example, as may be seen in Dr. Mueller's sixth 

 plate. Thus it would appear that all secretion is of that kind 

 to which old physiologists gave the name of diapedesis or 

 transudation, in which the fluid is supposed to be merely strained 

 through the sides or open mouths of the vessels, and upon which 

 Blumenbach remarks, that " physiologists have lately given 

 different explanations of this mode of secretion. Some assert 

 that every fluid is formed by passing merely through inorganic 

 pores from the blood : others altogether deny the existence of 

 these pores. I think much of this is a verbal dispute. Because, 

 on the one hand, I cannot imagine how inorganic pores can be 

 supposed to exist in an organised body, for we are not speaking 

 here of the common interstices of matter, in physics denominated 

 pores; and I am persuaded that every opening in organised bodies 

 is of an organic nature, and possesses vital powers exactly corre- 

 spondent. On the other hand, these openings or pores, which 

 indisputably exist in the coats of vessels, I think but little differ- 

 ent, in function at least, from the cylindrical ducts through which 

 fluids are said to percolate in conglomerate glands and secreting 

 viscera : for this percolation depends less on the form of the 

 organ than on its vital powers? f But Dr. Mueller asserts that 



e De glandularum secernentium structura penitiori earumque prima forma- 

 tions in homine atque animalibus. Lipsiae. 1830. folio. 



f " Consult, among others, Schreger, Fragmenta, p. 37. sq. already recom- 

 mended. 



