264 SECRETION. 



variously diversified, complicated, and arranged, and collected 

 into a small space, so as to resemble vegetable forms, and be as 

 various. These all consist of a single membrane, belonging to 

 the class of the mucous ; with some of which, indeed, it usually 

 at last becomes continuous. When the conjunction of tubes has 

 produced the portion called excretory duct, one or more addi- 

 tional coats unite with the essential membrane. The secreting 

 membrane is always white, whitish grey, or whitish yellow, what- 

 ever the colour of the fluid secreted. Very different secretions 

 are produced by glands of similar structure, as by the kidneys 

 and testes; the same secretion is produced by very different 

 structures in different animals, as the saliva, bile, urine, and semen. 

 Some appear to require an immense surface to produce a given 

 quantity ; others one not of great extent. The secreting surface 

 of the vessels, which by their union form the hepatic duct, must 

 be immense : the gastric juice proceeds from the limited inner 

 surface of the stomach ; and this shows also that, for an important 

 secretion, no gland, that is, no tube, or aggregation of tubes, is 

 necessary. The saliva, on the other hand, is produced by several 

 elaborately formed glands. Complexity of gland merely implies 

 a greater extent of secreting surface ; a larger number of canals 

 being aggregated, or longer canals coiled up together. Amount 

 of secreting surface is, of course, proportionate, not merely to 

 the complexity of the tubes, but to the bulk of the whole organ. 

 The most elaborate fluids, as the semen, bile, urine, are pro- 

 duced by the most complicated glands, that is to say, such 



fol. ; and published likewise elsewhere,) considered the miliary globules, which 

 are easily discoverable in most glands, as acini, according to his expression, 

 internally excavated," and having arteries open into them and excretory ducts 

 begin from them to carry off the fluid first poured forth from the arteries and then 

 fully elaborated in the acini. " Ruysch, on the contrary, contended that these 

 supposed hollow acini were nothing more than glomerules of blood-vessels" which 

 were continued into the excretory ducts. The acini are merely the blind extre- 

 mities of ducts, and the blood-vessels, conglomerated into granules, as we saw 

 when considering the liver. Duverney (Comment. Ac. Sc. Petrop. 1750.) 

 showed that the lactiferous ducts of the hedgehog began as vesicles arranged like 

 a bunch of grapes. Mascagni and Cruikshank afterwards demonstrated the 

 same in the human breast, and Mascagni admitted that the excretory ducts began 

 as blind extremities and had no arteries opening into them. He assumed the 

 existence of inorganic pores. Dr. Mueller justly proposes to banish the term 

 acini altogether, and speak only of the elementary parts of glands or blind be- 

 ginnings of the secreting tubes. 



