GLAND. 



487 



of Glisson's capsules, and, lastly, of those 

 which quit the organ. The limitation thus 

 established, and which I have no doubt is 

 strictly correct, may be applied to all those 

 glands, such as the salivary, mammary organs, 

 &c., which, like the liver, possess distant 

 lobules. In the kidney the true secreting 

 structure is probably restricted to the serpen- 

 tine tubes contained in the cortical texture, 

 (catKilct corticate*, or ducts of Ferrein,) the 

 straight canals (tub. Kclliniani) constituting 

 the cones, which bodies, in a minute injection 

 of the bloodvessels, are nearly colourless, 

 being merely for excretion.* The testis, with 

 its appendix the epididymis, presents an in- 

 tricate arrangement ; it is probable, however, 

 that the principal secreting part consists of the 

 seminal tubes which form the lobules, and that 

 the vasa recta and efferentia are merely ex- 

 cretory in their office; in the upper part of the 

 epididymis a second secreting structure is met 

 with, constituting the coni vasculosi, whilst 

 the lower part containing the convolutions of 

 the vas deferens is of the excretory character. 



Jt is proper to observe that the process of 

 secretion is incessantly going on ; but with the 

 exceptions of the mucous, sebaceous, and a 

 few other glands, the fluids produced are 

 destined to be poured out only at stated in- 

 tervals ; it is, therefore, evident that some con- 

 trivance is required, by which the several secre- 

 tions may be retained, till the moment arrives 

 when it is necessary they should be discharged. 

 The liver may be selected to illustrate this 

 principle : one of the most essential functions 

 of that organ being the decarbonization of 

 venous blood, its constant action is no less 

 indispensable, indeed considering the whole 

 animal series, is even more indispensable than 

 that of the lungs themselves; and yet the pro- 

 duct of that action, the bile, is only designed 

 to be poured into the duodenum during the 

 process of digestion. In order to obviate the 

 irritation of the bowels that would result from 

 the incessant discharge of the bile, and at the 

 same time to economise that fluid, the gall- 

 bladder is provided, which, receiving the se- 

 creted fluid in the intervals of digestion, fulfils 

 all the conditions required. The absence of 

 the gall-bladder in several classes of annuals 

 can scarcely be admitted as being incompatible 

 with this explanation ; fur the majority of 

 these instances of deficiency occur in non- 

 ruminant vegetable feeders, in scvenil i;eiir::i 

 of the Pachydermata and Uodentia for example, 

 in which it is evident that as the process of 

 digestion must occupy a considerable period, 

 a prolonged flow of bile is requisite, and a 

 special reservoir is less necessary ; in addition to 



* It is stated by Miiller that all his researches 

 induce him In conclude that the serpentine tuhnli 

 of the cortical part constitute the true secreting 

 texture; an opinion which is corroborated by a 

 very rurious preparation contained in the museum 

 of the Webb-Street School of Anatomy, in lii<-h 

 a sac has been formed on tin- oiurr sunate of the 

 kidury, containing a number of small calculi, and 

 having no connexion whatever either with the 

 straight tubes or with the iniuudibulu. 



which it is known that in some of these case*, 

 as in the horse and elephant, the principal 

 trunk of the biliary duets i> very larre, and 

 may in some degree supply the place of a gall- 

 bladder.* 



Tlie urinary bladder is a provision rather 

 of convenience than of necessity, enabling the 

 animals that possess it to retain the urine as it 

 flows from the ureters, until a considerable 

 accumulation takes place. These are the only 

 instances in the human body of a distinct re- 

 servoir being provided ;f but every gland, by 

 retaining its secretion in the excretory ducts, 

 has a power of emitting the fluid, under certain 

 circumstances, in larger quantities than usual, 

 as in the case of the salivary and lachrymal 

 glands; a similar accumulation must take place 

 in the seminal tubes and prostatic ducts, and 

 especially in the lactiferous tubes and their 

 terminal sinuses. In animals the examples of 

 distinct reservoirs are too numerous to be here 

 enumerated. 



Structure of the secreting canals and eicre- 

 tory ducts. ft is now certain that all these 

 tubes are com|x>sed essentially of a prolonga- 

 tion of the mucous membrane. The former, 

 according to Muller, consist only of a single 

 coat, but it must be presumed that they pos- 

 sess in addition a tunic, having, independently 

 of elasticity, a power of contraction by which 

 their contents are propelled often in a direction 

 opposed to gravity, and in obedience to the 

 application of a mechanical stimulus to the 

 surface on which the ducts terminate. In the 

 excretory ducts the internal membrane is sur- 

 rounded by a fibrous structure, which is very 

 apparent in some of the larger canals, and 

 probably exists in all. The fibres of this coat 

 are of a greyish white or brownish colour, and 

 are often so fine and compact that they ar 

 distinguished with great difficulty. The real 

 character of this structure is not known ; in 

 appearance there is little or no resemblance to 

 proper muscle; the action, however, of the 

 excretory canal seems to require a contractile 

 power; and Meckel states that he has dis- 

 tinctly perceived circular fibres in the vas 

 deferens, which tube is said to be distinctly 

 muscular in the bull. 



Bloodvessels. If it be recollected that the 

 arteries carry to the glands the materials of 

 their various secretions, and if the large quan- 

 tity of fluid formed by those viscera be called 

 to mind, we shall not be surprised to find that 

 with a few exceptions, such as the lungs and 

 the brain, there are no organs so abundantly 

 supplied with arterial blood. This supply is 

 in proportion to the activity of secretion, rather 

 than to the size of the gland ; thus the kidneys, 



Cams, Trait* Elem. d'An. Comp. ii. p. 269. 

 It may be proper to state that the Otter, according 

 to Daubenton, possesses the above dilatation of tho 

 duct in conjunction with a gall-bladder. 



t Some anatomists conceive that the vesicmla 

 seminales are merely receptacles of the semen j 

 but this opinion has been to a great extent aban- 

 doned in England since the observations of Hunter. 

 Si-e the works of Hunter, edit, by Palmer, vol. iv. 

 p. 20. Note, p. 26. 



