154 PHYSIOLOGY 



accumulated through the whole night, it was supposed that it was 

 secreted by the dark-coloured lymphatic glands that lay about the 

 trachea ; but it certainly is not, these being truly lymphatic glands. 



Of the Natural Lubricating Fluids. 



All cavities have a fluid of some kind for the more easy motion 

 of their sides [and contents] upon one another. Some have it in large 

 quantity, as the ventricles of the brain, pericardium, joints, &c. ; some 

 have it in small quantity, as the thorax, abdomen, tunica vaginalis 

 testis, and common cellular membrane. Where it is in large quantity 

 we can judge of its nature, but where it is in very small quantity it is 

 almost impossible to judge. One way of judging is by comparing one 

 with the other in the diseased state of both ; or in that disease which 

 produces an increased quantity [of the natural fluid]. When there is an 

 increased quantity of fluid in the ventricles of the brain (excepting 

 from inflammation), the fluid is the same as when it is only natural in 

 quantity. The same [may be said] of the pericardium and of the joints. 



From thence it is reasonable to suppose that when there is an 

 increased quantity in the thorax, abdomen, tunica vaginalis testis, and 

 cellular membrane, this fluid is similar to the natural [fluid]. 



Our internal canals are passages for our secretions, as also for extra- 

 neous matters, as faeces, air, &c, in which cases they may be said to be 

 both passive and active, although [they are] probably never entirely 

 passive. They are most passive in the ureters and urethra, but princi- 

 pally the last, as also in the trachea ; and are most active in the intes- 

 tines. But even where they would appear to be least active, as in the 

 trachea and urethra, yet they are active so far as concerns their own 

 matter or themselves. Thus the urethra acts to press its secretions, or 

 any extraneous matter forwards, and I am persuaded the trachea has 

 the same power. 



It is more easy to conceive how a flexible canal, such as an intestine 

 or urethra, may have this power, when we know that they can act so 

 as to shut up the canal. Their actions are all directed one way, 

 beginning at one end ; but, as we cannot suppose that the trachea can 

 shut up its passage so closely as to compress its contents forwards, and 

 if they do come forwards and even upwards, mostly in the human sub- 

 ject, then there must be some other mode of action to effect this. That 

 the mucus of the trachea is gradually conveyed up is a fact, and the 

 only thing wanting is the mode in which it is done l . I conceive that 



1 [The action of the vibratile cilia of the tracheal mucous membrane would have 

 been a welcome spectacle to Hunter, if it could have been shown to him in a good 

 modern microscope : it would have realized his pre-conception ' of a constant action 

 of the mucous surface, tending to convey the mucus upwards.'] 



