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collected, that the chemical composition and the uses of the synovia, are 

 not precisely the same as those of the fluids secreted by the pleura or the 

 peritoneum ; and that, besides, the analogy between two objects, does 

 not constitute their identity. The human mind being naturally indolent, 

 loves to discover analogies that support it in its weakness, and that may 

 save it the trouble of seeking points of difference. I am aware, that to 

 prove that the mechanism of the synovial secretion, which exactly resem- 

 bles that of the fluid which moistens the inside of the great cavities, re- 

 quires like it but a simple membranous apparatus, it is customary to re- 

 peat, in every possible way, that Nature is scanty in her means, and la- 

 vish in her results; that she produces from the same cause, a variety of 

 different effects, Sec. but without pointing out the manifest absurdity of 

 admitting metaphysical arguments in the natural sciences, is it not much 

 more reasonable to acknowledge with philosophers, that the primitive 

 cause may vary in many ways, and that its innumerable modifications, 

 whence arise the difference in the effects, exceed the limited powers of 

 our understanding? 



CI. When a gland is irritated, it becomes a centre of fluxion, towards 

 which the fluids are determined from every part; it swells, hardens, con- 

 tracts, is in a kind of state of erection, bends on itself, and acts on the 

 blood conveyed by its vessels. Secretion, depending on the peculiar and 

 inherent power of the glandular organ, is promoted by the slight motion 

 which it receives from the neighbouring muscles. The gentle pressure 

 of those parts on the glandular organ, is sufficient to keep up their excite- 

 ment, atid to assist in the separation and excretion of the fluid. Bordeu, 

 in his excellent work on the glands and on their action, has shown that it 

 is not in consequence of the compression which is produced on them by 

 the neighbouring muscles, that they part with the fluid they have pre- 

 pared, that physiologists were therefore very much in the wrong, in say- 

 ing, that the excretion of a fluid consisted merely in its expression, and 

 in comparing, under that point of view, the glands to sponges soaked 

 with a fluid which they give out, on being squeezed. 



The excretory ducts of organs absorb or reject the secreted fluid, ac- 

 cording as it affects their inhalent mouths : these canals partake in the 

 convulsive state of the gland, undergo a degree of erection, and contract 

 on the fluid to expel it. Thus, the saliva starts from the parotid duct, 

 at the sight, or in the recollection of food that has been longed for; thus, 

 the vesiculae seminales and the urethra (for the reservoirs in which the 

 fluids lie some time before being expelled, may be considered as forming 

 apart of the excretory ducts), contract, become straighter, and lengthen 

 themselves to force to a distance the spermatic fluid. 



The thin and transparent ureters in fowls have been seen to contract 

 on the urine, which, in these animals, concretes on the slightest stagna- 

 tion. 



After remaining a certain length of time, in that state of excitement, 

 the glands relax, their tissue collapses, the juices cease to be conveyed 

 to it as plentifully, they fall into a state of repose or sleep, which restores 

 their sensibility, exhausted by too much action. It is well known, that a 

 gland over-stimulated, becomes, like any other part, insensible to the 

 stimulus, the continued appplication of which parches and exhausts it. 



From what has just been said relative to the mechanism of the secretions, 

 it will be seen that this function may be divided into three very distinct pe- 

 riods ; IstMhat of irritation, characterised on the growth of the vital pro- 



2 E 



