find a contrivance to prevent regurgitation. In fome cafes valves are used; in other cafes, amongst which is that now before us, a different expedient is reforted to: which may be thus defcribed. The gall duct enters the duo, denum obliquely after it has pierced the first coat, it runs near two fingers breadth between the coats, before it open into the cavity of the inteftine *. The fame contrivance is used in another part, where there is exactly the fame occafion for it, viz. in the insertion of the ureters in the bladder. These enter the bladder near its neck, running obliquely for the space of an inch between its coats t. It is, in both cafes, fufficiently evident, that this ftructure has a necessary mechanical tendency to resist regurgitation; for whatever force acts in fuch a direction as to urge the fluid back into the orifices of the tubes, muft, at the fame time, stretch the coats of the veffels, and, thereby, comprefs that part of the tube, which is included between them.

IV. Amongst the vessels of the human body, the pipe which conveys the faliva from the place where it is made, to the place where

*Keill's Anat. p. 62.
9

+ Chef. Anat. p. 260.

it is wanted, deferves to be reckoned amongst the most intelligible pieces of mechanism with which we are acquainted. The faliva, we all know, is used in the mouth; but much of it is manufactured on the outfide of the cheek, by the parotid gland, which lies between the ear and the angle of the lower jaw. In order to carry the fecreted juice to its deftination, there is laid from the gland on the outside, a pipe, about the thickness of a wheat straw, and about three fingers breadth in length; which, after riding over the maffeter muscle, bores for itself a hole through the very middle of the cheek; enters by that hole, which is a complete perforation of the buccinator mufcle, into the mouth; and there discharges its fluid very copiously.

V. Another exquifite ftructure, differing indeed from the four preceding inftances, in that it does not relate to the conveyance of fluids, but ftill belonging, like these, to the clafs of pipes or conduits of the body, is feen in the larynx. We all know, that there go down the throat two pipes, one leading to the ftomach, the other to the lungs; the one being the paffage for the food, the other for the breath and voice: we know alfo that both

these

thefe paffages open into the bottom of the mouth; the gullet, neceffarily, for the conveyance of food; and the windpipe, for fpeech and the modulation of found, not much less fo: therefore the difficulty was, the paffages being fo contiguous, to prevent the food, efpecially the liquids, which we fwallow into the ftomach, from entering the windpipe, i. e. the road to the lungs; the confequence of which error, when it does happen, is perceived by the convulfive throes that are inftantly produced. This bufinefs, which is very nice, is managed in this manner. The gullet (the paffage for food) opens into the mouth like the cone or upper part of a funnel, the capacity of which forms indeed the bottom of the mouth. Into the fide of this funnel, at the part which lies the lowest, enters the windpipe, by a chink or flit, with a lid or flap, like a little tongue, accurately fitted to the orifice. The folids or liquids which we swallow, pass over this lid or flap, as they defcend by the funnel into the gullet. Both the weight of the food, and the action of the mufcles concerned in fwallowing, contribute to keep the lid clofe down upon the aperture, whilft any thing is paffing; whereas, by means

of

of its natural cartilaginous fpring, it raises itself a little, as foon as the food is paffed, thereby allowing a free inlet and outlet for the refpiration of air by the lungs. And we may here remark the almoft complete fuccefs of the expedient, viz. how feldom it fails of its purpofe, compared with the number of inftances in which it fulfills it. Reflect, how frequently we swallow, how conftantly we breathe. In a city feast, for example, what deglutition, what anhelation! yet does this little cartilage, the epiglottis, fo effectually interpofe its office, fo fecurely guard the entrance of the windpipe, that, whilst morfel after morsel, draught after draught, are courfing one another over it, an accident of a crumb or a drop flipping into this paffage, (which nevertheless must be opened for the breath every fecond of time,) excites, in the whole company, not only alarm by its danger, but furprise by its novelty. Not two guests are choked in a century.

There is no room for pretending, that the action of the parts may have gradually formed the epiglottis: I do not mean in the fame individual, but in a fucceffion of generations. Not only the action of the parts has no such

tendency,

tendency, but the animal could not live, nor confequently the parts act, either without it, or with it in a half formed state. The species was not to wait for the gradual formation or expansion of a part, which was, from the firft, neceffary to the life of the individual.

Not only is the larynx curious, but the whole windpipe poffeffes a ftructure, adapted to its peculiar office. It is made up, (as any one may perceive by putting his fingers to his throat) of ftout cartilaginous ringlets, placed at small and equal distances from one another. Now this is not the cafe with any other of the numerous conduits of the body. The ufe of these cartilages is to keep the paffage for the air conftantly open; which they do mechanically. A pipe with foft membranous coats, liable to collapse and close when empty, would not have answered here; although this be the general vascular structure, and a ftructure which ferves very well for those tubes, which are kept in a state of perpetual diftenfion by the fluid they inclose, or which afford a paffage to folid and protruding fubstances.

Nevertheless, (which is another particularity well worthy of notice,) these rings are not complete,