which the diftraction of its fibres produces. He then proceeds to fhew, that as, from the effects of nauseous and irritating fubftances, the ftomach is even convulfed in confequence of its contents; that as opium, which renders our nerves and fibres infenfible of irritation, allays thofe irregular commotions; and as a fudden and very plentiful ingeftion produces naufea and vomiting, fo the ordinary vermicular motions of this bowel is chiefly to be afcribed to the gentle irritation of its ordinary contents.

The fimilar peristaltic motion of the guts he justly attributes to the fame irritating caufes, with an additional irritation from the bile; and proves this from the effects of purging medicines; from pricking the guts of living animals into stronger contractions by the application of pointed inftruments, or acrid liquours: from the effects of opium in leffening or deftroying the periftaltic motion of the intef tines, Dr. Kaau having found it extremely flow and weak in a dog, who had taken fix grains of opium, and that it was not fenfibly increafed by pricking the furface of the guts with a needle. That the bile is particularly neceffary to render this motion complete, without which the guts would not be fufficient to overcome their diftention from contained air, is very probable, from their inflation in fuch as die of defperate jaundices; and from the cafe of a patient, who died from a wound in the gall-bladder, who was incurably coftive, and thofe inteftines, after death, were exceffively inflated.'

The natural exoneration of the inteftines he thinks effected by the continuance and propagation of their vermicular motion, joined to the acrimony and pondus of the Faces irritating and diftending the rectum; as its extreme irritation in a tenefmus is beft allayed by opiates and smooth glysters.'

He confiders the bladder of urine as a hollow muscle, which, having no proper antagonist, would reduce itfelf to its leaft capacity, were it not for the inftillation of urine from the ureters; the accumulation of which, by overfretching its coats, excites them into strong contractions, which are nevertheless unable to overcome the conftriction of the fphincter, but that being opened by the affiftance of feveral other mufcles, the contractile power of the bladder is then sufficient to the expulfion of its contents. He obferves, the urinary fluid, however acrid, in a healthy perfon where the bladder is duly varnished with its natural mucus, to act more from its diftending quantity, than its fimple ftimulation.

In

In his fixth fection, concerning the motions of the blood vessels, and several others of the spontaneous kind, he afcribes the dilatation of the arteries to the projectile force of the heart; and their fyftole to their elafticity, the contraction of their muscular coat, and the gentle ftimulus of the blood affecting their internal furface. Befides this, he fuppofes an ofcillatory motion in the fmaller veffels, and fecretory tubes of the glands where the force of the heart does not feem to extend, and elafticity is not concerned; but thro' which he fuppofes the circulation to be preferved by fuch vibrations of their vessels, as the gentle stimulus of the blood may excite. He conceives, that even the veins are not wholly inactive canals, but have their mufcular coat irritated into fuch weak contractions, as may, in fome meafure, contribute to the circulation. As fome proof of this fuggeftion, he remarks, that the contraction of the vena cava is vifible in diflected dying animals; tho' it may prove more fenfibly fo, on account of fome kind of alternate depletion which it fuffers. And hence we may infer, that the fluids are, in some fenfe, one caufe of their own motion.'

After rejecting, with fome other moderns, the erection of the penis from the action of the erector mufcles, he fuggefts, that as the fight or even remembrance of grateful food is known to occafion an uncommon derivation of faliva into the mouth of a hungry perfon; fo it is not improbable that the ftimulus of the femen, the fight, or even idea of lafcivious objects, may occafion an extraordinary flow of blood thro' the fmall arteries of the penis, by increasing their vibrations; from whence the red arteries will be all enlarged, and many even of the ferous ones will admit red blood; the arteries, which terminate in veins, will tranfmit their fluids to them with unufual impetuofity, and those, whofe orifices terminate in the cells of the penis, will effufe both lymph and red blood; which not being carried off by thofe abforbent veins, whofe orifices. are not proportionably enlarged, a diftenfion of the corpora cavernofa, and confequently an erection of the penis, must enfue. In fome extenfion of this hypothefts (which is very likely and analogical) to other phenomena of the body, and particularly to blufhing, he very juftly commutes the fuppofed ftagnation of the blood in the fuperficial veffels of the face (which ill agrees with the rofinefs and heat perceptible there) for a partially augmented circulation in them, from their accelerated vibrations. To this he adds, that why the affection of shame should produce

this change in the circulation, rather in the face than elfewhere, he cannot pretend to fay;- which naturally reminds a reader of the great humility and ingenuousness of the most ingenious men, and of the honeft fimplicity of Horace, in his

Quae non didici planè nefcire fateri.

This doctrine of a ftimulus is fo obviously extendible to the actions of the organs of generation in both fexes, that we may well difpenfe with a further enumeration, or even abridgement, of them.

In his feventh very curious and diffufe fection, of the motions of the pupil and mufcles of the internal ear, after obferving the neceffity of contracting and dilating the pupil, in order to distinct vifion, and an accurate defcription of the circular and radiated fibres of the iris, which answer thofe purposes, he remarks, that dilatation is the natural ftate of the pupil, as the longitudinal fibres are evidently ftronger than the circular; the contracting power of which is excited by the ftimulus of light, and augmented or remitted by the various degrees of it. This however he does not attribute to the immediate effect of light on the fibres of the uvea or iris, but in confequence of its affecting the very tender membrane of the retina; a certain proportion of light being neceffary to produce its regular function, and an extreme degree impairing it, and exciting an uneafy fenfation in it. This doctrine is confirmed by fome very curious, eafy and fatisfactory experiments; and accounted for from an obfervation, that the optic nerve, and the nervous fibrils of the uvaa, arife froin different parts of the brain, and have no communication in their progress to the eye; whence the light, that affects the retina, cannot affect the pupil from any commerce between their nerves, but the uneafy fenfation in the retina, from too much light, may excite the fentient principle, ever prefent and ready to act at the origin of the nerves, to determine the nervous influence into the sphincter pupilla, to mitigate the offending caufe, by a contraction of it; as in a fainter degree of light it ceafes to act this mufcle, and allows the curtain of the pupil fuch an aperture from the natural action of its longitudinal fibres, as admits a commodious quantity of this fubtile fluid. And this difpofition, or faculty, of the fentient principle cur author illuftrates, by the experiment of placing a lighted candle before the eyes, when, upon covering one, the pupil of the other is imVOL. VI.

mediately

mediately dilated. This he obferves to be inexplicable upon mechanical principles, as their nerves and blood-veffels have no other connexion, than in iffuing from the fame brain and the fame aorta; and the candle, the mechanical caufe of its contraction, acts with undiminished force. But admitting the contraction of the pupil from the energy of the min', in confequence of the fenfations excited in the retina, it follows, that the mind no longer excited to contract the pupil covered from the light, and then in its natural dilated ftate, the pupil of the eye expofed to the light is dilated from the meer force of that conftant habitude of our moving both eyes alike, and contracting their pupils at the fame time. For though the motions of thofe organs be voluntary, the mind may, according to his eighteenth principle, lofe the power of moving the voluntary mufcles, except in a particular way. This principle, however, is to be admitted with fome reftriction; fince, though there is a remarkable uniformity between the motions of each pupil, that expofed to the light is obferved to be fomewhat lefs than the other; though neither is as much contracted, as if both were equally affected by light.' But however our author terms the motions of our eyes' voluntary, it feems that the great contraction of the pupil depends principally on the ftimulus of light, as we fuppofe no one can look up at the fun, or any very luminous object, with a dilated pupil, whatever inclination he has to do it; for in this cafe the force of the ftimulus muft operate more potently than our contrary volition.

Dr. Whytt, after employing fome pages in pointing out the errors of fome writers on the caufes of the motions of the pupil, confirms his own theory by a remarkable hiftory, and fome judicious reflections on it.

Our learned author next afferts, that the motions of the pupil are as neceffary to the diftinct vifion of objects at different diftances, as to adapt the eye to different degrees of light. This he illuftrates by feveral curious and eafy autoptical experiments, many of which a reader may obferve in his own eyes, as probably the doctor did. He affirms, that in order to the diftinét vifion of near and lefs luminous objects the contraction of the pupil is neceflary; and this contraction refults principally from an effort of the mind, though, in the viewing of diftant objects, the dilatation of the pupil is entirely determined by the quantity of light applied to

the

the eye. In fhort, as he had before premised, the motions of the fibres of the uvea are of the mixed kind, being involuntary with regard to the flimulus of light, and fometimes moderated by the intervention of the will, tho' not always attended with a consciousness of volition.

In obferving the greater dilatation of the pupil in infants, from the lefs tranfparence, and greater thickness and flatnefs of the cornea, through a deficience of the aqueous humour, whence the rays of light are lefs transmitted to the retina, he takes no notice of that exquifite membrane, the velum pupilla, which has been demonftrated to exift in foetuffes, and which perhaps is not wholly effaced very foon after the birth, but may remain for fome little time, to defend the exquifitely tender and medullary retina from the ftimulus of light.

He obferves next, that in a paralyfis of the longitudinal fibres of the uvea, while the circular ones, retain their ufual force, the pupil will be always greatly contracted; whence the patient, being able to fee only in a great light, will have the spahoma, a remarkable inftance of which the doctor fubjoins. And on the contrary, if the circular-fibres are deprived of their energy, the pupil thence being much dilated, the eye will not be able to bear a great light, and being ufeful only in a faint one, the patient will fee beft in the fhade, or by a candle, which is that morbid affection of the fight termed by the Greeks vuntahOTIA.

The motions of the mufcles of the internal ear follow, as a fhort appendage to the fection of the motions of the pupil. And here our author obferves, that the ear muft have been incapable of diftinguifhing a diverfity of founds, were not fome of its parts capable of various degrees of tenfion; for as a mufical chord of a certain length and tenfion can vibrate harmonically but with one particular found, if there was no mechanifm, by which the membranes of the tympanum and feneftra ovalis could be variously ftretched or relaxed, they could be harmonically affected but by one found, and have only a more or lefs confufed perception of others. As the doctor very justly obferves, it may well appear wonderful how the ears fhould be fo adapted, by the actions of its mufcles, to fuch a vaft variety of founds; but with what exquifite fkill and amazing wifdom, fays he, is every thing in the animal frame adjufted! Here the ftimulus of found on the auditory nerves excites the mind to adapt,