« ПредыдущаяПродолжить »
which the distraction of its fibres produces. He then proceeds to shew, that as, from the effects of nauseous and irritating substances, the stomach is even convulsed in consequence of its contents; that as opium, which renders our nerves and fibres insensible of irritation, allays those irregular commotions; and as a sudden and very plentiful ingestion produces nausea and vomiting, so the ordinary vermicular motions of this bowel is chiefly to be ascribed to the gentle irritation of its ordinary contents.
The similar peristaltic motion of the guts he justly attributes to the fame irritating causes, 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 lessening or destroying the peristaltic motion of the intestines, Dr. Kaau having found it extremely flow and weak in a dog, who had taken fix grains of opium, and that it was not sensibly increased by pricking the surface of the guts with a needle. That the bile is particularly necessary to render this motion complete, without which the guts would not be sufficient to overcome their distention from contained air, is very probable, from their inflation in such as die of desperate jaundices; and from the case of a patient, who died from a wound in the gall-bladder, who was incurably costive, and those intestines, after death, were excellively infiated.'
The natural exoneration of the intestines he thinks effected by the continuance and propagation of their vermicular motion, joined to the acrimony and pordzis of the Fæces irritating and distending the rectum; as its extreme irritation in a teremus is best allayed by opiates and smooth glyfters.
He considers the bladder of urine as a hollow muscle, which, having no proper antagonist, would reduce itself to its least capacity, were it not for the inftillation of urine from the ureters; the accumulation of which, by overítretching its coats, excites them into strong contractions, which are nevertheless unable to overcome the constriélion of the Sphincter, but that being opened by the affance of several other muscles, the contractile power of the bladder is then fufficient to the expulfion of its contents. He observes, the urinary fluid, however acrid, in a healthy person where the bladder is duly varnished with its natural mucus, to act more from its distending quantity, than its simple stimulation.
In his fixth section, concerning the motions of the blood • vesels, and several others of the spontaneous kind, he ascribes the dilatation of the arteries to the projectile force of the heart; and their systole to their elasticity, the contraction of their muscular coat, and the gentle stimulus of the blood affecting their internal furface. Besides this, he supposes an oscillatory motion in the smaller vessels, and secretory tubes of the giands where the force of the heart does not seem to extend, and elasticity is not concerned; but thro' which he supposes the circulation to be preserved by such 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 muscular coat irritated into such weak contractions, as may, in some measure, contribute to the circulation. As some proof of this suggestion, he remarks, that the contraction of the vena cava is visible in diflected dying animals; tho' it may prove more sensibly so, on account of some kind of alternate depletion which it fuffers. And hence we may infer, that the fluids are, in some sense, one cause of their own motion.'
After rejecting, with some other moderns, the erection of the penis from the action of the erector muscles, he suggests, that as the fight or even remembrance of grateful food is known to occafion an uncommon derivation of saliva into the mouth of a hungry person; so it is not improbable that the stimulus of the semen, the fight, or even idea of lafcivious objects, may occasion an extraordinary flow of blood thro’ the small arteries of the penis, by increasing their vibrations; from whence the red arteries will be all enlarged, and many even of the serous ones will admit red blood; the arteries, which terminate in veins, will tranfinit their fluids to them with unusual impetuosity, and those, whose orifices terminate in the cells of the penis, will effuse both lymph and red blood; which not being carried off by those absorbent veins, whose orifices are not proportionably enlarged, a distension of the corpora cavernoja, and consequently an erection of the penis, must ensue. In some extension of this hypothesis (which is very likely and analogical) to other phænomena of the body, and particularly to blushing, he very juftly commutes the supposed ftagnation of the blood in the fuperficial versels of the face (which ill agrees with the rosiness 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 ellewhere, he cannot pretend to fay; - which naturally reminds a reader of the great humility and ingenuousness of the most ingenious men, and of the honest simplicity of Horace, in his
Quae non didici planè nefcire futeri. This doctrine of a stimulus is fo obviously extendible to the actions of the organs of generation in both sexes, that we may
well dispense with a further enumeration, or even abridgement, of them.
In his seventh very curious and diffuse section, of the motions of the pupil and muscles of the internal car, after onserving the necessity of contracting and dilating the pupil, in order to diitinct vision, and an accurate description of the circular and radiated fibres of the iris, which answer those purposes, he remarks, that dilatation is the natural ftate of the pupil, as the longitudinal fibres are evidently stronger than the circular; the contracting power of which is excited by the stimulus 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 consequence of its affecting the very tender membrane of the retina ; a certain proportion of light being necessary to produce its regular function, and an extreme degree impairing it, and exciting an uneasy fenfation in it. This doctrine is confirmed by some very curious, eafy and satisfactory experiments; and accounted for from an observation, that the optic nerve, and the nervous fibrils of the uvia, arise 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 uneasy sensation in the retina, from too much light, may excite the sentient principle, ever present and ready to act at the origin of the nerves, to determine the nervous inAuence into the sphinɛter pupilla, to mitigate the offending cause, by a contraction of it; as in a fainter degree of light it ceases to act this muscle, and allows the curtain of the pupil such an aperture from the natural action of its longitudinal fibres, as admits a commodious quantity of this subtile fluid. And this difpofition, or facul. ty, of the fentient principle our author illuftrates, by the experiment of placing a lighted candle before the eyes, when, upon covering one, the supil of the other is imVOL. VI.
mediately dilated. This he observes to be inexplicable upon mechanical principles, as their nerves and blood-vessels have no other connexion, than in issuing from the same brain and the same aorta ; and the candle, the mechanical cause of its contraction, acts with undiminished force. But adinitting the contraction of the pupil from the energy of the min', in consequence of the fenfations excited in the reiina, it follows, that the mind no longer excited to contract the pupil covered from the light, and then in its natural dilated state, the pripil of the eve exposed to the light is dilated from the meer force of that constant habitude of our moving both eres alike, and contracting their pupils at the same time. For though the motions of those organs be voluntary, the mind may, according to bis eighteenth principle, lose the power of moving the voluntary muscles, except in a particular way: This principle, however, is to be admitted with some restriction; tince, though there is a remarkable uniformity between the motions of each pupil, that exposed to the light is observed to be somewhat less 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 seems that the great contraction of the pupil depends principally on the fimuides of light, as we fuppofe no one can look up at the fun, or any very luminous objeci, with a dilated pupil, whatever inclination he has to do it; for in this case the force of the stimulus must operate more potently than our contrary volition.
Dr. Whytt, efter employing some pages in pointing out the errors of some writers on the causes of the motions of the pupil, confirms his own theory by a remarkable history, and fume judicious reflections on it.
Our learned author next aflerts, that the motions of the pupil are as necesary to the distinct vision of objects at different distances, as to adapt the cye to different degrees of light. This he illustrates by several curious and easy autoptical experiments, many of which
observe in his own eyes, as probably the doctor did. "He affirms, that in order to the diftinét vifion of nicar and less luminous objects the contraction of the pupil is neceflary; and this contraction results 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
a reader may
the eye. In short, as he had before premised, the motions of the fibres of the uvea are of the mixed kind, being involuntary with regard to the stimulus of light, and sometimes moderated by the intervention of the will, tho' not always attended with a consciousness of volition.
In observing the greater dilatation of the pupil in infants, from the less transparence, and greater thickness and flatness of the cornea, through a deficience of the aqueous humour, whence the rays of light are less transmitted to the retina, he takes no notice of that exquisite membrane, the velum pupillæ, which has been demonstrated to exist in fætuffes, and which perhaps is not wholly effaced very soon after the birth, but may remain for fome little time, to defend the exquisitely tender and medullary retina from the stimulus of light.
He observes next, that in a paralysis of the longitudinal fibres of the uvea, while the circular ones, retain their usual force, the pupil will be always greatly contracted; whence the patient, being able to fee only in a grear light, will have the suspecho toode, a remarkable instance 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 in bear a great light, and being useful only in a faint one, the patient will see best in the shade, or by a candle, which is that morbid affection of the fight termed by the Greeks νυκταλοπια. .
The motions of the muscles of the internal ear follow, as a short appendage to the section of the motions of the pupil. And here our author observes, that the ear must have been incapable of diftinguishing a diversity of sounds, were not some of its parts capable of va: ious degrees of tension ; for as a musical chord of a certain length and tension can vibrate harmonically but with one particular found, if there was no mechanism, by which the membranes of the tympanuni and fenestra ovalis could be variously stretched or relaxed, they could be harmonically affected but by one sound, and have only a more or less confused perception of others. As the doctor very justly observes, it may well appear wonderful how the ears should be so adapted, by the actions of its mus. cles, to such a vast variety of founds; but with what exquisite skill and amazing wisdom, says he, is every thing in the animal frame adjusted! Here the stimulus of found on the auditory nerves excites the mind to adapt,