My dear Darw⁠⟨⁠in⁠⟩⁠

Any information I m⁠⟨⁠ay⁠⟩⁠ be able to supply is heartily a⁠⟨⁠t⁠⟩⁠ your service—² The difficulty is to speak absolutely of inside & outside where the conditions we are in the habit of observing occur under such very complicated forms— If a particle of dust irritates the front of the eye, the eye waters— it also reddens—and becomes to some extent intolerant of light—that is, the bloodvessels of the lacrymal gland are dilated & filled by a reflection of the stimulus through their nerves & through the lacrymal branch ⁠⟨⁠    ⁠⟩⁠ nerve (of common ⁠⟨⁠    ⁠⟩⁠)— the vessels of the irritated surface of the globe & lids also fill— Then the whole ciliary region within the globe—or that concerned in the accommodation of the Eye to distances, and in the alterations of the iris under varying light, is excited, the ordinary play of the accommodative movements is disturbed, the pupil contracts— The retina too sympathizes & cannot bear the ordinary light— The effort of looking is painful & difficult, perhaps even the passive reception of an image on the retina is a source of animation ⁠⟨⁠&⁠⟩⁠ exc⁠⟨⁠it⁠⟩⁠ement⁠⟨⁠.⁠⟩⁠ With these disturbances of the ciliary ap⁠⟨⁠p⁠⟩⁠aratus of the retinal fu⁠⟨⁠n⁠⟩⁠ction, ⁠⟨⁠it is⁠⟩⁠ highly probable that the ⁠⟨⁠    ⁠⟩⁠ of these internal parts are exci⁠⟨⁠  ⁠⟩⁠ ⁠⟨⁠  ⁠⟩⁠ted.

No dou⁠⟨⁠bt⁠⟩⁠ ⁠⟨⁠the⁠⟩⁠ degree of these acces⁠⟨⁠sor⁠⟩⁠y disturbances of tissues & structural arrangements within the eye ball are liable to vary in accordance with the intensity & persistancy of the external irritation & with the part of the conjunctival surface touched—also with the character of the irritation whether in the nature ⁠⟨⁠of⁠⟩⁠ scratching, cutting, rubbing &c—i.e. I suppose the minute mechanical differences in the impact of the irr⁠⟨⁠i⁠⟩⁠tant on the nervous tissue ⁠⟨⁠    ⁠⟩⁠ w⁠⟨⁠h⁠⟩⁠ich receive the imp⁠⟨⁠re⁠⟩⁠ssions.

⁠⟨⁠The⁠⟩⁠ most common cause ⁠⟨⁠of⁠⟩⁠ e⁠⟨⁠xcess⁠⟩⁠ive lacrymation is that ⁠⟨⁠    ⁠⟩⁠ meet with in unhealthy children when the surface of the cornea is affected by a small ulcer—or raised speck following an ulcer, when it has existed for some time & unhealthy vessels form in it. With this there is intense photophobia the lids being forcibly closed against the least admission of ⁠⟨⁠light⁠⟩⁠, and any attempt at opening them, or even of exposing them when closed to ordinary daylight being attended by a profuse flow of tears.

Here the lacrymation ⁠⟨⁠is a⁠⟩⁠ssociated with a peculiar heightening of the sensibility of the retina to light— the child will scream violently if exposed to it and I think it is probable that the ciliary region as well as the retina must be unduly vascular, although such excitement does not rise to inflam^tn. or do these parts cause permanent damage, or even alter their structure temporarily. It was what we call functional disturbance—yet it seems necessary as a factor when this great lacrymation exists.

Now there are very numerous other morbid states & structural alterations of the same surface or conjunctival membrane, and even very terrible inflammations of it which may be attended with little or no lacrymation— or the same inflammation may be attended at one time with much watering, at another time with none at all,—as we say, the excitement of the lacrymal apparatus may accompany or not various forms and stages of such inflammations.

Again a somewhat undue secretion of tears very often follows the habitual strain of accommodation & the sustained effort to distinguish small print of which the retinal image is indistinct, and the too bright illumination of the small objects looked at, when persons who ought to begin convex glasses to aid their failing accommodation, omit their use. Attending this is commonly an undue contraction of the pupil which is apt to become rigidly small. The retinæ also here are liable to become too sensitive, light to be distressing, es⁠⟨⁠pe⁠⟩⁠cially gas or artif^l light—& much more to the same effect might be said.

With respect to the influence of morbid states of the internal tissues of the eye on the lacrymal secretion, it is a very large subject— Speaking generally, affections of the retina and of the choroid, and of the sclerotica, are not attended with so much liability to lacrymation, as those of the conjunctiva & corneal surface and accommodative apparatus. Mere undue hardness of the eyeball not rising to inflammation, but implying a want of balance betⁿ the fluids poured out from the intraocular vessels & the fluids they take ⁠⟨⁠u⁠⟩⁠p again & carry away—does not usually cause any lacrymation or intolerance of light— When the balance is on the other side & the eye soft, there is more tendency to lacrymation and also to intolerance. The most destructive changes may go on in the retina & optic nerve without tears flowing—and so of many other parts—but when intolerance of light is an accompinment of any eye tissue inflamed or irritated, I think lacrymation is more likely to occur.— Still I agree that some excitement by too strong light is not following in any special way by lacrymation—unless indeed the exposure is followed by vascular excitemen⁠⟨⁠t⁠⟩⁠ & persistent intolerance to light.— Then the eyes water as a part of the result. The conjunctiva in such cases may have become injected, & from this injection & irritation, the lacrymal apparatus may have taken up its irritation in turn.—

The conjunctive surface you must bear in mind is essentially a part of this lacrymal surface—& I suppose that all parts of this surface are in special association with each other by its nerves. Irritation of the nipple by the suctiⁿ. of the infant excites the mammary secretion—so excitement of the conjunctival surface, which may be regarded as an open expansion of the excreting lacrymal channels, excites the lacrymal secretion.

I am afraid I have been dreadfully prolix, but you see I am used to look at these things from the point of view of the practitioner—partly also (but I am afraid less of late years) from that of the Anatomist & Physiologist & one’s powers of observation are applied in that direction & are too apt to be narrowed & limited to that— I should like much to try to observe some of the matters from your point of view if I could   I am an entire believer in the doctrine of development—being satisfied that slight as is our present proof of it, to what it will be hereafter, it is in that direction that all true knowledge will expand & grow among the infinite details of organized structures—& if you will let me know what you want me to try to observe I will gladly be a student to help you in my little field.— I am wrong in saying little, for no doubt it is immense, but in the impact which the wide sphere your genius is able to embrace—

Yrs very tr⁠⟨⁠uly⁠⟩⁠ W Bowman