tion than that here the nervous stimulus passes from one element to the other. It appears to me to be at least worth while to throw out the suggestion that the nervous energy resembles a static electrical charge, in the fact that the discharge takes place most readily through points. Wherever nerve endings have been.demonstrated, the breaking up into finer and finer branches, which end freely, has been shown to take place.

On the view suggested, each nerve element resembles an electrical condenser capable of charging itself, and being suddenly discharged by an appropriate stimulus. It is interesting to note what would happen upon some such theory as this, supposing one of the elements D to be stimulated in any way. Imagine, for instance, the element D Thorax IX to be caused to discharge, either by an impulse from a sensory nerve, or from the brain. The main discharge would, we must suppose, pass into one of the motor elements such as E, by means of the fine branches which both send to the neuropile, and the muscle innervated by that particular fibre would be stimulated. A portion of the charge would, however, pass to the lateral and terminal tufts of D Thorax IX, and we may suppose that in this way D Thorax VIII and D Thorax X are made to discharge, as well as C Thorax VI. If we suppose the C elements to influence some other motor element in the neuropile, say F, then it will be seen by following out the result in the figure, that all the E and F elements upon one side of the thorax would be stimulated by the single stimulus to D Thorax IX.

With regard to elements of the class A, which start from a cell in one of the anterior ganglia and send a fibre down the cord, the fact that they give off collateral branches to the neuropile of each ganglion would seem to indicate that they in some way control an element, which also sends branches into that particular ganglion, and the suggestion would be that by their means a series of elements are stimulated all along the body by an impulse from the brain. On the other hand, by means of elements B, a particular ganglion would be placed in direct communication with the brain. This communication would be independent or correlated with a stimulus to (or from) all the ganglia through which the fibre passes, according to whether the absence of collaterals is the true condition of the element, or is due merely to imperfect staining.

A similar consideration will apply to the giant fibres (A(a) Br). It has been already stated that no collaterals have ever been observed on these fibres. According to Retzius branches go directly from their ends to the nerves of the last abdominal ganglion, and it, therefore, seems probable that they serve the purpose of putting some organ into direct communication with the brain. The most obvious suggestion would be that it is by their means that the muscles of the tail-fin, the steering apparatus of the animal, are controlled by the brain. There

is, of course, the possibility that the other fibres, which have been followed all along the cord to the last abdominal ganglion, also send branches through the nerve roots of this ganglion and serve a similar purpose. The presence of collateral branches upon them seems to me to be opposed to this view. The problems, however, suggested by the foregoing remarks can only be finally solved by means of physiological research.

My observations were made in the laboratory of the Marine Biological Association at Plymouth, with the assistance of a grant made by the Government Grant Committee. I hope shortly to publish a more detailed account with fuller illustrations.

V. "The Refractive Character of the Eyes of Horses." By Veterinary-Captain F. SMITH, F.R.C.V.S., F.I.C., Army Veterinary Department. Communicated by Professor MCKENDRICK, F.R.S. Received March 7, 1894.

The eyes of the horse are of great physiological interest, for there are certain features in connection with them not found in other animals.

While equine vision is principally monocular, it is quite undoubted that the horse can see objects situated to the right and left of his body at one and the same time; it is also equally certain that by directing both eyes forward, and producing a powerful internal squint, vision may be rendered binocular for objects situated directly to the front.

The corresponding points in the retina of the human eye, do not hold good in the case of animals which have their eyes situated laterally in the head and at some distance apart.

The pupil of the horse is a horizontal slit, which in full sunlight becomes so contracted that, with the presence of the corpora nigra, it is difficult to understand how sufficient light finds its way to the retina. A monocular retinal image in full sunlight must be a broken or imperfect one, for, filling up the centre of the horizontal pupil, are some large, black, soot-like bodies, the corpora nigra just referred to. These are attached principally to the superior pupillary margin of the iris, and completely block out the centre of the pupil when the latter is closely contracted. The horse is, I believe, the only animal possessing corpora nigra, and the function of these bodies is quite

obscure.

It is essential that the horizontal pupil should always be kept horizontally placed, no matter what position the head may occupy ; every upward and downward movement of the head necessitates a rotation of the eyeballs to ensure the pupils remaining horizontal,

and this rotation is produced by the oblique muscles of the eye. As the horse can move its head vertically through an angle of about 90°, (the pupil all this time remaining horizontal) it can be seen how important is the function of these oblique muscles.

Though in direct sunlight the pupil closely contracts, yet in ordinary artificial light it dilates. This phenomenon can be turned to practical advantage in an ophthalmoscopic examination. Further, I have found that if daylight be thrown into the eye by means of a mirror the iris does not, as a rule, contract; this may be proved by placing a horse in a stable and excluding daylight from all but one source, and utilising that source to illuminate the eye by means of a mirror; a complete examination of the fundus can now be made, owing to the dilatation or, at any rate, non-contraction of the pupil.

For the last two years I have used no artificial light in my ophthalmoscopic work, and in the enquiry I am about to detail the whole was carried out in broad daylight. I have met with cases where the pupil has contracted on throwing a beam of daylight into the eye; such do not amount to more than 6 per cent. or 8 per cent. of the eyes examined.

Horses no doubt possess the power of accommodation, but I have never been able to satisfy myself that the instillation of atropine paralyses this power as it does in man; in fact, I think I can go so far as to say that in many cases no action is produced by this drug on the ciliary muscle, though the pupil be widely dilated. small proportion of cases I believe I have been able to observe some defect in accommodation under atropine.

In a

My attention to this point was especially directed by a remark made by Messrs. Lang and Barrett* to the effect that a cat, the pupils of which they had thoroughly dilated with atropine, had no difficulty in pursuing and catching a mouse. Though it is difficult to apply such a satisfactory test to the horse, yet I have adopted measures which should, undoubtedly, have demonstrated any paralysis of accommodation had such existed.

The physiological pecularities of the horse's eye, which I have ventured briefly to draw attention to, induced me to examine the nature of the refraction, especially as a knowledge of it was calculated to be of practical importance and utility.

Disorders of vision are very common amongst horses; the most careful examination of such eyes often fails to reveal any pathological condition, but I think it will be possible to show that the nature of their refraction is calculated to throw light on an obscure condition. We must accept it as practically correct that disorders of vision in

"The Refractive Character of the Eyes of Mammalia," by W. Lang, F.R.C.S., and J. W. Barrett, M.B., Royal London Ophthalmic Hospital Reports,' vol. 11, Part II.

horses commonly exhibit themselves by "shying," a vice which may be attended by considerable danger. I shall hope to show that there is now a reasonable prospect of determining the cause of "shying” in the absence of pathological changes in the eye; further, that in that important branch of the veterinary art-the examination of horses for soundness-it should be possible to detect the kind of eye against which an intending purchaser should obtain a special warranty; and, lastly, as it is possible not only to determine the nature of the refraction, but also the amount of error which exists, we shall be able-when the needful public prejudice is overcometo prescribe glasses for horses with every reasonable prospect of improving their vision.

This is not the first time the refraction of the horse's eye has been examined. Professor Berlin, of Stuttgart, published two papers en the subject,* of these I have only been able to consult the second one. About the same time J. Hirschberg published his observations on the refraction of horses' eyes.t

In this country Messrs. Lang and Barrett, to whose paper I have already alluded, published an account of the refraction of the eyes of auimals, including the horse. It is from their paper that I obtained two of the above references.

These observers obtained results which I have not been able to verify, and I think the reason of this admits of easy explanation:In the case of Berlin the refraction was obtained by a direct examination of the fundus of the eye with the ophthalmoscope, a procedure the result of which appears to depend very largely on the refraction of the examiner, and his capacity for relaxing his accommodation.

In Lang and Barrett's excellent work the refraction was measured by means of "retinoscopy." They devoted their attention to the eyes of mammalia generally, but the number of horses tested by them was too small to admit of any general deductions being drawn.

According to Berlin, the majority of horses are slightly hypermetropic; he only met with eight cases of myopia in as many years. The amount of hypermetropia, according to this observer, is small, as a rule, lying between 1 D and 2 D. The largest amount found by him was 2.25 D.

"Refraction und Refractions-Anomalien von Thieraugen," Tagblatt der 52 Versammlung Deutscher Naturforscher und Aertze in Baden-Baden,' p. 317. (Extract in Nagel's Jahres-Bericht.')

"Ueber den Physikalisch-optischen Bau des Pferdeauges," Zeitschrift für vergleichende Augenheilkunde,' 1882, Heft I.

+ "Zur vergleichenden Ophthalmoscopie," Vortrag gehalten in der Berliner Physiologischen Gesellschaft am 10 Feb., 1882; ' Archiv f. Anat. und Phys.,' 1882; 'Phys. Abth.,' Heft 1 and 2, p. 81.

Of myopia, Berlin found in one case as much as 3 D, and in another, 2.75 D.

Hirschberg, adopting the same method of enquiry as Berlin, viz., direct examination of the fundus with the ophthalmoscope, found a considerable amount of astigmatism in the eyes of horses.

Lang and Barrett, in the six eyes they examined, found one was emmetropic, three hypermetropic and astigmatic, and two had slight mixed astigmatism. The astigmatism only once equalled 1D, and the horizontal was always the least curved meridian.

In the enquiry I am recording, the number of horses examined was 54; these 54 horses, from one cause or another, furnished me with 100

eyes.

Of the 54 animals, 31 were mares and 23 were geldings; their ages varied from 4 years to 16 years; the largest number of horses of one age (5 years) was 13, 7 horses were 6 years old; 7 horses 7 years, 6 horses 8 years; the remaining numbers for each age varied from 1 to 4.

Before recording the results of this enquiry,. I think it necessary to describe the method by which retinoscopy is carried out on the horse.

It has been shown by Messrs. Morton and Barrett* that the results of retinoscopy are liable to considerable error unless practised at the right portion of the fundus.

The observations made by these and other observerst are of the greatest value, for they have drawn attention to the difference in the reflex obtained in the human subject at the yellow spot and at the optic disc, and have settled that retinoscopy is only reliable when practised at the visual axis of the eye.

In the horse I found very early in the enquiry, that confusing and conflicting results might be obtained with the same eye when retinoscopy was practised at different parts of the fundus, for the reflex given at the optical axis was not the same as that obtained when the fundus was looked at obliquely. We have no knowledge whether the optical axis and visual axis of the horse are identical-for the purpose of this enquiry it has been assumed that they are.

The horse has no yellow spot, so that a valuable guide is lost in determining the position on the fundus at which to judge the reflex; on the other hand, the error of taking the reflex at the disc is not likely to occur, owing to the difficulty of seeing the disc with the head in the ordinary position.

* "A Clinical Investigation of the Methods of Practising Retinoscopy," 'British Medical Journal,' 16th January, 1886. See also 'Refraction of the Eye,' A. S. Morton, M.B., F.R.C.S.

The Shadow Test in the Diagnosis and Estimation of Ametropia.' W. B. Beaumont.