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too sharp. At the same time reach forward with the point of the needle and pierce the suspensory ligament and iris in order to open the aqueous chamber. In doing this, care is taken not to injure structures in the plane of the desired sections. A cannula of suitable size, being connected with a siphon from A or A', is filled with the liquid and inserted. The cannula should have a fine smooth point. Great care is taken in inserting it so that the stream of fluid is not directed behind the retina to float it off. A hole is now made in the opposite side of the eye, the aqueous chamber again pierced and all aqueous and vitreous humor allowed to run out. In some animals this humor is very much more gelatinous than in others, and requires much more pressure to remove it. The hole below is then stopped with a small glass plug (Fig. 2, B), and the eye immersed in hardening fluid (Fig. 1, B). The bottles are now covered as tightly as possible with tinfoil to prevent evaporation and entrance of dust particles. The cannula and stopper should fit so tight that there is no leak. In every case the orientation of the eye is marked before it is removed from the

B

FIG. 2.

Pig Hol

19 Cammula above Ivory outal See Right

head. This is done by sewing a small tag to the outer layers of the sclerotic (Fig. 2, C).

The pressure varies greatly with the kind of eye used. Those with thin walls, or containing much cartilage, birds and amphibians, require little pressure, while mammals, in general, can receive much higher. The pressures which I have found to work best vary between 28 and 36 cm.

The hardening fluid used is Perenyi's, in which the eye is allowed to remain twenty-four hours, when it is changed to 70 per cent. alcohol.

In making changes of liquids, great care should be taken that no air get into the eye, and that all the former liquid is

replaced with fresh by removing the stopper in the lower part of the eye. After remaining twenty-four hours in each of the following liquids: 80, 90, 95 per cent., absolute alcohol and absolute ether (1 part each), it is then changed to celloidin. Best results are obtained when three grades of celloidin are used-1st, very dilute; 2d, less dilute; 3d, as thick as will run. It is allowed to remain from four to six days in the first, six to eight days in the second, and ten to fifteen days in the third. If the eye is kept well under pressure throughout this process, the retina will be well preserved and lie smoothly against the choroid.

I have tried other liquids for hardening the eye whole, but with poor success. Have tried the method of Barrett and of Cuccati, but, in each case, the retina was very much wrinkled and folded, while the whole eye was much shrunken and out of shape. In vapors of osmium, I have had fairly good results with the retina, but the same trouble, due to the shrinking of the whole eye, is present. Chievitz says that a fish's eye may be preserved whole, with retina lying nicely back, by simply immersing it, or even the whole head, in 80 per cent. alcohol. The hardening agent which he generally uses is 2.5 per cent. nitric acid.

Another method which I have employed with small animals, especially birds, in order to demonstrate quickly the presence or absence of a fovea, is to immerse the whole head in Perenyi's fluid for from three to five hours. This will harden the eyes so that the. cornea, lens and vitreous humor may be removed, leaving the posterior half in situ. With birds I have had good results, the retina lying back smoothly so that the fovea and entrance of the nerve, marked by the pecten, may be easily seen. Fig. 3 represents diagrammatically the appearance of the retina after the front of the eye has been removed.

In order to show the angles which the lines of vision make with the median plane, sections were made through the whole head of several animals (fish, amphibians, reptiles, birds and

J. H. Chievitz, Untersuchungen über die Area centralis retinae. Archiv für Anatomie und Entwickelungsgeschichte, Sup., Band, 1889, p. 141-142.

small mammals), the plane of the section passing through each fovea on the centre of the area centralis. Fig. 4 represents such a section through the foveæ a and b of a chickadee's head (Parus atricapillus), while the lines G H and G I show

FIG. 3.

Snow-bird (Junco hyemalis) x 3.

A, Fovea centralis.

B, Entrance of optic nerve.

P, Pecten.

FIG. 4.

Chickadee (Parus atricapillus) x 3.
A and B. Foveæ.

C, C, Entrance of optic nerves.
G H and G I, Axes of vision.

the axis of vision. The dotted lines c mark the position of the optic nerves which enter in a plane much lower down. In order to harden the whole head, and, at the same time, decalcify the bone, it must remain longer in Perenyi's fluid (about thirty-six hours), and to preserve the cornea and lens in position, a window is made in the top of the eye that the fluids may enter.

Having had good success with simple immersion of the head, this method was tried for hardening the small eyes, and with good success. In fact, the retina proved in good condition, if not better, than when taken through by the injection. method. The eye-ball, however, usually caves in when placed in 70 per cent. or 80 per cent. alcohol, but this may be prevented by simply making a small slit through the sclerotic

into the vitreous chamber before immersing in 70 per cent. alcohol to allow the liquids to pass in. Just before putting into celloidin, a window is made parallel to the plane of desired sections, and the hardened vitreous humor is easily removed without injury to the retina or other structures. This method is now used with small eyes instead of the injection, as it is so much easier of manipulation.

In order to show the relation of the retinal arteries to the area and fovea centralis, they were injected with the gelatine.carmine mass of Ranvier. In small animals this injection was made in the carotid arteries, while with large animals the eyes were removed and the injection made into that branch of the ophthalmic artery which supplies the retina. After injection, the eyes were at once cooled and hardened in alcohol. When hardened, the front half of the globe and the vitreous humor were carefully removed, exposing to view the retina, arteries, entrance of nerve, and area and fovea centralis, when present. The fovea is at once seen if it be present, but the area is sometimes very difficult to discern, and, were it not for the bloodvessels acting as land-marks, it might be overlooked altogether. Drawings were made of this posterior half, great care being taken to orient it, so that one would look into it along the axis of vision.

The results of these injections only serve to substantiate Müller's observation. He states that mammals are the only class of vertebrates which possess, in the true sense, a retinal circulation, while with many mammals only a meagre circulation is present (horse and rabbit). Fish and amphibians possess a good circulation in the hyaloid membrane, while birds and many reptiles have the circulation of the pecten. Huschke states that these vessels of the hyaloid membrane and the pecten correspond to the retinal vessels in mammals. They do not, however, penetrate the retina.

With animals which have neither retinal nor hyaloid vessels, it would appear that the retina is nourished by the choroidal vessels. In fact, in animals with good retinal circulation, the capillaries do not penetrate deeper than the outer 'H. Müller, Anatomie und Physiologie des Auges, p. 117.

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molecular layer, thus leaving the rod and cone, and outer nuclear layers without blood-vessels.*

Investigations show that not all vertebrates possess foveæ, but that each class has a representative which does. When there is no fovea, a well-defined area centralis is usually present. However, in some vertebrates, even an area has not been observed.

The following condensed tabulation will show the frequency of the area and fovea centralis in the eyes which have been examined.

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From this tabulation it is readily seen, so far as experiments have gone, that in mammals the presence of a fovea is the exception while an area is the rule. The primates are the only mammals in which a fovea has been found. Most of the mammals examined have a well-defined area which is easily seen, but, in some, an area has not been demonstrated. The arrangement of the retinal vessels, however, indicates the presence of an area which is free from blood-vessels, and may correspond to the area centralis of other animals.

H. Müller, Anatomie und Physiologie des Auges, p. 103.

5 These results are partly obtained from the tabulation of J. H. Chievitz in his article: Ueber das Vorkommen der Area centralis retinae in den vier höheren Wirbelklassen. Archiv. f. Anat. u. Entwick., 1891, p. 321–325.

Simple.

Band like or

trough-like

7

Double.

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