PRIZE ESSAY.

VISION, AND SOME OF ITS ANOMALIES, AS REVEALED BY THE OPHTHALMOSCOPE.

THE following essay purports to consider two subjects, which are intimately connected: viz., the anatomy and physiology of the parts concerned in vision, and some of the derangements attending it. For the purpose of better illustrating these latter, the late discoveries effected by the ophthalmoscope will be entertained.

That branch of science which treats of the nature and laws of vision, is called optics, and is generally considered in three points of view, viz:

A. Perspective Vision, which treats of the apparent size of objects, from the theory of the laws of the straight lined motion of light.

B. Catoptric Vision, which embodies the laws of the reflection of light, embracing always the principle, that the angle of incidence is equal to that of reflection.

c. Dioptric Vision, signifying the power of seeing through transparent media, involving the laws of refraction, which embraces the principle of the bending of light as it passes from a rarer to a denser medium, or vice versâ.

The two former have so slight a connection with our task, that it is needless to discuss the formulæ involving their demonstration. But as the third division enters very materially into the explanation of the modus operandi of vision, it is deemed advisable to enter into its discussion, when, further on, the subject will be brought up in its proper place.

The eye is the most complete of all optical apparatus, and, although it is perfected in man, he is not the sole possessor of

this "window of the soul," for many of the lower animals, the invertebrata as well as the vertebrata, are endowed with the sense of vision.

As we propose to take a rapid and cursory glance of the optical apparatus of the invertebrates, it may not be amiss or foreign to the subject, to sketch the various groups of the invertebrates, with regard to the structure of the visual apparatus. The vertebrate mammals are represented by man, whose eye will be more fully described, both as regards its anatomy and its physiology.

Before entering into any details concerning the invertebrata, we will give Siebold and Stannius"

CLASSIFICATION OF THE INVERTEBRATE ANIMALS.

The invertebrate animals are organized after various types, the limits of which are not always clearly defined. There is, therefore, a greater number of classes among them than among the vertebrates. But, as the details of their organization are but yet imperfectly known, they have not been classified in a natural manner.

There are among them many intermediate forms, which make it difficult to decide upon the exact limits of various groups. The following division, however, from the lowest to the highest forms of organization, appears at present the best.

ANIMALIA INVERTEBRATA.

INVERTEBRATE ANIMALS.

Brain, spinal cord, and vertebrate column, absent.

First Group.-PROTOZOA.

Animals in which the different systems of organs are not distinctly separated, and whose irregular form and simple organization are reducible to the type of a cell.

Class I. Infusoria.

II. Rhizopoda.

Second Group.-Zoophyta.

Animals of regular form, and whose organs are arranged in a

ray-like manner around a centre, or a longitudinal axis; the cen

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Anatomy of the Invertebrata, by C. Th. v. Siebold. Translated from the German by Waldo I. Burnett, M. D.

tral masses of the nervous system forming a ring, which encircles the oesophagus.

Animals with an elongated symmetrical body, and whose organs are arranged along a longitudinal axis; so that right and left, dorsal and ventral aspects may be indicated. The central nervous mass consists of a cervical ganglion, with or without a chain of abdominal ganglia.

Animals of varied form, and whose bodies are surrounded by a fleshy mantle. The central nervous mass consists of ganglia, some of which surround the œsophagus, and others, connected by nervous filaments, are scattered through the body.

Class X. Acephala.

XI. Cephalophora. "XII. Cephalopoda.

Fifth Group.-ARTHROPODA.

Animals having a perfectly symmetrical form and articulated organ of locomotion. The central masses of the nervous system of a ring of ganglia surrounding the oesophagus, from which proceeds a chain of abdominal ganglia.

Class XIII. Crustacea.

66 XIV. Arachnida.
XV. Insecta.

Having premised the above classification, we propose to enter upon the visional apparatus of the Invertebrata, and will begin with the Protozoa.

Most of the Infusoria and Rhizopoda have simple red pigment spots, capable only of distinguishing light from darkness, and