from the entrance of the optic nerve nearly as far as the lens. On examining the retina at the back of the eye by an instrument called an ophthalmoscope, we observe, directly in a line with the axis of the globe, a circular yellow spot called, after its discoverer, the yellow spot of Sömmering. The only mammals in which it exists are man and the monkey. It is the point of distinct vision. When we read a book, we run the eye along the lines so as to bring portions of the line successively on the yellow spot. If, on the other hand, we carefully fix our attention on a word in the middle of the line, we see the word distinctly, because it is on the yellow spot, while the words towards each end of the line are less distinct, being on other portions of the retina. The structure of the retina, as revealed by the microscope, is seen in fig. 9.

The transparent media through which rays of light must pass before they form on the retina the images of external objects are :

Immediately behind the transparent cornea is the aqueous humour, which fills up the chamber between the cornea and the lens. It is nearly pure water, with a trace of chloride of sodium.

The crystalline lens lies opposite to and behind the pupil, close to the iris, and its posterior surface is received into a depression on the forepart of the vitreous humour (see fig. 8). In form, it is a double-convex lens, with surfaces of unequal curvature, the posterior being the most convex, and the curvature is also less at the centre than towards the margin.

The vitreous humour lies in the concavity of the retina, and occupies about four-fifths of the eye posteriorly.

The appendages of the eye are:

1. The muscles by which the eye is moved are four straight (or recti) muscles, and two oblique (the superior and inferior). By the duly associated action of these muscles, the eye is enabled to move (within definite limits) in every direction.

2. The eyelids are two thin movable folds placed in front of the eye, to shield it from too strong light, and to protect its anterior surface. The eyelashes intercept the entrance of foreign particles directed against the eye, and assist in shading that organ from an excess of light.

3. The lachrymal apparatus consists of the lachrymal gland, by which the tears are secreted; two canals, into which the tears are received near the inner angle of the eye; the sac, into which these canals open; and the duct, through which the tears pass from the sac into the nose. The constant motion of the upper eyelid induces a continuous gentle current of tears over the surface, which carry away any foreign particle that may have been deposited on it.

The various uses of the different structures of the eye are readily understood. Assuming a general knowledge of the ordinary laws of geometrical optics, we will trace the course of the rays of light proceeding from any luminous body through the different media on which they impinge. If a luminous object, as, for example, a lighted candle, be placed at about the ordinary distance of distinct vision (about ten inches) from the front of the eye, some rays fall on the sclerotic, and being reflected, take no part in vision; the more central ones fall upon the cornea, and of these some also are reflected, giving to the surface of

the eye its beautiful glistening appearance; while others pass through it, are converged by it, and enter the aqueous humour, which probably, also, slightly converges them. Those which fall on and pass through the outer or circumferential part of the cornea are stopped by the iris, and are either reflected or absorbed by it; while those which fall upon its more central part pass through the pupil. The rays now impinge upon the lens, which, by the convexity of its surface, and by its greater density towards the centre, very much increases the convergence of the rays passing through it. They then traverse the vitreous humour, whose principal use appears to be to afford support to the expanded retina, and are brought to a focus upon that tunic, forming there an exact, but inverted image of the object.

Accommodation of the Eye to Distance.-It will be found on experiment that we cannot see a distant and a near object at the same moment. For example, if we look through a railing at a distant church spire, and fix our attention on the spire, we do not distinctly see the railing; and vice versa. This was early observed; but, until recently, the mechanism by which the eye accommodates or focuses itself for different distances was unknown. Cramer was the first to point out that if we bring a candle-flame near the eye in a dark room, we may see three images-1st, an erect image reflected on the cornea; 2d, an erect image on the anterior surface of the lens; and 3d, an inverted and very faint image on the posterior surface of the lens. He also shewed that when the eye looks quickly at a near object, after having been for some time directed to a distant one, the middle image moves forward nearer to the first, and also becomes smaller, shewing that for near vision the anterior surface of the lens becomes more convex. Helmholtz afterwards, by means of an instrument called the ophthalmometer, measured the sizes of those reflections, and, from certain data, calculated by mathematical formulæ the radii of curvature of the reflecting surfaces; and he shewed conclusively that the accommodation of the eye for different distances is effected by changes in the curvature of the anterior surface of the lens. The physiological explanation is as follows:

The lens, which is elastic, is kept habitually in a state of tension by the pressure of the suspensory ligament, and consequently has a flatter form than it would take if left to itself. When the ciliary muscle contracts, it relaxes the ligament, and thereby diminishes its elastic tension upon the lens. The lens, consequently, becomes more convex, returning to its former shape when the ciliary muscle ceases to contract.

There are two common forms of defective vision which require notice-namely, short-sightedness or myopia, and long-sightedness or presbyopia. They are due to an abnormality either in the curves or in the density of the refracting media. In short-sightedness from too great a refractive power from either cause, the rays from objects at the ordinary range of distinct vision are brought too soon to a focus, so as to cross one another, and to diverge before they fall on the retina; the eye in this case being able to bring to the proper focus on the retina only those rays which were previously diverging at a large angle from a very near object. The correction for this deficiency is accomplished by interposing between the eye

a curvature sufficient to throw the images of external objects at the ordinary distance of distinct vision backwards upon the retina. In long-sightedness, on the other hand, there is an abnormal diminution of the refractive power, so that the focus is behind the retina. This defect is corrected by a convex lens, which increases the convergence of the rays of light.

Position of Objects on the Retina.-In consequence of the bending of rays of light by the refractive media, the image of an external object is inverted on the retina, and yet we see objects erect. The probable explanation is, that the mind may perceive as correctly from an inverted as from an erect image. When we glance at a column from top to base, we move the eyeball downwards so as to bring successive parts on the yellow spot, and it is the feeling of movement which informs us which is top and which is base, not the inverted position on the retina, of which we are really unconscious.

Single Vision with two Eyes.-This phenomenon is explained by the fact that there are corresponding points on the retina, so that when, by the regular action of the muscles of the eyeball, an image is formed on a corresponding point in each eye, the mind is conscious of one image. If we alter the direction of the axis of one eye by pressing gently on the ball, an image is formed on à point of the retina of that eye which does not correspond, and consequently we squint, or see two images.

cates with the back of the throat by the Eustachian tube, the function of which is to equalise atmospheric pressure on both sides of the vibrating membrane. When this tube becomes stopped mechanically by enlargement of the tonsils, partial deafness is the result, and when cleared so as again to allow air to pass into the tympanum, hearing at once returns to its normal state. Across the tympanum, we find a chain of small bones, one of which, the malleus, or hammer, is attached by a long handle to the drum; this unites by a joint with another, the incus, or anvil; which in turn bears the stapes, or stirrup, the base of this being fixed to a small oval membrane closing an aperture, called the fenestra ovalis, which communicates with the internal ear. The function of this

side and below:

Fig. 10.-General view of the External, Middle, and Internal Ear, shewing the interior of the auditory canal, tympanic cavity, and Eustachian tube :

4, the auditory canal; b, the tympanum; c, the Eustachian tube, leading to the pharynx: d, the cochlea; and e, the semicircular canals and vestibule, seen on their exterior by the removal of the surrounding bony tissue.

presents elevations and depressions, the functions of which are to receive and reflect the vibrations of the air which constitute sound, and to transmit these by a tube, partly cartilaginous, partly bony, called the auditory canal, to the middle ear. The middle ear is named the tympanum or drum. It is a cavity in the petrous or hard portion of the temporal bone. It is shut off from the auditory canal by the membrane of the drum, a thin structure capable of vibrating when acted on by the

h, the manubrium or handle; sc, the short crus, and le, the long crus of the incus ; a, the position of the lenticular process, through the medium of which it articulates with the head of the stapes; s, the base of the stapes. Magnified three diameters.

chain of bones is to convey vibrations from the membrane to the internal ear. The internal ear, or labyrinth, so called on account of its complexity of structure, is the essential part of the organ of hearing, because here we find the filaments of the auditory nerve which are ultimately to receive impulses originally produced by vibrations of the air, and which are conveyed by the intermediate structures already described. It is made of three parts-the vestibule, or central part; the semicircular canals, three in number, which communicate posteriorly by five openings with the vestibule; and the cochlea, so called from its resemblance to a snail-shell. Each of these parts is excavated from the substance of the bone, and forms the bony or osseous labyrinth ; but within this we have a fibrous structure, exactly corresponding in shape, the membranous labyrinth. The osseous is separated from the membranous labyrinth by a fluid called the perilymph, and within the membranous portion there is another fluid, called the endolymph. The terminations of the auditory nerve are distributed on the walls of the membranous portion, and by the presence of the two fluids just mentioned, the most delicate vibrations of the air communicated directly to the drum and chain of bones, or indirectly through the bones of the head, are conveyed to the nerves. The structure of the cochlea is very remarkable. It consists of a central pillar, round which a tube makes two and a half coils. This tube is divided into two compartments

W0OLOGY (from Greek zoon, an animal, and logos, a discourse) treats of the form and structure of animals, and the characters by which they may be distinguished from each other.

All natural bodies may be divided into two great groups-mineral or inorganic, and living or organic. The organic are again subdivided into vegetables and animals. Hence arises the division of all natural objects into three great kingdoms-namely, the Mineral, Vegetable, and Animal.

Inorganic substances never live. Chemically, they may be simple or compound, such combinations usually forming binary or ternary compounds. Their physical condition may be solid, fluid, or gaseous; but they are homogeneous in texture, that is, any detached portion exactly resembles the remainder in composition and properties. They may be amorphous, without distinct forms; or crystalline, that is, having distinct geometrical forms, bounded by plane surfaces, which have a definite relation to each other. They increase by the addition of like particles to their surface, which is termed accretion or juxtaposition. Their atoms are at rest, unless set in motion by some physical force acting from without: they initiate no change or motion.

An organic being either lives or has lived during some part of its existence. Chemically, it consists of few elements, which unite to form ternary and quaternary compounds. It consists of solid and fluid parts, which exercise a reciprocal action on each other. It is bounded by curved lines, and has convex and concave surfaces. Each organised being, under the influence of life, assumes a characteristic, though not absolutely definite shape. It increases or grows by receiving into its interior matter which it elaborates and assimilates. The old particles are being constantly removed, and replaced by new ones, so that all its parts are in constant motion, and are ever changing. It arises from some pre-existing organism of the same kind, by means of a germ, which becomes separated, and enjoys an individual existence.

We do not know 'life' apart from matter; some material substratum or 'physical basis' is required for its manifestation. This, according to Huxley, is furnished by what he calls protoplasm, which is a homogeneous, structureless substance, endowed with contractility, and having a chemical composition nearly allied to that of albumen: it is composed of carbon, hydrogen, oxygen, and nitrogen. This term life indicates a very special property indeed; and at present, looking at it from a purely material point of view, we are scarcely justified in regarding life as more than that condition of an organised being in which the products of chemical and physical changes taking place within it are stamped with a specific form. By the term 'moulding of specific form is meant the building up of a complicated and heterogeneous organism, which repeats the characters which have been transmitted to it through a germ, by a parent,

every molecule of every part having thus a direct relation in form, in position, and in composition, to every other molecule of the body.

In

It is impossible to draw a distinct line of demarcation between vegetables and animals, as the lowest forms of both kingdoms seem to meet and merge into each other. It was on this account that Professor Ernst Haeckel of Jena constructed a fourth kingdom, called by him 'Protista,' into which he proposed to put all the organisms of doubtful affinity. But this, at present, seems scarcely justifiable. Of course, the conspicuous members of the vegetable kingdom can never be confounded with the higher animal forms. the latter, the presence of a nervous system, the possession of a mouth and digestive cavity, as well as the power of voluntary locomotion, are sufficient to distinguish them from the former, in which all these are absent. In the simplest groups in each kingdom, however, these grand distinctions are lost. Chemically, plants consist chiefly of ternary compounds-that is to say, compounds consisting of three elementary ingredients, as starch and cellulose, which are different combinations of oxygen, hydrogen, and carbon. In an animal, quaternary, and still more complex compounds, as albumen, fibrine, and gelatine, make up the bulk of the body; while ternary compounds, though, indeed, not wanting, play a subordinate part. The general plan of nutrition is strongly contrasted in the two kingdoms; but in some low animal forms, as the Gregarinæ, nutrition is effected by absorption through the external surface, just as is the case in plants. Most plants are permanently fixed in the ground by roots, but some low forms of algæ are locomotive; and although most animals can move about from place to place, others are incapable of progression, as the branched and tree-like sponges. It may be said, in a general sense, that organs of relation are present in animals, and absent from plants. But some phenomena connected with climbing plants, and with the process of fertilisation, are very difficult to explain without admitting some low form of a general harmonising and regulating function, comparable to such an obscure manifestation of reflex nervous action as we have in Sponges and other animals in which a distinct nervous system is absent.

Plants can secrete and store in their leaves and other parts a substance called 'chlorophyl,' by means of which, under the influence of light, they absorb carbonic acid from the atmosphere, decompose it, and, when the carbon is in a nascent state-that is, just liberated from combination-can combine it with the elements of water (hydrogen and oxygen), or with the elements of water and ammonia, likewise reduced to a nascent state by the same agency. plant thus gains from the air and from the soil certain elementary substances, chiefly carbon, oxygen, hydrogen, and nitrogen. These, unde the guidance of a vital property, and through the

The

medium, of protoplasm contained in its cells, it combines into still more complex organic compounds, which contribute to the development or maintenance of the special specific form of the organism of which it is a part. Plants can assimilate no elementary substance except oxygen, unless it is presented to them in the nascent condition.

An animal stands in exactly the same relation to the binary compounds, carbonic acid, water, and ammonia, which, along with salts, form the food of plants. An animal cannot assimilate these substances directly; they must first be elaborated to the condition of ternary and quaternary compounds, which can be done only by the cells of plants. This is the broad and practical distinction between the vegetable and the animal kingdom. Plants possess the power of absorbing, modifying, and organising inorganic substances; while animals are entirely dependent for their support upon the organic substances thus prepared.

The pale growing parts of plants have precisely the same vital properties and relations as animal protoplasm. It is only in cells in which protoplasm elaborates and incorporates with itself colouring-matter (endochrome), which seems to be a more powerful catalytic agent, capable of disengaging the component atoms of the more stable binary compounds, when loosened by the vibrations of light, that the special function of the vegetable cell is manifested. Further, in the interior of the cells of some plants, as Chara, the movements of the protoplasm are so special and characteristic as to prove its absolute identity with the protoplasm of the Rhizopods.

A living being is a complicated machine, which does a great deal of various work. One of the higher animals, to take an example, is made up of a great many parts, each of which does its own special part of that work- - thus, the stomach digests, and the eye sees. These several parts are called organs, and the thing which an organ does is called its function. If we remove these organs, performing each its function, one by one, the whole animal disappears. The animal body therefore consists of the sum of its organs.

Living beings may be studied under three principal aspects -the Morphological (morphe, form, and logos, a discourse), the Physiological,

and the Distributional.

Morphology, which treats of the form and structure of living beings, includes anatomy, both naked-eyed and microscopic; to the latter, the term Histology (histos, a web, and logos) has been applied. It also embraces Embryology, or the study of the forms of living beings in all stages of development, from their earliest or immature condition, till they reach their mature or adult state. Physiology treats of the functions of the organism as a whole, or of its separate component parts, organs, or tissues; of what an animal does, or of what its different parts do. The functions of an organism are divisible into-1. Function of Nutri

tion.

2. Function of Generation or Reproduction. 3. Function of Irritability or Correlation.

The function of nutrition has reference to the support and maintenance of the body. Matter is introduced into the interior of the body; there it undergoes certain changes, which assimilate it to, and fit it to be incorporated with, the textures which compose the body. The function of reproduction serves the purpose of perpetuating the

species. The function of nutrition and the function of reproduction have been called the functions of 'organic' or 'vegetative' life, because they are possessed both by plants and animals. The functions of relation—including irritability, consciousness, sensation, and volition, with all the movements depending upon the will-bring the organism into connection with the outer world, and the outer world into connection with the organism, so that thus the one reacts upon the other. This group of functions is possessed by animals alone, so that they have been called the 'functions of animal life.'

Distribution treats not only of the areas of the globe over which organisms are distributed, and the conditions under which they exist, but it also relates to the history of life in bygone ages, as furnished to us by the evidence of fossil remains. The former is called distribution in space, or geographical distribution; the latter, distribution in time.

Classification.-On looking at the multitude and variety of animal forms around us-such as we are familiar with as inhabitants of this country, or as natives of other climates collected for our observation-the mind naturally associates together those which have the greatest general resemblance, and separates these (although differing in some degree amongst themselves) from those with which they have greater dissimilarity. Now, it is necessary that some system of arrangement or classification should be adopted, by bringing together those animals which most closely resemble each other, not so much in external appearance as in internal structure, in order that the mind shall be the better able to grasp the facts of zoological science. A classification, therefore, will be correct in proportion to the number of ascertained facts upon which it is founded. Zoologists, in seeking to classify animals, are in the habit of using several terms, one of the most important being species. It is by no means easy to define this term. It is generally understood to mean an assemblage of animals that resemble each other in all essential points of structure, and which are supposed all to have descended from the same parent stock. A test to which much importance has been assigned, is founded on the supposed fact, that when the animals of different species breed together, their offspring is barren. This offspring is called a hybrid-thus, a mule is a hybrid between a horse and an ass. This, however, is not an absolutely satisfactory or conclusive test of specific identity, as hybrids between undoubtedly distinct species have been known, though rarely, to breed and produce fertile offspring.

Until lately it has been the almost universal belief among naturalists that species are permanent within very narrow limits of variation; that is to say, that any group of animals which present the same specific characters, characters which lead an educated observer to set them down as the same thing for example, all common partridges, or, to take a variable species, all domestic dogs-are descended from an ancestry which can by no possibility include anything except partridges or dogs, and can never, under any circumstances, or through the lapse of any amount of time, give origin to anything except dogs or partridges. This view, of course, involves