ANIMAL PHYSIOLOGY.—II. THE EYE (Continued). THROUGHOUT those classes of animals which are called vertebrate, because they have an internal skeleton, the main central portion of which consists of a back-bone of pieces jointed to one another in a long row stretching from one end of the body to the other, the eye is essentially of the same structure as in man. It is true there are differences in the proportion and shape of the parts, and in some cases additional parts are found, while in others the eye is so reduced and degraded as to be of little VERTICAL SECTION OF THE EYE OF A SOARING BIRD. 1, Sclerotic; 2, Choroid; 3, Retina; b, Pecten; 4, Vitreous humour; 5, Bony support of sclerotic or hard coat; 6, Iris; 7. Cornea; 8, Lens; 9, Aqueous humour; 10, Lens ligament; 11, Ciliary processes; 12, Optic nerve. or no use; but in the majority of cases in brutes, reptiles, and fishes, and in all birds, the eye is well developed, and even where it can be of no use, still indications of it are found. Our English mole is an instance of an animal with a degraded condition of eye. It is in this animal smaller than a pin's head, and has to be looked for carefully in the midst of the velvet fur. Of course, to an animal which lives underground, burrowing continually in soft earth, an eye would be useless, and even inconvenient; yet the rudiment of an eye is found. Besides man, only apes (and some lizards, such as the chameleon, and perhaps some fish) have the yellow spot of distinct vision. Vision in some apes must be very powerful, for it is said a gentleman who owned a baboon used to ride away across the plain until he could only just see his dog-ape with the naked eye; then using his telescope, he made a number of gestures, which were immediately mimicked with precision by the animal. In looking into the open eye the white is part of the opaque sclerotic. The coloured part is the iris seen through the transparent cornea and aqueous humour, while the pupil is the hole through the middle of this, which seems black because of the dark non-reflecting choroid at the back of the eye. place in the choroid of pigment of metallic brilliancy. This may be well seen at the bottom of the eye of the ox inside; in others, the sclerotic is coloured, as any visitor at the Zoological Gardens may see to be the case in the eye of the chimpanzee. These diversities, with many others, such as the contraction of the iris of the cat, so as to leave a slit instead of a circular opening, are interesting, but by no means so functionally important as others to be mentioned hereafter, when we describe eyes suited to conditions altogether different, such, for instance, as the fish's eye, which is constructed to see in water. Birds, some of which are almost exclusively denizens of the air, and most of which have the power of betaking themselves to flight occasionally to escape pursuit, to hunt active prey, to search for new feeding-grounds, or to select a more genial climate at the change of the seasons, must have eyes suited to distant vision. Hence the lens is of a very flattened form, and does not increase in density from the outside to the inside as it does in mammalia, and more strikingly in fish. The distance from the lens to the back part of the eye is small, and to the cornea large relatively; in other words, they have a larger amount of aqueous and a smaller amount of vitreous humour than brutes have. The back part of the eye too is flatter, and is a portion of a larger sphere in relation to the rest of the eye than in animals. The shape will be best seen by the aid of the diagram of the vertical section of the eye of a soaring bird. When the eye is spherical and distended with fluid, as in man, there is no tendency of the pressure within to alter the shape of the ball; but when, as in the case of birds, it has any other form, the internal pressure would strain the elastic capsule of the eye in some parts more than in others. This strain can only be prevented by rendering those parts of the capsule which are exposed to the extra pressure more solid. In the case of the bird, this is effected by means of a series of bony plates which encircle the sclerotic, bedded in its substance, and stretching from the rim of the cornea to the circumference of the large segment of the eye, on the inside of which the retina is spread out. The structures described above, conducive to long sight in a thin medium, are more especially to be remarked in soaring, raptorial birds, like the eagles, vultures, and hawks. These, as they wheel round at a great height, survey a large extent of The iris gives the colour to the eye. When there is only a layer of pigment on the back part of this, the eye is blue; but when, in addition, specks or sheets of pigment are distributed through the substance of the iris, eyes of various colours are produced. Thus, fair people have usually blue eyes, and black eyes accompany an olive complexion and dark hair. In other 1, words, people that have a surplus of internal paint elsewhere have it in the iris too. Again, the lack of pigment is sometimes so great that even the choroid has none, and then the pupil looks red because the blood-vessels of the choroid can be seen through its front layer. Albinos, as individuals with the last peculiarity are called, are found among rabbits, mice, cats, and many other species, and are especially prone to occur under domestication. These creatures present an appearance which is very ethereal and fairy-like, so that artists have often intoduced them into their fanciful pictures, as in Landseer's "Bottom and Titania." But however they may grace the ideal creation of the painter, they are less suited to this working-day world than their coarser brothers. On the other hand, in some species a further deposit takes VERTICAL SECTION OF THE EYE OF A FISH. Sclerotic; 2, Choroid, 2, Inner layer of Choroid; 3, Retina; a, Choroid gland; 4, Vitreous humour; 5, Bony support of sclerotic or hard coat; 6, Iris; 7. Cornea; 8, Lens; 9, Aqueous humour; 10, Lens ligament; 11, Ciliary processes; 12, Optic nerve. country; yet their sight is so keen at that elevation that no young unprotected animal, or maimed and disabled prey, escapes their sight. So keen is the sight of the condor of the Andes, that if a carcase be exposed where the naked eye can detect none of these creatures in the horizon, yet in a few minutes they are seen streaming in from all directions straight towards their hoped-for meal. But though birds be long-sighted, it is also highly necessary that they should see minute objects at a short distance. No entomologist will deny that an insectivorous bird must have keen eyes for short distances, if it is to get its living with ease. A microscopic sight is scarcely less requisite for a grain feeding bird. The swallow, which plunges with such reckless impulse through the air, will nevertheless seize a small insect as it dashes along with almost unerring certainty. Usually the prey is so small, that the wonderful powers of the bird displayed in the chase cannot be observed; but sometimes, when the insect has large wings, this dexterity may be seen. The writer has seen a swallow seize, while in headlong flight, the beautiful, scarce swallow-tail butterfly, and shear out its sapid body from between the wide wings, and let them float severally down; and then, not satisfied with a feast so little proportioned to the splendour in which it was dished up, glance round and seize again the several pieces before they had time to reach the ground. How, then, is a long sight and a keen short sight to be obtained from the same eye? This is done mainly by the aid of the bony plates already described. These are so disposed that the edge of one is capable of sliding over the edge of its next neighbour, so that when the fibres of the muscle which unites them contract they compress the eye all round and make it more tubular, while the humours of the eye, thus subjected to pressure, cause the cornea to protrude more, and also the retina to be removed further from the lens. These motions are, in addition to the adjustment for distance, found in mammals. in small quantity, and the result of this is that the fish can see distant objects as well through the air as through the water; and this is important, because almost all fish are surface fish; many feed on flies, and most have to be on their guard against aerial foes. The reader, then, need not be surprised when the sun-loving shoals of carp or chub all plunge headlong into the depths when he appears on the river bank. As a singular instance of the adaptation of means to ends, it is found that all animals, whether reptiles, birds, or brutes, which are amphibious, or which spend much time in the water, have eyes which, though they differ from those of fish, in some things have the same relation of the cornea and lens. Thus the whale and the dolphin (which are but brutes which have taken to the sea), the cormorant and diver, the frog and the crocodile, have all spherical lenses and flat corneæ. Fish and frogs have on the outer layer of the choroid a layer of silvery or golden crystals, and this layer, which is continued round till it occupies the front layer of the iris, gives to the toad so metallic and bright an eye as to countenance the legend that it has a jewel in its head. So Shakespeare "The toad, ugly and venomous, Wears yet a precious jewel in its head." LESSONS IN GERMAN.-IV. SECTION VIII.-INDEFINITE ARTICLE. Intimately connected with this pressure upon and alteration of the dimensions of the humours of the eye, is another pecuharity in the eye of a bird. This is a puckered, purse-like membrane, which is attached to the optic nerve, which in this class enters into the eye by a slit-like opening. This membrane THE indefinite article is less varied than the definite, having for is sometimes called a marsupium, from its resemblance to a the masculine and neuter nominative but one form, aspurse, and sometimes a pecten, from its supposed likeness to a comb. It stretches to the interior of the eye to a different extent in different birds, and is composed of a tangled mass of blood-vessels, mixed with pigment granules. Whether this is simply an erectile organ, which can rapidly contract and enlarge suddenly as it is deprived of or injected with blood, or is capable of feeding the vitreous humour with liquid strained by it from the blood, and draining it off again as circumstances require, is not known. The eyes of reptiles are so different from one another, ranging in structure between the eye of the bird and that of the fish, that it is better at once to pass on to a description of an eye adapted to sight in water. A fish, living as it does in an atmosphere which is many hundred times denser than air, and by no means so transparent, must have an eye suited to look at near objects. It must therefore be able to concentrate the rays of light rapidly; yet it is under this disadvantage, that as it is only when passing from a rare into a dense transparent convex substance that diverging rays are bent towards one another, and the original rays pass through a dense medium, the cornea and aqueous humours can play no part in the bending of the rays towards one another, for they are of about the same density as water. The whole duty of refraction must thus be done by the lens. This is very dense, and of the sheets of which it is made up the inside are denser than the outside, while it is so convex both before and behind as to become a perfect globe. Both the consistence and shape of the round lens may be seen by squeezing it out of the eye of a cooked fish, even by those whose taste for comparative anatomy is only stimulated at the dinner-table. In connection with this kind of lens we have a shallow eye. In other words, if the cornea, through which light enters, be turned upwards, the back of the eye on which the retina is spread resembles a saucer, and not a cup as it does in animals and birds. This is so much the case, that even though the hard capsule is shallower than in brutes, there is still left a large space between this and the choroid, and even this latter has between two of its layers a horse-shoe shaped "gland" composed of blood-vessels, something like the pecten of a bird, though in a different place, and with exactly a converse function. The hard outer coat is strengthened and held to its form by a cup-shaped bone or cartilage, which occupies the parts which are left unoccupied by the bird's eye-bones; because while the latter are used to elongate the eye this maintains a shortened axis. The cornea, or window, and the watery fluid behind it being useless to collect the rays are left, the one flat and the other Masculine: ein Mann, a man. Neuter ein Glas, a glass. DECLENSION OF THE INDEFINITE ARTICLE MASCULINE AND Masculine. NEUTER WITH NOUNS. N. Ein Mann, a man; Neuter. ein Kind, a child; G. Eines Mannes, of a man; D. Einem Manne, to, fora man; A. Einen Mann, a man; eines Kindes, of a child; einem Kinde, to, for a child; ein Kind, child. ner. 6. Der Vater dieses Attributive. Predicative. Gewehr. 5. Wer hat den Pflug des Bauere? SECTION IX.-DECLENSION OF ADJECTIVES. Mann Jetes glücklich-c Kind ist Jener schön-e Baum ist groß. Jenes groß-e Pferd ist schön. Mancher gut-e Mann ist arm. The hard iron is useful. This beautiful bird is white. The adjective has thus far been employed only predicatively, Manches schön-e Mädchen ist eitel. in which use it is unvaried in form, as Stahl ist hart, steel is hard, Blei ist weich, lead is soft. The terms attributive and predicative have, in grammar, a strictly conventional sense, and should be distinctly understood. If we say, The deep river is here (der tiefe Fluß ist hier), the adjective deep is attributive: for the quality, depth, is there referred to as a known and recognised attribute of the river. If we say, The river is deep here (ter Fluß ist hier tief), the adjective is predicative, for we then merely affirm or predicate of the river that it has the quality, depth. When used attributively, the adjective is varied by the addi tion of suffixes. 1. When not affected by a preceding word, the adjective is inflected according to Solcher fein-e Stahl ist kostbar. Every happy child is contented. Yonder (that) beautiful tree is large. Yonder (that) large horse is Many a good man is poor. EXERCISE 10. 1. Ist dieser junge Mann der Sohn des Capitains? 2. Nein, er ift det SECTION X.-DECLENSION OF ADJECTIVES (continued). the adjective adds, in the nominative masculine and in the nominative and accusative neuter, the letter c, and in all the other cases en; and is inflected according to the adjective has, in the nominative masculine and in the nominative and accusative neuter, the terminations of the old declension, and, in all the other cases, those of the new, and is said to be of 1. In the preceding list of words, ein, mein, dein, ., it will be seen that their form for the masculine and neuter is the same; and hence that they do not (like the previous class, der, dieser, ., and like adjectives of the old declension) indicate the gender of the nouns which they precede. The adjective, therefore, by taking the characteristic terminations (er for the masculine and es for the neuter) assumes the office of pointing out the gender of its noun, as Aber, but. OF Masculine: Gin groß-er Stein, a great stone. THE NEW Dach, n. roof. All hard steel is useful. All useful iron is hard. The useful steel is hard. Faul, lazy, idle. Holländer, m. Dutch man. Ihr, your. Ein warm-er Rock ist gut. A warm coat is good. 1. Ist Ihr guter Freund, der Capitain, noch ein junger Mann? 2. Ja, er ist noch jung, aber sein guter Freund, der Holländer, ist alt. 3. Haben Sie einen schönen, großen Hund? 4. Nein, ich habe ein schönes, großes Pferd. 5. Hat Ihr kleines Kind mein neues Meffer? 6. Nein, aber Ihr guter Sohn hat Ihren neuen Stock. 7. Hat der Fleischer ein fettes Schaf? 8. Ja, und sein guter Sohn hat ein schönes, weißes Lamm. 9. Ist Ihr Freund, der junge Hollänter, reich oder arm? 10. Er ist nicht reich, aber er ist zufrieden. 11. Ein zufriedener Mann ist auch reich. 12. Ein reicher Mann ist nicht stets ein zufriedener Mann. 13. Ihr großes Haus hat ein steiles Dach und einen tiefen Keller. 14. Von wem haben Sie Ihr neues Sopha? 15. Ich habe es von einem guten Freunde. 16. Das Schwein ist ein faules, fettes Thier. 17. Ein treuer Freund ist ein starker Schuß. a с LESSONS IN PENMANSHIP.-V. HITHERTO the attention of the learner has been confined to letters based on the elementary stroke called the "pot-hook' or "bottom-turn." He may now proceed to copy the next elementary stroke, called the "top-turn" or "hanger," as shown in Copy-slip No. 11. This stroke will be found to enter into the composition of three letters only, and therefore plays by no means so important a part in the formation of the writing alphabet as the bottomturn, which, as it has been already said, enters into the composition of no less than nine. It consists of a fine hairstroke, commenced on the central line c c, and carried upwards in a direction bending gradually towards the right, as far as the upper line a a, where it is turned and changed into abroad down-stroke, which is brought downwards, with an equal pressure of the pen throughout, as far as the lower ne b b. It is needful, therefore, for the learner to become acquainted with a third elementary stroke before he can proceed to the formation of any new letters, and this he will find in the top and bottom-turn shown in Copy-slip No. 12. This stroke enters into the composition of six letters of the writing alphabet, as the learner will find in future lessons. It consists of a fine hair-stroke, commenced at the central line c c, brought upwards towards the right in a gentle curve, and turned at the upper line a a into a broad down-stroke, which is again narrowed as it approaches the lower line b b into a fine hair-stroke that is turned and carried upwards towards the right. It may be de-. scribed as being formed of the upper half of the top-turn and the lower half of the bottom-turn, joined together on the line cc. Examples of all these elementary strokes will be found in No. 1 of "Cassell's Graduated Copy-Books." When the learner can make these strokes with ease, he will find that he is in a position to form two more letters of the writing alphabet without any difficulty whatever, while he has also advanced more than half-way towards the formation of the seven other letters that are partly made by the aid of these strokes. He may now The top-turn may be described as being precisely the reverse of the bottom-turn; or, in other words, the bottom-turn reversed, as may be seen by turning the page upside down, and examin-proceed to copy the letters n and m, as shown in Copy-slips ing the stroke in this position. It is only used in combination with other elementary strokes in forming letters, for unlike the bottom-turn, there is no letter of the writing alphabet which is formed of this stroke alone, or even by its repetition or any modification of it. Nos. 13 and 14, observing that the letter n consists of a combination of these two strokes only, the top-turn being made first, and the top-and-bottom-turn added to it, while in the letter m the top-turn is repeated twice, and the letter is then completed by the addition of the top-and-bottom-turn. LESSONS IN ARITHMETIC.—V. DIVISION. 1. THE process of finding how many times one number is contained in another is called Division. The number to be divided is called the Dividend. The number by which we divide is called the Divisor. The result-viz., the number of times which the Dividend contains the Divisor-is called the Quotient (Latin quoties, "how often "). The sign placed between two numbers means that the first is to be divided by the second. Thus, 195 means 19 divided by 5. If the Dividend does not contain the Divisor an exact number of times, it will contain it a certain number of times (the Quotient) with a number left over, which will be less than the Divisor. The number left over in this case is called the Remainder. Thus, when we say that 5 is contained in 19 3 times and 4 over, 19 is the dividend, 5 the divisor, 3 the quotient, and 4 the remainder. This fact may be exhibited in the following form:19 3 x 5 +4 2. It will readily be perceived that division is, in reality, only a short method of performing a series of subtractions, in the same way as multiplication is a convenient method of perform ing a series of additions. For instance, to find how many times 5 is contained in 19, subtract 5 (the divisor) continually from 19 (the dividend), until the number is exhausted, or a number less than 5 is left; then, counting the number of these subtractions, we shall get the quotient. Thus, 5 from 19 leaves 14, 5 from 14 leaves 9, 5 from 9 leaves 4. Since 5 has been subtracted 3 times from 19, leaving 4 as a remainder, we see that 19 divided by 5 has 3 for its quotient, leaving 4 as a remainder. N.B. It is evident, from the nature of division, that the product of the quotient and divisor, added to the remainder, is equal to the dividend. 3. Method of Division.-The method we are about to explain depends upon the truth of the following principle: If the dividend be split up into any number of parts, of which the sum is equal to the dividend, then, if we divide each part separately by the divisor, the sum of all the quotients so obtained will be the quotient required. For instance, 18 is equal to the sum of 9 and 6 and 3. The quotients of these, divided respectively by 3, are 3, 2, and 1, which, added together, make 6, the quotient of 18 divided by 3. Similarly, 36 is 28 +8, and therefore 36 divided by 4 is the sum of the separate quotients of 28 and 8 by 4, which are 7 and 2 respectively. Hence 7+ 2, or 9, is the required quotient. It must be observed that if, the quotient of a given dividend and divisor being known, the dividend be increased by annexing any number of ciphers to it, the new quotient is obtained by annexing the same number of ciphers to the quotient. Thus, 28 divided by 4 has the quotient 7; and 28000 divided by 7 18 4000. Now 13 contains 4 three times, with remainder 1; therefore 13 hundreds contain 4 three hundred times, with remainder 1 hundred. Add this remaining 1 hundred to the 5 tens, thus making 15 tens. Now 15 contains 4 three times, with remainder 3: therefore 15 tens contain 4 thirty times, with remainder 3 tens, or 30. Add this remaining 30 to the 6 units, thus making 36 units. Therefore 1 thousand, 3 hundreds, three tens, and 9 units are the number of times the parts into which 5356 has been divided contain the divisor 4 respectively. Their sum, therefore, is the required quotient: this is 1 thousand + 3 hundreds + 3 tens + 9 units, i.e. 1339. 5. The above is the analysis of the following shorter process, and will be seen fully to explain it :Write down the dividend and divisor as in the margin; 4)5356 then say 4 in 5 is contained 1 time, with 1 over. the quotient 1 under the 5, and placing the remaining I before the next figure of the dividend 3, say, 4 in 13 Write 1339 Write the quotient 3 is contained 3 times and 1 over. under the second figure in the dividend, and prefixing the remaining 1 to the 5, say, 4 in 15 is contained 3 times and 3 over. Write the quotient 3 under the third figure in the dividend, and prefixing the remaining 3 to the 6, say, 4 in 36 is contained 9 times, with no remainder, and write down the 9 under the last or unit's figure of the dividend. It will be seen that when, to get the first figure of the quotient, we say 4 in 5 is contained once, with remainder 1, we really indicate that 4 is contained in 5000 1000 times, with remainder 1000, which 1000 we carry on to add to the next three of the dividend, which really indicates 300, and so on; as will be seen by comparing the process with the analysis of the method in Article 4. 6. To divide 7499 by 9. 9)7499 833-2 Here, since 7, the first figure of the dividend, is less than the divisor, 9, we take two figures of the dividend, and say, 9 in 74 is contained 8 times, with a remainder 2, and put down the 8 under the second figure of the dividend (reckoning from the left hand). Then, proceeding as in the previous example, we say, 9 in 29 is contained 3 times and 2 over; and again, 9 in 29 is contained 3 times and 2 over. This last 2 is 2 units, and is therefore the remainder left after dividing 7499 by 9. It is generally written after the quotient, as above. divisor is a small number (generally one figure), is called Short This method, which is only conveniently applicable when the Division. EXERCISE 8. (1.) Divide 658 by 2; 537 by 3; and 7891011 by 6. (2.) Divide 4389127 by 8; 407792 by 11; and 5349279 by 9. (3.) Divide 41239789 by 12; and 54937862 by 5. (4.) Divide each of the numbers contained in the square in Ex. 4, page 23, successively by 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12. (5.) Divide each of the numbers contained in the square in Ex. 4, page 23, successively by 2, 3. 4, 5, 6, 7, 8, 9, 10, 11, 12. (6.) Divide each of the numbers 1010421690, 7689768432134, 54932684736856, and 428571428571496, by all the numbers from 2 to 12 inclusive. contained in 229 is 6 (which must be found by trial). Then say, the greatest number of times that 35 is Put down the 6 to form the next figure in the quotient, and subtract 6 times 35—i.e., 210—from 229, leaving a remainder 19. To this annex the last figure (8) of the dividend, making it 198. 23 Then say, the greatest number of times which 35 is contained in 198 is 5. Write down the 5 to form the next figure in the quotient, and subtract 5 times 35-i.e., 175-from 198, leaving 23. 265 is the required quotient, and 23 is the remainder. Hence 9298265 × 35 + 23. 8. A careful examination of the above process will show that what we have really done is equivalent to saying: 35 is contained in 92 hundreds two hundred times, with a remainder 22 hundred; then, subtracting 200 times 35-i.e., 7 thousandfrom 9298, we have 2298 left. Next we say 35 is contained in 229 tens sixty times, with a remainder of 19 tens; then subtracting 60 times 35-i.e., 2100 -from 2298, we have 198 left. Next we say: 35 is contained five times in 198, with a remainder 23. the dividend, again, 60 times from what is left, and again, 5 times Hence we see that after taking away 35, first, 200 times from from what is left, we have 23 units over, a number which is less than 35. Hence we see that 35 is contained in 9298 200+60+5-i.e., 265 times-with a remainder 23. |