ness of the circle of light thus formed. By experiments of this kind, the time has been found to vary, in different individuals, or in different states of the same individual, from about 1-4th to 1-10th of a second: that is, the impression must be repeated from four to ten times in each second to insure the continuousness of the image.

971. The impressions of variety of color, are produced by the differently-colored rays, which objects reflect or transmit to the eye. It is curious that some persons, whose sight is perfectly good for forms, distances, &c., are unable to discriminate colors. This curious affection has received the name of Daltonism; from the circumstance that the celebrated Dalton was an example of it. There are numerous modifications of it; the want of power to discriminate color being total in some; whilst in others it extends only to certain shades of color, or to the complementary colors.

972. When the retina has been exposed for some time to a strong impression of some particular kind, it seems less susceptible of feebler impressions of the same kind; thus if we look at any brightly-luminous object, and then turn our eyes upon a sheet of paper, we shall perceive a dark spot upon it: the portion of the retina, which had received the brighter image not being affected by the fainter one. Again, when the eyes have received a strong impression from a colored object, the spot which is seen when the eyes are directed upon a white surface exhibits the complementary color; for the retina has been so strongly affected in the part that originally received the image, by its vivid hue, that it does not perceive the fainter hue of the same kind in the object to which it is then turned, and it is impressed only by the remaining rays forming the complementary colors. This explanation applies to the phenomena of the colored shadows which are often seen at sunset, and of those which may be seen in a room whose light enters through colored glass or drapery. For if the prevailing light be of one color, -orange or red for instance,-the eye will not take cognizance of that color in the faint light of the shadows; and will see only its complement, blue or green. If the shadow be viewed through a tube, in such a manner that the general colored ground is excluded, it presents the ordinary tint.

CHAPTER XIV.

OF THE VOICE AND SPEECH.

973. THERE is one particular application of Muscular power in Man, which deserves special consideration, as being that by which he effects his most complete and intimate communication with his fellows;—that, namely, by which his organ of Voice is put into action. In all airbreathing Vertebrata, the production of sound depends upon the passage of air through a certain portion of the respiratory tubes, which is

so constructed as to set it in vibration, as it passes forth from the lungs. In Reptiles, the vibrating apparatus is situated at the point, where the trachea opens into the front of the pharynx; it is of very simple construction, however, being only composed of a slit bounded by two contractile lips; and few of the animals of this class can produce any other sound than a hiss, which, owing to the great capacity of their lungs, is often very much prolonged.-In Birds, the situation of the vocal organ is very different. The trachea opens into the front of the pharynx as in Reptiles, by a mere slit; the borders of which have no other movement than that of approaching one another, so as to close the aperture when necessary. This appears to be the instrument for regulating the ingress and egress of air, in conformity with the wants of the respiratory function. The vocal larynx of Birds is situated at the lower extremity of the trachea, just where it subdivides into the bronchial tubes; and it is of very complex construction, especially in the singing birds.-In Mammalia, on the other hand, the vocal organ and the regulator of the respiration are united in one larynx, which is situated at the top of the trachea. There are few, if any, of this class, which have not some vocal sound; but the variety and expressiveness which can be given to it, differ considerably in the several orders; being by far the greatest in Man, who alone, there is reason to believe, has the power of producing articulate sounds, or proper language.

974. The Larynx is built up as it were, upon the Cricoid cartilage (Fig. 165, x w r u), which surmounts the trachea, and which might be

Bird's-eye view of Larynx from above, after Willis:-G E H, the thyroid cartilage, embracing the ring of the cricoid rux w, and turning upon the axis x 2, which passes through the lower horns; N F, NF, the arytenoid cartilages, connected by the arytenoideus transversus; TV, TV, the vocal ligaments; xx, the right crico-arytenoideus lateralis (the left being removed); vkf, the left thyro-arytenoideus (the right being removed); N 1, N 2, the crico-arytenoidei postici; B B, the crico-arytenoid ligaments.

considered as its highest ring modified in form, its depth from above downwards being much greater posteriorly than anteriorly. This is embraced, as it were, by the thyroid cartilage (G E H); which is arti

culated to the sides of the Cricoid by its lower horns, round the extremities of which it may be considered to rotate, as on a pivot. In this manner, the front of the Thyroid cartilage may be lifted up, or depressed, by the muscles which act upon it; whilst the position of its posterior part is but little changed. Upon the upper surface of the back of the Cricoid cartilage, are seated the two small Arytenoid cartilages (N F); these are so tied to the cricoid by a bundle of strong ligaments (BB), as to have a sort of rotation upon an articulating surface, which enables them to be approximated or separated from each other, -their inner edges being nearly parallel in the first case, but slanting away from each other in the second. To the summit of these cartilages are attached the Chordae vocales, or vocal ligaments (T U), composed of yellow fibrous or elastic tissue. These stretch across to the front of the Thyroid cartilage; and it is upon their condition and relative situation, that the absence or the production of vocal tones, and all their modifications of pitch, depend. They are rendered tense by the depression of the front of the Thyroid cartilage, and relaxed by its elevation; by which action the pitch of the tones is regulated. But for the production of any vocal tones whatever, they must be brought into a nearly parallel condition, by the mutual approximation of the points of the arytenoid cartilages to which they are attached; whilst in the intervals of vocalization, these are separated, and the rima glottidis, or fissure between the chorda vocales, assumes the form of a narrow V, with its point directed backwards.

975. Thus there are two sets of movements concerned in the act of vocalization;-the regulation of the relative position of the Vocal Cords, which is effected by the movements of the Arytenoid cartilages; -and the regulation of their tension, which is determined by the movements of the Thyroid cartilage. The Arytenoid cartilages are made to diverge from one another by means of the Crico-arytenoidei postici of the two sides (N 1, N 1), which proceed from their outer corners and turn somewhat round the edge of the Cricoid, to be attached to the lower part of its back; their action is to draw the outer corners of the Arytenoid cartilages outwards and downwards, so that the points to which the vocal ligaments are attached are separated from one another, and the rima glottidis is thrown open. The action of these muscles is antagonized by that of the Arytenoideus transversus, which draws together the Arytenoid cartilages; and by that of the Crico-arytenoidei laterales of the two sides (N X), which run forwards and downwards from the outer corners of the Arytenoid cartilages, and tend by their contraction to bring together their anterior points, to which the Vocal ligaments are attached.-The depression of the front of the Thyroid cartilage, and the consequent tension of the Vocal ligaments, is occasioned by the conjoint action of the Crico-thyroidei of the two sides, which occasions the Thyroid and Cricoid cartilages to rotate, the one upon the other, at the articulation formed by the inferior cornua of the former; and this action will be assisted by the Sterno-thyroidei, which tend to depress the front of the Thyroid cartilage, by pulling from a fixed point below. On the other hand, the elevation of the front of the Thyroid cartilage, and the relaxation of

the Vocal ligaments are affected by the contraction of the Thyro-arytenoidei of the two sides (v k f), whose attachments are the same as those of the Vocal ligaments themselves; and this is aided by the Thyrohyoidei, which will tend to draw up the front of the Thyroid cartilage, acting from a fixed point above.

976. The muscles which govern the aperture of the glottis,-those namely, which separate and bring together the arytenoid cartilages, and thus widen or contract the space between the posterior extremities of the vocal ligaments,-have important functions in connexion with the Respiratory actions in general; standing as guards, so to speak, at the entrance of the lungs. We can entirely close the glottis through their means, by an effort of the Will, either during inspiration or expiration; and it is a spasmodic movement of this sort, which is concerned in the acts of Coughing and Sneezing, the purpose of which is to expel by a sudden and powerful blast of air, any irritating substances, whether solid, liquid, or gaseous, which have found their way into the air-passages. These muscles appear to be under the sole direction of the inferior or recurrent laryngeal nerve; which seems to possess exclusively motor endowments. When this nerve is divided, on each side, or when the par vagum is divided above its origin, the muscles of the larynx (with the exception of the crico-thyroid) are paralysed; and the aperture of the glottis may remain open, or may be entirely closed, according to the manner in which its lips are affected by the currents of air in ingress or egress. It is found that, under such circumstances, tranquil respiration may be carried on; but that any violent ingress or egress of air will tend to drive the lips of the glottis (these being in a state of complete relaxation) into apposition with each other, so as completely to close the aperture. The character of the superior laryngeal nerve appears to be almost exclusively afferent; no muscle, except the crico-thyroid, being thrown into contraction when it is irritated; whilst, on the other hand, if it be divided, neither the act of coughing, nor any reflex respiratory movement whatever, can be excited, by irritating the lining membrane of the larynx.

977. During the ordinary acts of inspiration and expiration, the Chordæ vocales appear to be widely separated from each other, and to be in a state of the freest possible relaxation. In order to produce a vocal sound, they must be made to approach one another, and their inner faces must be brought into parallelism; both of which ends are accomplished by the rotation of the Arytenoid cartilages; whilst, at the same time, they must be put into a certain degree of tension, by the depression of the Thyroid cartilage. Both of these movements take place consentaneously, and are mutually adapted to each other; the vocal ligaments being approximated, and the rima glottidis consequently narrowed, at the same time that their tension is increased. There is a certain aperture, which is favorable to the production of each tone, although the pitch itself is governed by the tension of the Vocal Cords; and it is, perhaps, to a want of consent between the two, that the peculiarly discordant nature of some voices, which appear incapable of producing a distinct musical tone, is due.

978. It has been fully proved, by the researches of Willis, Müller,

and others, that the action of the Vocal ligaments, in the production of sound, bears no resemblance to that of vibrating strings; and that it is not comparable to that of the mouth-piece of the flute-pipes of the Organ; but that it is, in all essential particulars, the same with that of the reeds of the Hautboy or Clarionet, or the tongues of the Accordion or Concertina. All the phenomena attending the production of Musical tones are fully explicable on this hypothesis; except the production of falsetto notes, which has not yet been clearly accounted for.-The power which the Will possesses, of determining, with the most perfect precision, the exact degree of tension which these ligaments shall receive, is extremely remarkable. Their average length in the Male, in the state of repose, is estimated by Müller at about 73-100ths of an inch; whilst, in the state of greatest tension, it is about 93-100ths; the whole difference, therefore, is not above 20-100ths, or one-fifth of an inch. In the female glottis, their average dimensions are about 51100ths, and 63-100ths, respectively; so that the difference is here only 12-100ths, or less than one-eighth of an inch. Now the natural compass of the voice, in most persons who have cultivated the vocal organ, may be stated at about two octaves, or 24 semitones. Within each semitone, a singer of ordinary capability could produce at least ten distinct intervals; so that, for the total number of intervals, 240 is a very moderate estimate. There must, therefore, be at least 240 different states of tension of the vocal cords, every one of which can be at once determined by the will, when a distinct conception exists of the tone to be produced (§ 905); and, as the whole variation in their length is not more than one-fifth of an inch, even in Man, the variation required, to pass from one interval to another, will not be more than 1-1200th of an inch. And yet this estimate is much below that, which might be truly made from the performance of a practised vocalist. The celebrated Madame Mara is said to have been able to sound 50 different intervals between each semitone; the compass of her voice was at least 40 semitones, so that the total number of intervals was 2000. extreme variation in the length of the vocal cords, even taking the larger scale of the Male larynx, not being more than one-fifth of an inch, it may be said that she was able to determine the contractions of her vocal muscles to the ten-thousandth of an inch.

The

979. It is on account of the greater length of the Vocal cords, that the pitch of the voice is much lower in Man than in Woman: but this difference does not arise until the end of the period of childhood,—the size of the larynx being about the same in the Boy and Girl, up to the age of 14 or 15 years, but then undergoing a rapid increase in the former, whilst it remains nearly stationary in the latter. Hence it is that Boys, as well as Girls and Women, sing treble; whilst Men sing tenor, which is about an octave lower than the treble; or bass, which is several notes lower still. The cause of the variation in the timbre or quality in different voices is not certainly known; but it appears to be due, in part, to differences in the degree of flexibility and smoothness in the cartilages of the larynx. In Women and children, these cartilages are usually soft and flexible, and the voice is clear and smooth; whilst in men, and in women whose voices have a masculine roughness,