Of the Cartilages, Ligaments, &c.

If the moveable bones were not connected and kept firm by some strong substance, they would be luxated at every motion of the joints; and if their hard, rough, unequal surfaces were to play on each other, their motion would not only be difficult, but the loss of substance from attrition would be great. Therefore, ligaments are made to obviate the first, and cartilages to prevent the other inconveniency. But, because ligaments and cartilages turn rigid, inflexible, and rough, unless they are kept moist, a sufficient quantity of a proper liquor termed synovia, is supplied for their lubri

cation,

Of the Muscles.

By the name of muscular or moving fibres in the human body, we call bundles of reddish coloured threads, which perform all the motions visible in the human body. When many of these fibres are collected together, and appear more evidently red, they are called a muscle.

In every muscle, we meet with long soft threads, or fibres, some what elastic, or extensible, and almost constantly disposed parallel with each other; and these, being surrounded with a good deal of cellular substance, are by that fastened together into little bundles, called lacertuli; which are again tied together into larger bundles, by a more loose cellular net-work, that contains some fat; and between these, we constantly perceive membranous partitions and stripes of the cellular substance, removing them farther from each other, till at last a number of them combined together, either parallel or inclined, are surrounded with a more thin and dense cellular membrane, continuous with that of their partitions; and this being again surrounded by a thicker plate of the cellular substance, parts the whole bundle from the adjacant flesh, and gives it the denomi nation of a single or entire muscle.*

* The total number of Muscles in the human body is 207. Their various names and uses may be known by consulting the works of medical men,

The Skin.

Cuticula, or scarf-skin, is that thin insensible membrane, which is raised by blisters in living bodies. It is extended over every part of the true skin, unless where the nails are. It appears, when viewed in a microscope, a very fine smooth membrane, only with some inequality of surface, where the rete mucosum adheres to it.

The filamentary substance called the body of the skin, is the almost universal integument of the body, and the basis of all the other cutaneous parts.

The skin is able to resist external injuries to a certain degree, and defend the parts over which it extends from the effects of friction, blows, &c. to which the human body is often liable. The papilla are the organs of feeling, and also contribute to an universal evacuation, called insensible perspiration. They likewise serve to transmit from without, inwards, the subtile particles of substances applied to the skin.

The most important office of the skin is, to exhale moisture from the body, and to absorb vapours from the atmosphere.*

The Ear.

It is composed of cartilage, a kind of substance likely to preserve its form, without being liable to break. Its use is to collect sounds, and direct them into the meatus auditorius, which is the passage that leads to the tympanum or drum.

At the farther extremity of the meatus auditorius lies the mem-. brana tympani, which is extended upon a bony ridge, almost circular. Its situation, both in men and quadrupeds, is nearly horizontal, inclined towards the meatus auditorius, which appears to be the best position for receiving sounds; as they generally are reverberated from the earth. In men, it is concave outwardly, but in birds it is convex, so as to place the upper side of it nearly perpendicular to the horizon, and render them more capable of

• The pores of the skin are so minute, that a grain of sand would cover 125,000 of these little funnels, which are incessantly employed in carrying off a superfluous vapour from the blood. The discharge from the skin is upwards of three pounds daily of excrementitious fluid, and in quantity the evacuation by the pores exceeds that of the urine.

FELTHAM.

hearing each other's sounds, when aloft in the air, where there can be but little reverberated sound.

From the cavity behind the tympanum, which is called the barrel of the ear, goes the eustachian tube, or iter ad palatum; it ends cartilaginous behind the palate. This passage seems to be exactly of the same use as the hole in the side of the common drum; that is, to let the air pass in and out from the barrel of the ear to make the membrane vibrate the better; and perhaps, in the ear, which is closer than a common drum, to let air in and out, as it alters in density; and further, if any fluid should be separated in the barrel of the ear, to give a passage to that also. This tube, when obstructed, as sometimes happens from the growth of a polypus behind the uvula, occasions great difficulty of hearing.

Besides the fenestra ovalis, there is near it another, somewhat smaller, called rotunda. These two holes lead to a cavity called vestibulum, which opens into other cavities called cochlea, and three semicircular canals, all together named the labyrinth, in which are spread the auditory nerves, to receive and convey the impulse of sounds to the brain. Besides these, the chorda tympani, which is a branch of the fifth pair of nerves, may also convey these sensations to the brain. The two holes called fenestra ovalis and rotunda, are closed with a fine membrane, like that called the drum, and the larger being occasionally covered and uncovered by the stapes, causes sound to be modulated more or less, as best serves the purposes of hearing; and this advantage being added to that of a lax or tense tympanum, the effect of sounds may be greatly increased or lessened upon the auditory nerves, expanded in the labyrinth.

If sounds propagated in the ear were heard less, we might often be in danger before we were apprized of it; and if the organs of hearing were much more perfect, unless our understandings were so too, we should commonly hear more things at once than we could attend to.

Of the Eye.

The general situation, figure, and use of the eyes, together with the eye-brows, eye-lashes, and eye-lids, being well known, it is

only necessary for us to describe what is usually shown by dissection. The orbit of the eye, or bony socket in which it is contained, is in all the vacant places filled with a loose fat, which is a proper medium for the eye to rest in, and serves as a socket for it to be moved in.

In the upper and outer part of the orbit is seated the lachrymal gland. Its use is to furnish a watery secretion called the tears, which answer the end of washing off dust, and keeping the outer surface of the eye moist, without which the transparent cornea would be less pellucid, and the rays of light less distributed in their passage to the retina; and that this liquor may be rightly disposed of, we are continually closing the eye-lids to spread it equally, even when we are not conscious of doing so. At the inner corner of the eye, between the eye-lids, is a caruncle, which seems placed there to keep that corner of the eye-lids from being so totally closed as to hinder the discharge of tears or gummy matter from under the eye-lids during sleep. Close to this caruncle are situated the puncta lachrymalia, which are little holes, one in each eye-lid, designed to carry off the superfluous tears into the nostril.

The first membrane of the eye is called tunica conjunctiva. It covers so much of the eye as is called the white, and being reflected all round, lines the two eye-lids. Being thus returned from the eye to the inside of the lids, it effectually hinders any extraneous bodies from getting behind the eye into the orbit, smooths the parts covers, and thus renders the friction less between the eye and the eye-lids. This coat is very full of blood-vessels, as appears when it happens to be attacked with inflammation.

it

Tunica scelerotica, and cornea; these make together one firm and nearly globular case for the attachment of the other coats of the eye, and to contain its humours. The fore-part of this strong coat being transparent and like horn, is called cornea, and the rest scelerotis. Under the cornea lies the iris, which is an opaque membrane, like the tunica choroides, but of different colours in different eyes, such as blue, grey, black, or hazel. The middle of the iris is perforated for the admission of the rays of light, and is called the pupil.

Immediately next to the tunica choroides, lies the tunica retina, which is the optic nerve expanded and co-extended with the choroides. Rays of light striking upon this membrane, the sensation is conveyed by the optic nerves to the common sensorium. These nerves. do not enter at the middle of the bottom of the eyes, but nearer the nose; for those rays of light being ineffectual for vision, that fall upon the entrance of the optic nerves, it is fit they should so enter, as that the same object, or part of any object, should not be unperceived in both eyes, as would have been the case, had they been otherwise inserted; which appears from a common experiment of part of an object being lost to one eye, when we are looking towards it with the other shut.

The inside of the eye is filled with three humours, called aqueous, crystalline, and vitreous. The aqueous humour lies foremost, and seems chiefly of use to prevent the crystalline from being easily bruised by rubbing, or a blow; and perhaps it serves for the crystalline matter to move in, while we view near objects, and backward for remoter objects; without which mechanism, or, in the place of it, a greater convexity in the crystalline humour in the former case, and a less convexity in the latter, it is difficult to imagine, according to the laws of optics, how we could so distinctly see objects at different distances.

To paint objects distinctly on the retina, the cornea is required to have such a degree of convexity, that the rays of light may be collected at a certain point, so as to terminate exactly on the retina. If the cornea is too prominent, the rays, by diverging too soon, will be united before they reach the retina, as is the case with near-sighted people, or myopes; and on the contrary, if it is not sufficiently convex, the rays will not be perfectly united, when they reach the back part of the eye, and this happens to long-sighted people, or presbi, being found constantly to take place as we approach to old age, when the eye gradually flattens. These defects are only to be supplied by glasses adapted to their particular nature.

The Teeth.

In the fetus, about the tenth week after conception, there is no vestige either of an alveolar socket or nucleus of the teeth, but sim