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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
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 uuperceived 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 con
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
ply a gelatinous substance, included in a very vascular membrane, extended around the maxillary bones,
About the fourth or fifth month after conception, the lower jaw. bone has a distinct shallow groove extending through its whole length; anteriorly this is narrower and deeper, and posteriorly wider and more shallow; and in this canal appear little pulps, enclosed in delicate membranaceous sacs, surrounded with small cells of a rude form. These bags are the rudiments of the teeth to be produced. Nearly about this period, delicate bony partitions, separating these membranaceous pouches, are seen extending across this canal; which ultimately become more distinct, and form the alveolar sockets.
The sacs of the incisivi are first visible; next, those of the canini; and, lastly, those of the molares.
The nerves and blood-vessels run along the bottom of this channel immediately beneath the pulps of the teeth ; and at a more advanced age are encased in a bony canal, and send separate branches or filaments to the individual teeth.
The pulps attached to the bottom of the cells gradually assume the figure of the crowns of the respective teeth which they are to form, and then begin to be converted into bone. In the incisivi and canini, the vessels of the membranous bags commence with depositing ossific matter externally at their tips or points; the ossification then extends downwards, both on their exterior and interior surfaces, and also from without, inwards, pressing the pulp gradually downwards. As soon as the crown of the tooth is formed, which is named the enamel, the fang or root is produced, of a bollow tubular form, to afford Jodgement for the vessels.
In the molares, ossification begins at two points in the first two, or bicuspides, which are the milk or deciduous grinders, and at five points in the last three molares, or permanent grinders. In the molares, the striæ run on the upper surface from each of these tips to the centre, and from the same points downwards to the neck of the tooth ; ossification extends at the same time centrad, squeezing down the pulp, leaving a cavity, and producing the fangs in the same manner as the incisive or canine teeth did,
This process resembles the beautiful appearance displayed by the slow congelation of water, or the elegant surface of fibrous gypsum, or satin spar.
The ten teeth which are now formed, for we must exclude the three last molares, are termed milk or deciduous teeth, because they fall out, or are shed at a future period of life; they are also denominated temporary. We must here also recollect, tbat at this time the roots or fangs of these deciduous teeth are not yet formed; for they do not begin to be produced till after birth.
About the seventh month after conception, the pulps of the permanent incisores are seen almost in the same plane, only a little underneath the deciduous set, and next to the canini; and, lastly, the molares. At birth, therefore, a child has the rudiments of both the deciduous and permanent teeth lodged in the jaw-bone, and covered with the gum and linivg membrane of the mouth. The crowns of the deciduous teeth alone are ossified.
After the child is born, the roots or fangs of the teeth begin to be produced, so as to render them too long to be contained within the alveolar socket, and hence to raise the crown gradually towards the gum, which, giving less resistance than the alveolar socket, is cut more by mechanical pressure than by absorption. It is this mechanical puncturing, or cutting of the gum, which produces the severe symptoms and diseases of dentition.
At the age of the sixth or seventh month, the middle or mesial pair of the incisivi of the lower jaw protrude throngh the gum. In a few weeks the opposite pair of incisive teeth in the upper jaw also make their way through the gum. In a few weeks after the appearance of the two last, the two lateral incisores of the lower jaw force their way through the gum; but sometimes the corresponding ones of the upper jaw pierre first. This is the natural and general order in which the incisive teeth appear in the mouth ; but I find this order inverted by Richerand, while Albinus, Haller, Soemmering, Bichat, and others adopt it. "Richerand, I should presume, has either adopted this order from an individual case in nature, or he has followed Eustachius and some of the ancient authorities. The reason, that the teeth in the lower jaw come
sooner to perfection, appears to arise from the lower jaw being a more dense bone than the upper, and the vessels possessing the power of more easily and more readily secreting bone ; from which cause the vessels of the lower jaw can secrete a new bone when the old is dead, while those of the upper cannot.
Towards the end of the first year, the two first molares, or bicuspides of either the lower or upper jaw are visible; generally those of the lower jaw first.
During the second year, the canine teeth are pushed forth, and towards the end of the same year, the two secoud or remaining bicuspides make their appearance,
We have, therefore, the whole of the deciduous set, namely, twenty in number, present in the beginning of the third year.
The pulps of the permanent teeth begin to ossify about the sixth month after birth, and continue advancing to perfection till the seventh year, when the fangs of the deciduous teeth, and the alveolar floor which divided these from the permanent, are absorbed; and the deciduous teeth, having no longer any attachment, fall out. The permanent then make their appearance above the gum in an order similar to the deciduous teeth. The formation and perfection of this set of teeth occupy from twenty to thirty years. The permanent, incisive, and canine teeth are larger than the deciduous, that they may correspond to the increase of the jaw-bones. The molares wbich supplant the deciduous ones are smaller in size, that they may make room for the three which are to appear behind them. By the twelfth year, all the deciduous teeth have generally fallen out and been supplanted by the permanent. The anterior of the three backmost molares usually makes its appearance between the seventh and eighth year: the next or the fourth from the canine is ossified about the seventh year, and pushed forth about the eighteenth year; and the last or fifth from the canine appears between the eighteenth and thirtieth year, and sometimes even later.
Those vessels, through which the blood flows from the heart into every part of the body, are called arteries. The term, which is de
rived from two Greek words, signifying I hold air, was first adopted by the Alexandrian school, in consequence of the erroneous opinion which they entertained, that these vessels were designed for the distribution of air throughout the body.
All the arteries proceed from one great vessel, as the branches spring from the trunk of a tree.
The common carotid artery is destinéd for the supply of the head. It emerges from the chest by the side of the trachea ; mounts upwards in front of the vertebræ, and parallel with the trachea, till it reaches the upper margin of the thyroid cartilage, without sending off a single branch. At this part it divides into the external and internal carotid arteries, the former of which is distributed to the outside of the head; the latter to the brain.
The exteroal carotid continues its course upwards between the jaw and the ear, being imbedded in the substance of the parotid gland.
The internal carotid artery enters into the scull, through the canal formed in the substance of the temporal bone; and its branches ramify through the substance of the brain. All the arteries of the brain bave thinner coats than these vessels possess in any other part of the body.
The subclavian artery passes over the first rib, and behind the clavicle, into the cavity of the axilla. There it takes the name of axillary, and is covered by the pectoral muscles. Emerging from , the arm-pit, its name is again changed for that of brachial. This part of the trunk runs along the inside of the arm, close to the edge of the biceps muscle, until it reaches the elbow-joint, where it divides into the branches that belong to the fore-arm.
Tbe radial artery and the ulnar artery.
The radial artery runs along the fore-arm to the wrist, and sends off several branches to the palm and back of the hand.
The ulnar artery, when it has arrived at the wrist, passes forwards into the palm of the hand, more superficially than the radial, and forms the superficial arch of the palm.
The descending portion of the aorta passes through the diaphragm at the lower part of the chest, and takes the name of the