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 jawbone 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 hollow tubular form, to afford lodgement 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, that 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 lining 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 through 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 pierte 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 second 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 which 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.

The Arteries.

Those vessels, through which the blood flows from the heart into every part of the body, are called arteries. The term, which is de

G

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 destined 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 external 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 have 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.

The 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

abdominal aorta. It is still situated on the left side of the bodies of the vertebræ, and at the fourth lumbar vertebra it terminates by dividing into the two common iliac trunks.

The common iliac divides into the external and internal iliac branches, of which the former goes to the thigh, the latter enters the cavity of the pelvis.

The external iliac artery, having changed its name for that of femoral, runs along the front of the thigh, and then bends inward to the ham, where it takes the name of popliteal. It passes through the latter space to the leg, when it terminates by dividing into two, of which one runs along the front, and the other the back of the leg. There is another large arterial trunk in the body, besides the aorta, called the pulmonary artery; this rises from the right ventricle, and conveys the venous blood to the lungs, for the purposes of respiration.

The blood thrown from the heart dilates the arteries, and their own elastic force contracts them; by which means they vibrate, in proper places, very perceivably against the finger; bring advices of the utmost importance to the physician, and very much assist bim, both in discovering the nature of diseases, and prescribing for their respective cures.

The Veins.

The blood is constantly moving in the arteries from the trunks into the branches; in the veins it follows a directly opposite course, and flows from the branches to the trunks.

There are seven large venous trunks in the body, to which all the blood is returned: three of these, viz. the superior and inferior vena cava, and the coronary vein of the heart, return the blood, which has circulated through the body, into the right auricle of the heart; the other four are the pulmonary veins, and bring the blood back from the lungs to the left auricle.

The inferior vena cava is a very large trunk, running along the spine at the right side of the aorta. It returns the blood from all the lower parts of the body.