pass along the right side of this process; and the part in which they are situated is called the capsula Glissoni. Under the edge of this part is an opening, leading to the bag of the great omentum, and called the foramen epiploicum.

GALL-BLADDER AND BILIARY DUCTS.

The gall-bladder is a membranous bag, serving as a reservoir for the bile. Its shape is that of a pear, being broader at one end, and diminishing conically towards the opposite extremity. The broad end is called the fundus; and the smaller part the neck of the viscus. Its average capacity may be about one ounce. It is firmly bound to the surface of the liver by peritoneum. Its inner surface is elegantly reticulated, and furnishes a viscid mucus that mingles with the bile. The hepatic duct is continued in a straight course from the liver to the duodenum, in which it opens. It passes, however, in an oblique manner, between the coats of the intestine, before opening into its cavity. Hence the contents of the intestine cannot enter the duct; and the more fully the intestine is distended, the more completely is this prevented by the compression of the duct between the intestinal tunics. The neck of the gall-bladder is gradually contracted into a small tube, called the cystic duct, which joins the hepatic at an acute angle, after first running parallel with it. The remainder of the hepatic duct after the junction with the cystic, is often called the ductus communis choledochus. The surface of the cystic duct, as well as that of the neck of the gall-bladder, has numerous small folds of the internal membrane, which must retard and obstruct the course of the bile.

Pancreas-Is a gland of the conglomerate kind; that is, composed of numerous minute portions, united by cellular substance. It is connected by one end to the commencement of the duodenum, and extends across the vertebræ, behind the lesser arch of the stomach, to the spleen. Its length is about six inches; its breadth one and a half; and its thickness half an inch.

Each of the small molecules which compose this gland, has an excretory duct; these unite together into larger and larger trunks, and the main tube of all runs along the centre of the gland, and joins the ductus communis choledochus just before that duct opens into the duodenum.

Spleen-This part, which in common language is called the milt, is a soft and livid mass interposed between the great end of VOL. I.

the stomach and the diaphragm. It weighs about six or seven ounces. It consists of a congeries of cells filled with blood; as the arteries and veins of the organ communicate with them. It is closely connected to the great end of the stomach by vascular ramifications, which the splenic vessels send to the stomach. It has a concave and a convex surface; an anterior and posterior extremity; and an external peritoneal cover. ing.

ORGANS OF RESPIRATION.

As these are contained in the cavity of the thorax, we shall consider the subject in the form of a description of that cavity and its contents.

The cavity of the thorax is the space included by the dorsal vertebræ behind, by the ribs with their cartilages, the sternum, and intercostal muscles, at the sides and forepart; and by the diaphragm below. This cavity is lined by a membrane called the pleura, which has a smooth internal surface constantly moistened by a serous exhalation.

The cavity of the chest contains two distinct membranous bags, called the right and left bags of the pleura; each of these holds the lung of its own side, and is entirely separated from the opposite one. The pleura not only forms a bag which holds the lung, but is also reflected over the surface of the viscus, bestowing on it a smooth exterior investment. This is called the pleura pulmonalis, to distinguish it from the other, which is named pleura costalis.

If the cartilages of the ribs be divided on one side of the chest, the corresponding bag of the pleura will be opened; and it will then appear, that this is separated from the opposite one by a partition, which extends from the sternum in front to the vertebræ behind, and is known by the name of mediastinum. The pleura may be compared to two bladders placed laterally with respect to each other, but adhering only partially, and separated by various intervening bodies. Thus, the heart and adjoining large blood-vessels, the œsophagus and the division of the trachea into the two bronchi, are placed between the two pleuræ. The mediastinum then is the space included between the opposed surfaces of the two bags of the pleuræ, and containing the parts above mentioned. The name of anterior mediastinum is applied to a small interval left between the two pleuræ, just behind the sternum, and occupied only by a loose cellular texture. The posterior mediasti

num is a larger space in front of the bodies of the vertebræ; it contains the descending aorta, the vena azygos, thoracic duct, œsophagus, and the par vagum.

The capacity of the chest taken altogether varies according as we estimate it in a state of inspiration or expiration; being largest in the former, and smallest in the latter state. The right bag of the pleura is considerably larger than the left, as is also the right lung.

In the living state the lung is in close contact with the surface of the cavity, and follows all the motions of the sides of the chest. It is distended by the influx of air, when the chest is enlarged; and the air is expelled from the lung, when the chest is diminished. As soon as the thorax is opened in the dead subject, the lung falls down from the sides of the chest, or in technical language collapses; and then a large empty space is seen between it and the ribs. From this representation it should appear, that the lungs are quite passive in the business of respiration.

The lungs are two in number; one being contained in each bag of the pleura. They are loose and unconnected in these bags, except at one point, towards the upper and posterior portion of each viscus, where the great vessels enter them, and where the bag of the pleura is continuous with the reflected portion of the membrane. These are called the ligaments of the lungs.

Their colour varies considerably. It is always verging more to a red in proportion as the subject is younger; in the adult it has more of a spotted and livid cast. Towards the back of the lungs it is always much 'deeper from the gravitation of blood in the vessels in consequence of the position of the subject. It is lighter when the lungs contain much air.

The lungs are subdivided into lobes; of which the right contains three, and the left two. Their substance is composed of a congeries of minute membranous cells, about equal in size to a pin's head, and as these are more or less filled with air, they give the lung a peculiar spongy feel. These cells communicate with the ultimate ramifications of the air vessels, and receive air from that source. The pulmonary vessels ramify minutely in them, and thereby expose the blood to the effects of the contained air; and in this exposure the object of respiration is effected.

The windpipe. The tube, which conveys the external air into the lungs, may be divided into three parts; the larynx, the trachea, and the bronchi.

The larynx is a hollow cartilaginous organ, placed at the top of the trachea. The air which passes through this from the lungs, in expiration, produces the voice.

The cavity of the larynx opens above at the root of the tongue, and below into the trachea. The organ is composed of five pieces of cartilage: viz. the thyroid and cricoid cartilages, and epiglottis, and two arytenoid cartilages.

The thyroid cartilage is the largest, and consists of two irregularly quadrangular pieces, united in front at an obtuse angle. This part projects in the front of the neck, and much more conspicuously in the male than in the female sex: it is called pomum Adami.

The cricoid cartilage may be compared to a ring with a seal, of which the broad or seal part is placed behind, and the narrower portion in front. It is directly under the thyroid cartilage.

The arytenoid are two pyramidal portions of cartilage, connected by regular moveable articulations to the back of the cricoid.

The epiglottis is the softest cartilage of the larynx. It has a basis firmly tied to the thyroid cartilage, while its opposite extremity, which is very thin, is of a rounded figure, and stands directly upwards, except during deglutition, when it descends so as to cover the opening of the larynx.

The thyroid cartilage is tied by three ligaments to the os hyoides above, and by as many to the cricoid cartilage below: but the most important ligaments of these parts are the ligamenta glotcidis; which arise from the front of the arytenoid cartilages, and are attached to the posterior surface of the front portion of the thyroid. A longitudinal slit, called the rima glottidis, is left between these, and it is by the passage of the air through that slit that the voice is formed. Hence, from the great share which these ligaments have in forming the voice, the name of chordæ vocales has been given to them.

The larynx is lined by a vascular and very sensible membrane, copiously moistened with mucus, in order to defend it from the external air. It admits of free motion in the neck, and its parts are also moved on each other; particularly the arytenoid cartilages whose movements, by altering the size of the ruina glottidis, and the state of tension of the chorda vocales, contribu e most immediately to the variations in the tone of the voice.

The trachea is that portion of the aerial

tube, which is placed between the cricoid cartilages, and the origin of the bronchi. It is a cylindrical membranous tube, of from 5 to ths of an inch in diameter. It runs along the middle of the forepart of the neck, having the large blood-vessels of the head on each side, and being connected behind to the œsophagus. Soon after it has entered the chest, it divides into the two bronchi.

The tube of the trachea is furnished with hoops of cartilage, by which it is kept permanently open for the passage of the air: these are not however complete circles, being deficient behind. The lining of the tube is highly vascular and sensible, and covered with a copious mucous secretion, which is rendered necessary by the constant current of air to which it is exposed.

The bronchi are merely the two branches into which the trachea divides for the two lungs; and of these the right is the largest and shortest. They ramify through the lungs, dividing into smaller and smaller branches; and the ultimate ramifications communicate with the air-cells.

ORGANS OF CIRCULATION.

The heart is the centre of the circulating system; being the source of the arteries, and the termination of the veins. The younger the subject, the larger is the heart in proportion to the body. It is often smaller in tall and strong men, than under different circumstances.

It is connected at its posterior part, behind the sternum, by the large blood-vessels, being unattached every where else, and merely confined in its situation by the pericardium.

The pericardium is placed in the cavity of the chest, behind the second, third, fourth, and fifth ribs of the left side. It is covered to the right and left, by the bags of the pleura, which adhere by a loose cellular membrane. It is not actually connected by any part of its surface to the sternum. Below, it rests on the diaphragm, and adheres very firmly to the superior surface of the tendon of that muscle.

The cavity of the pericardium is larger than the heart, so that this viscus can move freely in it.

The bag of the pericardium in shape resembles the figure of the heart itself, being conical. Its substance is thick and compact, and it is much more dense and strong than the peritoneum or pleura. Where the great vessels are connected to the heart, this membrane becomes reflected over its surface; and hence the substance of the

heart has a close investment from this membrane, besides being contained loosely in the bag-like portion. A small portion of the large blood-vessels is included within the cavity of the pericardium; particularly of the aorta and pulmonary artery; which are consequently covered by the reflected portions.

The internal surface of the pericardium is moistened by a serous secretion from the exhalant arteries; which is collected after death into a few drops of a clear light yellow liquor. It is an unnatural increase of this that constitutes dropsy of the pericardium. This fluid in the living state lubricates the opposed surfaces of the heart and pericardium, and thereby facilitates their motion on each other, and prevents their accretion.

The heart, which is contained almost entirely in the left side of the chest, resembles a half cone; hence we distinguish in it a basis or broad part, and an apex or narrower portion; a convex and a flat surface. The basis is placed towards the right and backwards; the apex points obliquely to the left, forwards and downwards. The basis is opposite to the seventh or eighth.. vertebra of the back, and the apex points to the cartilage of the fifth or sixth left rib. The position however varies by the motion of the diaphragm in respiration, as it is drawn down in a strong inspiration, and again rises in expiration. Its position also seems to vary slightly, according to the situation of the body in lying.

A small portion of the left lung seems, as it were, removed just at the apex of the heart; so that that part of the viscus is not covered by the lung like the rest; but touches the front of the chest.

Those cavities of the heart which are called the right, are placed in front; and the left cavities are towards the back part; so that the epithets anterior and posterior, would correspond more nearly with the true position of these parts, than those of right and left.

The flat surface of the heart looks directly downwards, and rests on the tendon of the diaphragm; this, therefore, in point of position is inferior; the convex surface is turned upwards, forwards, and obliquely towards the left, so that it may be called the superior surface.

The weight of the human heart, when removed from the body, with its pericardium, is from 10 to 15 ounces.

Like the heart of all warm-blooded ani

mals, this organ consists of two hearts, closely and intimately connected. One of these is concerned with the circulation through the body, or the greater circulation; the other with the circulation through the lungs, or the minor circulation. These might perform their offices if separate and even distant from each other. Each of these hearts consists of two cavities; an auricle, or membranous bag, placed at the mouths of the veins; a ventricle, or strong muscular organ, placed at the orifice of the artery, and constructed for the purpose of driving the blood into that vessel and its branches.

The two auricles are placed at the basis or broadest part of the heart; and the two ventricles composing the chief bulk of the organ, are found in front of the former cavities.

In the following description of the structure of the heart, we shall trace the parts in the same order in which the blood passes through them. This fluid, then, after circulating through the blood-vessels of the body, after serving the various purposes of nutrition, secretion, &c. is returned into the right auricle of the heart by three large veins, viz. the superior and inferior vena cava, and the great coronary vein. The properties of this blood have been so altered in its course, that it is necessary for it to be subjected to the action of the atmosphere in the lungs, before it is again fit to be sent into the arteries of the body. The right auricle derives its name of auricle from a small fringed process, which is found at its anterior part; the rest of the cavity is called the, sinus of the vena cava. The

lining of this bag, as indeed that of all the other parts of the heart, consists of a smooth and polished surface. The muscular fibres of the auricle are not numerous nor large; they are arranged in parallel fasciculi, which have been compared to the teeth of a comb; and hence the epithet of musculi pectinati has been given to them.

The right auricle transmits the blood into the right, anterior, or pulmonary ventricle, through a large circular orifice, called the annulus venosus, or the auricular orifice of the ventricle. When this latter cavity contracts, the blood would be driven back towards the auricle, were not this prevented by a valve, called the tricuspidal or triglochine. This valve is formed by a production of the lining of the heart, divided into three pointed portions. These are tied by tendinous strings to certain projecting packets of the muscular fibres,

The pulmonary artery, which arises from the upper and anterior part of this ventricle conveys the blood into the lungs. The opening of this artery, which is called the arterial orifice of the ventricle, is furnished with three valves, called sigmoid or semilunar, which prevent any retrograde motion of the blood from the artery towards the heart.

The venous blood, by being exposed to the atmospheric air in the lungs, is altered in its properties, and becomes arterial blood, in which state it is returned to the left auricle of the heart by four pilmonary veins, two of which belong to each lung. This left or posterior auricle consists of a large cavity, called the sinus of the pulmonary veins; and of a smaller process or auricula. It is situated quite at the upper and back part of the heart, and transmits the blood through the auricular orifice of the left ventricle into that cavity. This opening is perfectly similar in all essential circumstances to the corresponding part on the right side of the heart. But its valve, being divided into two portions only, is called mitralis, from a comparison with a bishop's mitre.

The left ventricle is much thicker and stronger than the right. It feels externally almost like a solid mass of flesh; while the right is comparatively thin and flabby. The

reason of this difference is obvious. The left ventricle has to drive the blood to the most remote parts of the body, whereas the right only sends it through the lungs. The aorta arises from the left ventricle, and its mouth is guarded by three semi-lunar valves. This is the trunk from which the arteries of the whole body arise.

STRUCTURE OF 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 derived from an, air, and τηρέως I hold, was first adopted by the anatomists of 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.

The larger arteries have thick and elastic

sides, so that they remain open when divided, and present a regularly circular aperture. The sides may be separated into three strata of dissimilar substances, which are technically called coats. The innermost, which is generally termed the cuticular coat, is very thin, but strong and inelastic. Upon this circumstance depends the regularly circular form of an injected artery; for if the cuticular coat be burst from the employment of too great a force in injecting, the exterior tunics are distended into an irregular and uncertain figure. The internal surface of this coat is perfectly smooth, so that the blood glides along it without impediment; the external surface is connected to that coat, which surrounds it. The middle, or, as it is called, the muscular coat, is composed of a congeries of circular fibres, separable into numerous strata, but not much resembling muscular fibres as found in other situations. The external, or elastic coat of the artery, is made of condensed cellular substance; it is powerfully elastic; and is resolved into a looser texture, which unites these vessels to the neighbouring parts.

It appears that the larger vessels have the greatest elastic power, with the smallest muscular force; while these properties exist in reversed proportions in the smaller vessels. In the large arteries muscular power is unnecessary, for the force of the heart is fully adequate to the propulsion of the blood; but in the smaller arteries, where the effect of the heart's action declines, a proportionate muscular power is allotted to the vessel to urge on the circulating fluids.

The arteries have their nutrient arteries and veins, their absorbents, and their nerves.

All the arteries proceed from one great vessel, as the branches spring from the trunk of a tree; and we proceed to notice certain circumstances observable in their ramifications.

cal, the basis of the cone being in the extreme arteries, and the apex in the heart.

2. When a large artery sends off a branch, its course does not, in general, deviate further from that of the trunk than an angle of 45 degrees. Sometimes a branch, which has gone off at an acute angle, returns, and proceeds in a contrary direction to that of the trunk. Sometimes indeed a large artery does proceed from the trunk at nearly a right angle, as the renal arteries. Though the large arteries generally ramify at acute angles, there is great diversity in the branching of the smaller

ones.

3. Arteries in general do not pursue a straight, but a serpentine course; this is remarkably the case in some instances; as in the spermatics; those of the face and occiput, and in most of the smaller arteries.

4. Though the ramification of arteries may be compared to the branching of trees, yet it differs materially in this particular, that the different branches frequently conjoin. This conjunction is technically termed, if we borrow the term from the Greek language, their anastomosis; if from the Latin, their inosculation. This union of arteries rarely happens among the larger ones, but frequently among the smaller; and increases in number in proportion to the minuteness of the vessels. The utility of the inosculations of arteries is evident; were it not for this circumstance, if any arterial trunk were accidentally compressed, so that the current of blood in it should be for some time obstructed, the parts, which it supplied, must perish. But in consequence of the frequent communications of these tubes with each other, the blood can pass from the adjacent arteries into all the branches of any one accidentally obstructed.

When arteries inosculate, two currents of blood, moving in opposite directions, must come together, and retard each other's motion. This probably is the reason, why larger arteries, in which the blood flows with rapidity, so seldom conjoin; whilst the smaller ones, in which the blood's motion is more tardy, communicate in surprising numbers, and with a frequency proportionate to their minuteness. The very frequent communications of the minute arteries prevent the prejudicial consequences of obstruction of the trunks almost as effectually, as if those arteries themselves communicated by more direct and larger channels. All these minute arterial tubes are capa