to lessen the atmospheric pressure upon parts they touch. 59 balanced by a column of water varying in height from one foot to a foot and a half; and in rabbits and cats, by a column of water varying in height from six to ten inches a.

If the lungs were inelastic, but admitted of being unfolded to an indefinite extent on the enlargement of the chest, the pressure of the atmosphere upon the inner surface of the chest would be the same as elsewhere; but it is clear that in proportion as the lungs have a tendency to resist the atmospheric pressure, or in other words to recede from the pleura reflexa, the weight of the atmosphere must be lessened upon all the parts against which the lungs are applied. Thus it happens that the outer surface of the heart is not at any time exposed to the same degree of pressure with parts external to the chest; and that the degree of pressure is yet further reduced, when upon the dilatation of the chest the lungs become further expanded, and their elastic resiliency increased.

The heart is of a conical figure: the septum, which divides its cavities, is disposed nearly in its long axis, but gives the apex of the heart to the left ventricle exclusively. The shape of each chamber of the right cavity is triangular, of the left oval: the contents of each are about two ounces. The auricles are of a thin substance; the ventricles are of considerable thickness; the muscular fibres of the right auricle are disposed in parallel lacerti, prominent inwards, called musculi pectinati: a like appearance is not seen in the left auricle. In the appendage of each auricle the lacerti are reticu

a Phil. Trans. vol. cx. p. 42.

60

Structure of the Heart.

larly interwoven. The external layer of muscular fibres in the left ventricle, extends spirally from the base and superior longitudinal furrow forwards and towards the left, and turning round the margin of the heart reaches the longitudinal furrow upon its under surface. The external layer of muscular fibres upon the right ventricle, extends in a like manner from the base and inferior longitudinal furrow obliquely forwards to the superior. In the middle layer no regular disposition seems observed. The lacerti of the inner layer again intersect each other reticularly, without any exact order, except that in the left ventricle two fleshy columns, and in the right three or four, project towards the auricle. The aperture of either ventricle towards the artery, which rises from it, is perfectly smooth.

Either cavity of the heart is lined with a thin transparent membrane, which is readily separable from the inner surface of the auricles and ventricles, and is found to be continued along the artery, which terminates the latter, and along the veins that open into the former. This membrane is in a degree firmer and more opake upon the left side of the heart than upon the right. In the arteries it appears of a more brittle texture than in the veins: it is every where in contact with the blood, and is usually classed among the serous membranes.

At the opening of the inferior cava into the right auricle, the inner membrane is raised along the left margin of the vein, so as to form a crescentic fold, which is termed the Eustachian valve. By this provision, useful only in the foetal state, the inferior cava is made to open exactly opposite to the fossa ovalis. Muscular fibres are often contained in the Eustachian valve. At the opening

Structure of the Heart.

61

of the coronary vein, another semilunar fold of membrane forms a valve to guard its oblique aperture, and to prevent the regurgitation of blood from the auricle into the vein. No valve is placed upon the entrance of the superior cava into the right auricle, or of the pulmonary veins into the left.

The valve between each auricle and ventricle is a reduplicature of the inner membrane, thickened by intervening fibrous substance. Its floating margin is irregular, and presents three points in the right, two in the left ventricle; whence the former is termed the tricuspid, the latter the mitral valve. The floating edge of the valve is attached by short tendinous threads, called chordæ tendineæ, to the fleshy columns of the ventricle. In general each columna carnea receives all the tendinous chords from the opposite edges of two adjoining points of the valve. The valve at its points is about nine or ten lines in depth, intermediately about five. By the action of the fleshy columns the floating margin of the valve can be drawn together, and the passage closed from the ventricle into the auricle. The margin of the valve is thickened with numerous little granular bodies, called corpora sesamoïdea.

The valves at the root of the aorta and of the pulmonary artery are of a different description; they consist of three semilunar folds of the inner membrane attached by their circular margin, each along a third of the circumference of the artery. They are so disposed that when blood issues from the ventricle, they are thrown up, and lie in contact with the parietes of the artery: but upon the reflux of the blood towards the heart, they are thrown down and sacculated, while their floating

62

Nature of Arteries

margins, the centre of each of which is strengthened by a corpus sesamoïdeum, meet as tense chords, describing three radii of the circular aperture of the vessel. These valves are termed the sigmoïdal or semilunar valves of the aorta and pulmonary artery.

The heart is supplied with blood from the two coronary arteries, which are the first branches of the aorta; it has a large supply of lymphatic vessels, its nerves are derived from the sympathetics and nervi vagi.

An artery is a cylindrical and highly elastic tube; its thick texture is separable into, 1. An inner sérous coat; 2. A middle fibrous coat, of a yellow colour in the larger trunks, of a reddish brown in the smaller branches, composed of fibres, which are disposed transversely, but seem in some degree interwoven; they are very elastic, and at the same time so brittle, that the pressure of a ligature tied upon an artery cuts through the fibrous together with the inner coat; and 3. An outer cellular coat, consisting of tough white elastic fibres closely interwoven, which the pressure of a ligature does not divide. Dr. Hales ascertained that the force required for bursting one of the carotids of a dog, is equal to that of a column of water one hundred and ninety feet high. He does not remark that the artery became dilated, but observes that with this force the artery burst at once.

A vein is a flexible tube of great strength, but of little elasticity, separable into an inner serous tunic, and a dense external coat of white and closely interwoven fibres. The inner coat is thrown at intervals into semilunar folds that occur in pairs, and are attached by their convex margins each to half the circumference of a vein: as the blood flows towards the heart, these valves lie against

the sides of the vessel; upon its reflux they are thrown down and their edges meet. Valves are not found in the vena cava or in the visceral veins: they are found in the iliac veins, in the veins of the neck and head, and of the extremities, especially in those which are subcutaDr. Hales found the jugular vein of a mare to burst with a force equal to that of a column of water one hundred and forty-four feet high".

neous.

When an artery divides, the two branches have a common area larger than that of the trunk, and in most instances diverge at an acute angle; the same is observed of veins. The arterial and venous trunks generally are distributed together: the largest arteries have one accompanying vein, the smaller arteries two. In the neck and extremities a superficial set of veins is added to that which accompanies the arteries. The area of the venous system is greater than that of the arterial, in the proportion of four to one, according to Borelli. The ratio between the capacity of individual arteries and veins in different parts, is very various: between the carotid and internal jugular 196:441, between the subclavian artery and vein 81:196, between the crural artery and vein 3844:7396, between the aorta and vena cava 9:16, between the splenic artery and vein 156:676°.

Arteries and veins have their vasa vasorum, and are supplied with nerves derived from the sympathetic and from the spinal nerves, if not indiscriminately from all but the first, second, and portio mollis of the seventh. If the chest and pericardium be laid open in an animal

Hæmastatics, p. 151.

Haller. Elementa Physiologiæ, vol. i. p. 131.