Mechanical Obstructions, which give rise, according to circumstances, either to preponderance of dilatation or preponderance of hypertrophy.

A. Mechanical Obstructions In The Ostia Of The Heart

The number of diseases produced by these causes is probably the greatest. They are consequent on various diseases of the arterial and auriculo-ventri-cular valves, more especially on the secondary effects of endocarditis (Bouillaud's chronic endocarditis), and admit of being generally referred to contraction (stenosis) of the ostium, and to insufficiency of its valves. Whether the latter has a tendency to give rise more frequently to dilatation, and the former to hypertrophy, has not yet been determined.

As these diseases of the valves are far more frequent in the left than in the right side of the heart, so also are the diseases of the heart to which they give rise. The auricle or the ventricle becomes the more acutely affected according to the seat of the valvular disease, while both are simultaneously attacked where the arterial and auriculo-ventricular valves are alike diseased. Owing to the impediment presented to the emptying of the heart's cavities on the left side, and the consequent obstruction of the capillary circulation through the lungs, disease extends to the right ventricle, and from thence to the right auricle, commonly manifesting itself as hypertrophy with excessive dilatation, and occasionally - more especially in the auricle - as passive dilatation.

B. Mechanical Obstructions In The Arterial Trunks

Mechanical Obstructions In The Arterial Trunks, occurring at different distances from the heart, promote the development of cardiac disease in proportion to their greater vicinity to that organ.

These obstructions are of various kinds.

Congenital contraction of one or other of the vascular trunks is not unfrequent, and is generally manifested in the aortic trunk by an insufficiency of calibre which extends to the branches, and probably also to the more delicate ramifications. This condition induces very considerable dilatation with more or less hypertrophy in the left ventricle, and subsequently leads to similar dilatations of the left auricle, and of the cavities on the right side of the heart.

To this class belong acquired contractions of the vascular trunks and their main branches, together with their final obliteration, dependent on alterations of texture.

Further, the contractions caused by compression or expansion, and the obstructions presented to the current of the blood by the elongation, angular curvature, twisting, etc, of the large arteries.

Lastly, there are also dilatations of these arterial trunks, which, as is well known, appear frequently, and in the most excessive degrees, in the trunk of the aorta and its main branches, under the various forms of aneurism. These conditions generally induce active dilatation of the left ventricle, with a rapidity and intensity proportional to their vicinity to the heart and to their importance. It must be observed, that although insufficiency of the valves of the diseased trunk may simultaneously Occur, its existence is not necessary to the formation of cardiac disease, which is then owing to the obstacle opposed to the advance of each successive blood-wave by the mass of blood accumulated in the dilated and paralyzed trunk.

C. Similar (Mechanical) Obstructions In The Capillaries

Obstructions in the capillaries of the pulmonary artery are tolerably evident, inducing active dilatations of the right ventricle, and, subsequently, dilatation of the right auricle, having in some cases a more active and in others a more passive character. To these belong: a. The obstruction presented to the circulation through the capillary system of the lungs by contraction of the thoracic cavity, and the consequent excessive thickness of the pulmonary texture. This condition presents a great degree of intensity and a character of constancy in malformations of the thorax and contractions of its cavities consequent on curvature of the spine, more especially extreme scoliosis and kyphosis, and in cases of rachitic chicken breast. When the above-named diseases of the right side of the heart attain a great degree of intensity, they commonly give rise to highly developed hypertrophy.

B. Next, we must notice an increased condensation and atrophy in a more or less considerable portion of the lung, in consequence of compression from pleuritic exudation and of its healing, and of indurated pneumonia. The development of the heart will be proportional to the actual atrophy of the pulmonary texture, to the extent of the surface atrophied, and to the degree in which the capillaries have been destroyed by obliteration.

r. Atrophy of the pulmonary texture associated with extended and considerable bronchial dilatation.

3. The obstruction presented to the circulation through the capillaries of the lungs by their emphysema (emphysema vesiculare). This condition depends at first on the continued excessive expansion of the pulmonary cells, and in the more advanced stages on the obliteration of the capillaries which occurs in an uniform degree with the atrophy of the texture of the lung. The importance of the heart-disease depends on the extent, degree, and period of duration of the emphysema; occasionally the disease is very intense.

e. The obstruction opposed to the injection of the lungs by the pulmonary artery, owing to the insufficient emptying of the pulmonary veins in consequence of disease of the left side of the heart, and of the habitual over-filled condition of the capillaries of the lungs. Hence arise the greatest number of diseases of the right side of the heart, and all the numerous consecutively developed dilatations of the right ventricle and auricle, which originate in the left cavities, and are extended by means of the capillaries of the pulmonary system. The most important of these forms of dilatation are those which, in the manner already indicated, depend upon a contraction of the auriculo-ventricular opening on the left side of the heart.

It is only in comparatively rare cases that tuberculosis of the lungs and tuberculous pulmonary phthisis give rise to even a very moderate degree of active dilatation of the right side of the heart. A certain degree of diminution in the size of the heart is more frequent, and is then manifested in the form of simple or even concentric atrophy, which corresponds with the general tabes and the wasting of the mass of the blood.

On the other hand, similar obstructions in the capillaries of the aortic system are either wholly unknown, or are so obscure, that although à priori conjectures may be hazarded regarding their persistent or transient existence, no physical (anatomical) demonstrable facts can be established in reference to the subject.