ATROPHY.

Atrophy of the bone-marrow is not infrequent in old age or in marasmic conditions.

HYPERTROPHY.

Hypertrophy of the bone-marrow occurs in various anemie diseases. Strictly speaking, there is atrophy or disappearance of the fat-cells of the yellow marrow of the long bones and hyperplasia of the true lymphoid elements. The bone-marrow, therefore, assumes more and more the appearance of the red or lymphoid marrow of early life. The highest grades of this transformation are met with in progressive pernicious anemia (Plate 3, Fig. 1) and in leukemia, but similar changes occur in cancer, tuberculosis, and various other cachectic conditions. The marrow becomes soft and red in color, and in extreme cases may be darkred and liquid.

Microscopically the fat-cells are much reduced in number and may be wholly wanting. In their place are found marrow-cells of various sizes, many of them containing neutrophilic and eosinophilic granulations. Cells containing pigment-granules or red blood-corpuscles, and nucleated red blood-corpuscles of different sizes, are seen in varying numbers.

In leukemia the marrow often has a mottled appearance, lightcolored areas composed largely of white corpuscles arrested in the marrow or formed in loco by active proliferation, alternating with darker areas of congestion or hemorrhagic extravasation (Plate 3, Fig. 2). The light-colored areas may predominate and may present a puriform appearance (pyoid marrow). More rarely the appearance of the marrow in leukemia is similar to that seen in the other anemic diseases. Microscopically a curious constituent is discovered the Charcot-Neumann crystals (see Leukemia). It has been held that leukemia is dependent primarily upon the disease of the bone-marrow (myelogenous leukemia), but the marrowchanges, in some cases at least, are secondary.

INFLAMMATION.

Osteomyelitis, or inflammation of the marrow, is infectious. in nature, and may occur in the course of various diseases, such as typhoid fever, relapsing fever, small-pox, septicemia, and the like; or as a result of traumatism and direct infection. The marrowdisease may be the only expression of an infection which has arisen in an obscure manner (infectious osteomyelitis). In the cases occurring in the course of infectious diseases, the changes are comparable to those which occur in the spleen under the same circumstances. The marrow assumes a redder color than normal, and it may be studded with punctate hemorrhages. In other

cases areas of necrosis and granular degeneration of the cells may be present, and increased numbers of lymphoid cells or white blood-corpuscles may be discovered. Sometimes the marrow is quite purulent (see Diseases of Bone).

INFECTIOUS DISEASES AND TUMORS.

(These will be referred to in the discussion of Diseases of the Bones.)

THE THYMUS GLAND.

Anatomy and Development.-The thymus gland at its earliest period consists of endodermie epithelium arranged somewhat like that of an epithelial gland. Later, mesoblastic lymphoid cells and connective tissue infiltrate it; and at birth and for several years thereafter it is composed largely of lymphoid tissue arranged like the follicles of lymphatic glands. Here and there in the center of these may be seen concentric whorls (the corpuscles of Hassall), the remains of the original epithelial cells. After the second year of life retrogressive changes take place, and by the age of adolescence the gland is converted into a mass of fatty connective tissue.

Congenital Abnormalities.-Complete absence or various minor irregularities of the thymus may occur; at times it is found enormously hypertrophied. In the latter case the root of the great vessels, the pericardium, and heart may be covered over by the enlarged thymus, and sudden death seems at times due to this cause. Thymic asthma, so-called, is rarely, if ever, due to enlargement of the thymus.

Circulatory Disturbances.-Intense congestion and punctate hemorrhages may be found in cases in which death has occurred from asphyxia.

Inflammation of the thymus as a primary disease is of doubtful occurrence, but abscesses may occur in cases of general pyemia, or from extension of suppurative affections of adjacent parts.

Infectious Diseases.-Syphilis occurs in the form of gummata, especially in the new-born. Caseation and softening of the gumma may occur, and a resemblance to abscess is thus produced. Miliary or caseous tuberculosis occasionally invades the thymus gland.

Tumors. The thymus or its remnant is not infrequently the place of origin of lymphosarcoma of the anterior mediastinum (see Fig. 47). Tumors having this origin may be recognized by their shape and by the regularity of their outlines, the lymphosarcomata (lymphadenomata) of the lymphatic glands of the anterior mediastinum having a more irregular lobulated appearance. Ordinary round-celled sarcoma has been described, and epithelioma springing from the corpuscles of Hassall has been seen in a few cases.

CHAPTER III.

DISEASES OF THE CIRCULATORY SYSTEM.

THE HEART.

Development of the Heart.-At the earliest period of fetal life the heart is represented by a hollow tube, lying toward the ventral aspect of the neck. Later this assumes an S-shape, and still later a transverse constriction marks the position which the auriculoventricular grooves subsequently occupy. Finally, vertical grooves divide the lateral halves into the respective auricles and ventricles; and the truncus arteriosus, which is at one of the ends of the primitive tube, becomes divided into two parts, forming the pulmonary artery and aorta. The separation of the cavities within is accomplished by the outgrowth of septa springing from the walls of the primary cavities. The septum dividing the ventricles is the first to appear, and springs forward from the posterior wall. Next a budding is seen in the position which is later occupied by the tissue between the auriculoventricular orifices of the two sides; still later the auricular cavity is divided into two parts by a process beginning at the lower and posterior part. The septum which divides the truncus arteriosus is essentially connected with or is a part of the septum which separates the ventricles. All of these changes begin from the seventh to the ninth or tenth week of fetal life.

Anatomic Considerations.-The heart consists of three layers, the endocardium, muscular layer, and pericardium. The endocardium is the inner lining of the organ, and is composed of a layer of endothelial cells resting upon a stratum of connective tissue. It is continuous with the lining membrane of the arteries, and by duplications forms the valves. The muscle of the heart is arranged in lamellæ. The fibers of the heart-muscle are peculiar in being branched and in being devoid of a sarcolemma. The pericardium resembles the endocardium in structure.

The blood-vessels supplying the heart-muscle are branches of the coronary arteries. They divide and subdivide, and afford abundance of blood proportionate to the needs of so active an organ. The lymphatic system is equally developed. The nervous system includes numerous ganglionic centers in the furrows between the ventricles and between the auricles and ventricles. Other ganglion-cells are found within the muscle itself.

Details regarding the gross structure of the organ need not be mentioned here. The weight of the heart in the adult male is about 300 g., in the adult female about 250 g. The volume of the entire organ in the adult male is about 290 to 310 cu.cm., in the adult female about 260 to 280 cu.cm.

CONGENITAL DISEASES AND DEFORMITIES.

Abnormalities of the heart may be discovered in the new-born, and are the result of developmental defects, or less commonly of