THE DUCTLESS GLANDS

These glands do not possess excretory ducts. They furnish materials which are added to the blood or lymph as it passes through them. The material from each gland is known as an internal secretion, or hormone. Some of these secretions are powerful materials and influence profoundly the body nutrition. The ductless glands are usually given as follows: the spleen, the lymph glands, the pineal gland, the pituitary body, the thyroid gland, the thymus gland, the adrenal glands, and the parathyroids. The spleen, the pituitary, the pineal, and the lymph glands are described in other sections.

The Thyroid Gland (Fig. 21, No. 16). Location.—The thyroid gland lies on the ventral side of the carotis communis at a point where the carotis communis touches the jugular vein, which is about the point of origin of the vertebral artery.

Shape.—The thyroid gland is small, oval or somewhat roundish, and red or rose-colored.

Structure.—The thyroid gland has a fibrous capsule, which sends into the interior septa which divide it into acini. These acini are closed and contain a fluid. The thyroid is a ductless gland. Short arteries from the carotis enter this gland, and some large veins connect it with the jugular vein. The lymph vessels which lie along the neck are closely connected with it and receive twigs from the gland. The minute lymphatic capillary endings are found in its septa and in its capsule. The acini are lined with a single or a double row of cuboidal secreting cells. There are two kinds of cells, namely, secreting and resting cells. The actively secreting cells secrete colloid.

The parathyroids consist of two small bodies attached to the lower pole of the thyroid.

The Thymus Gland (Fig. 47, No. 10).—The thymus gland is an organ of fetal and early baby chickhood. It soon undergoes retrogressive changes into fat and connective tissue. It is of epithelial 191origin, being formed in fetal life from the entodermal cells of the dorsal end of the throat fissure.

Location.—The thymus gland lies anterior to the thyroid, the latter lying the deeper.

Shape.—The thyroid consists of two lobes, which are united by connective tissue, and appears as a loop-like acinous gland lying along the neck and near the region of the bronchi and the jugular veins with fibrous extensions toward the head.

Structure.—The gland lobes are divided into lobules, which consist of a cortical and a medullary portion. The cortex consists of nodules of compact lymphatic tissue similar to those found in the lymph glands. These occupy the chambers formed by the septa of connective tissue. In the medulla there are a number of spherical, or oval bodies composed of concentrically arranged epithelial cells. These are known as Hassall’s corpuscles, and represent only the remains of the original glandular epithelium. They are characteristic of the thymus gland. The thymus appears to be a type of lymph organ. Lymph vessels are rare; a few blood-vessels on the upper side form capillary nets.

The Adrenal Gland (Fig. 54, F). Location.—The adrenal gland, often called the suprarenal capsule, lies just anterior to the front part of the anterior lobe of the kidney, adjacent to the testicles in the male and to the ovary in the female. It is loosely attached by connective tissue to the posterior aorta and to the vena cava.

Shape.—It is yellowish-brown or reddish-pink in color, small, and of irregular formation.

Structure.—The adrenal gland consists of a cortical and a medullary portion, although these two parts are not distinctly marked. The cortical portion has columns which extend deeply into the gland, and the medullary portion sends columns into the cortical portion. Therefore, the two substances, lying side by side, form a cord-like structure.

It is probable that the cortical portion is derived from the ingrowths of the peritoneum, and the medullary cords from the sympathetic ganglion.

The cells are cylindrical or polygonal in shape, with an eccentric substance between the columns. The cords, or columns, form between them, elongated channels which extend into the interior of the gland and end as blind or cæcal extremities. Large ganglionic nerve cells belonging to the sympathetic system occur near the surface 192of the gland. The blood-vessels are not well developed in the interior of the gland but are numerous and of good size in the outer parts. Lymph vessels are also present.

Function.—The adrenals are ductless glands. They secrete an internal secretion, or hormone, which influences the tonus of the blood-vessels. An extract from these glands is called adrenalin.

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