FIG. 1180 The hypophysis cerebri in position. Shown in sagittal section. (See enlarged image)
FIG. 1181 Median sagittal through the hypophysis of an adult monkey. Semidiagrammatic. (Herring.) (See enlarged image)
The hypophysis consists of an anterior and a posterior lobe, which differ from one another in their mode of development and in their structure (Fig. 1181). The anterior lobe is the larger and is somewhat kidney-shaped, the concavity being directed backward and embracing the posterior lobe. It consists of a pars anterior and a pars intermedia, separated from each other by a narrow cleft, the remnant of the pouch or diverticulum. The pars anterior is extremely vascular and consists of epithelial cells of varying size and shape, arranged in cord-like trabeculæ or alveoli and separated by large, thin-walled bloodvessels. The pars intermedia is a thin lamina closely applied to the body and neck of the posterior lobe and extending onto the neighboring parts of the brain; it contains few bloodvessels and consists of finely granular cells between which are small masses of colloid material. The pars intermedia in spite of the fact that it arises in common with the pars anterior from the ectoderm of the primitive buccal cavity is often considered as a part of the posterior lobe which arises from the floor of the third ventricle of the brain. Although of nervous origin the posterior lobe contains no nerve cells or fibers. It consists of neuroglia cells and fibers and is invaded by columns which grow into it from the pars intermedia; imbedded in it are large quantities of a colloid substance histologically similar to that found in the thyroid gland. In certain of the lower vertebrates, e.g., fishes, nervous structures are present, and the lobe is of large size.
FIG. 1182 Vertical sections of the heads of early embryos of the rabbit. Magnified. (From Mihalkovics.) A. From an embryo 5 mm. long. B. From an embryo 6 mm. long. C. Vertical section of the anterior end of the notochord and hypophysis, etc., from an embryo 16 mm. long. In A the buccopharyngeal membrane is still present. In B it is in the process of disappearing, and the stomodeum now communicates with the primitive pharynx. am. Amnion. c. Fore-brain. ch. Notochord. f. Anterior extremity of fore-gut, i. h. Heart. if. Infundibulum. m. Wall of brain cavity. mc. Mid-brain. mo. Hind-brain. p. Original position of hypophyseal diverticulum, py. ph. Pharynx. sp.e. Sphenoethmoidal. bc. Central. sp.o. Sphenoöccipital parts of basis cranii. tha. Thalamus. (See enlarged image)
From the pars intermedia a substance, no doubt an internal secretion, causes constriction of the bloodvessels with rise of arterial blood-pressure. This substance seems to have a stimulating effect on most of the smooth muscles, acting directly upon the muscle causing contraction. It also increases the secretion of the urine; of the mammary glands when in functional activity; and of the cerebrospinal fluid. Extracts of this lobe also influence the general metabolism of the carbohydrates by accelerating the process of glycogenolysis in the liver.
Enlargement of the hypophysis and of the cavity of the sella turcica are found in the rare disease acromegaly, which is characterized by gradual enlargement of the face, hands, and feet, with headache and often a peculiar type of blindness. This blindness is due to the pressure of the enlarging hypophysis on the optic chiasma (Fig. 1180).
Development of the Hypophysis Cerebri.This in the adult consists of a large anterior, consisting of the pars anterior and the pars intermedia, and a small posterior lobe: the former is derived from the ectoderm of the stomodeum, the latter from the floor of the fore-brain. About the fourth week there appears a pouchlike diverticulum of the ectodermal lining of the roof of the stomodeum. This diverticulum, pouch of Rathke(Fig. 1182), is the rudiment of the anterior lobe of the hypophysis; it extends upward in front of the cephalic end of the notochord and the remnant of the buccopharyngeal membrane, and comes into contact with the under surface of the fore-brain. It is then constricted off to form a closed vesicle, but remains for a time connected to the ectoderm of the stomodeum by a solid cord of cells. Masses of epithelial cells form on either side and in the front wall of the vesicle, and by the growth between these of a stroma from the mesoderm the development of the anterior lobe is completed. The upwardly directed hypophyseal involution becomes applied to the antero-lateral aspect of a downwardly directed diverticulum from the base of the fore-brain (page 744). This diverticulum constitutes the future infundibulum in the floor of the third ventricle while its inferior extremity becomes modified to form the posterior lobe of the hypophysis. In some of the lower animals the posterior lobe contains nerve cells and nerve fibers, but in man and the higher vertebrates these are replaced by connective tissue. A canal, craniopharyngeal canal, is sometimes found extending from the anterior part of the fossa hypophyseos of the sphenoid bone to the under surface of the skull, and marks the original position of Rathkes pouch; while at the junction of the septum of the nose with the palate traces of the stomodeal end are occasionally present (Frazer).