The pituitary and pineal bodies are two structures on the ventral and dorsal sides of the brain respectively, which require special consideration in this connection.
The pituitary complex has been termed the “master gland” in the endocrine system because the activity, particularly of the thyroid and of the gonadal glands, is interlocked with and initiated by this gland. It has been aptly called by the French physiologists “Vorgan enigmatique.”
It is a compound structure of double origin, located most inaccessibly just behind the optic chiasma in the space between the roof of the mouth - roof of the nasal cavity in amniotes - and the ventral side of the brain. In man and higher vertebrates it lies ensconced in the sella turcica, which is a cradle-like space hollowed out in the sphenoid bone of the cranial floor. Its dual origin is due to the fact that a part of it is composed of an up-pushing growth from the roof of the stomodaeum which meets and becomes incorporated with a down-pushing extension from the floor of the brain. The stomodaeal portion is the adenohypophysis, the nervous portion the neurohypophysis. Phylogenetically it is very old, being represented in all living vertebrates.
Its relative size decreases from the lower to the higher vertebrates. In adult man it is “about as large as a hazel nut,” or rather less than a cubic centimeter in bulk, weighing normally 0.5 gram. In myxinoids it discharges its secretions directly into the pharynx, but otherwise throughout the vertebrate series it has become a ductless gland, concerned solely with internal secretion.
The name “pituitary,” meaning phlegm, was given to this structure because Galen and the early anatomists, who had already discovered it, thought that it produced nasal secretions.
The pituitary body consists primarily of three parts: one posterior, pars nervosa; one intermediate, pars intermedia; and one anterior in position, pars anterior. In some vertebrates there is distinguishable, in addition, a so-called Uebergangsteil, containing transitional types of cells between those characterizing the anterior lobe and the posterior lobe, as well as a pars tuberalis associated mostly with the anterior lobe. The pars tuberalis has been found in amphibians and mammals but is absent in cyclostomes, some fishes, and most reptiles.
The posterior lobe, or pars nervosa, of the adult mammal consists of a neural portion derived from the brain (Fig. 417). It is formed from the infundibulum, which is a downgrowth of the diencephalon of the brain.
The hormone pituitrin, produced by the posterior lobe of the pituitary, has to do with fat formation, the production of urine, and the development of the sexual apparatus, the whole structure in the female increasing in size during pregnancy. Pituitrin stimulates smooth muscle cells to contraction. It therefore diminishes the caliber of the arterioles, thereby bringing about both an increase in blood pressure and a reduction in urine secretion. It is used in obstetrics to accelerate the contraction of the uterus and thereby hasten parturition, the delivery of the child. When administered at a later stage it assists the uterus to return to its resting condition.
By giving pituitrin to pigeons, Riddle succeeded in bringing about a sort of abortion in these birds, since it caused them to lay eggs at an exceptionally early stage of development.
The pars intermedia is the least vascular part of the pituitary mass, but it is nevertheless apparently secretory in character. It is well developed in lower vertebrates and in all vertebrates it separates the anterior lobe from the pars nervosa.
Both intermediate and anterior lobes have a common origin by way of an ectodermal e vagi nation, Rathke’s pouch, which pushes up from the roof of the mouth and eventually becomes cut off after coming in contact with the posterior neural lobe. Cushing compares the whole pituitary apparatus to a ball held in a boxing glove, in which the ball itself represents the posterior lobe, the cover on the ball the pars iptermedia, and the glove the enveloping anterior lobe, which increases in size and importance in the ascending vertebrate series.
The anterior lobe, which is the largest part of the pituitary apparatus except in cyclostomes and teleosts, is made up of at least three kinds of cells that stain differently and consequently are of diverse chemical nature.
On the periphery are basophil cells, which take basic stains readily. In the center are acidophil cells amenable to acid stains, while chromophob cells, that are not easily affected by either basic or acid stains, are scattered throughout the structure, completing the “ABC” trinity (acidophil, basophil, chromophob) of cell components.
The anterior lobe produces several hormones at least one of which regulates growth while others stimulate the gonads (gonadotropic hormone), the thyroid (thyrotropic hormone), or the adrenals. Insufficient growth hormone results in dwarfism while excesses of this substance bring about gigantism or, if produced after maturity, a condition known as acromegaly, the excessive growth of certain bones and joints.
The complete removal of the pituitary probably results in death, although the difficulty of access to it, and the resultant unavoidable injury to neighboring brain tissues, may be the reason for fatal results frequent in operations upon it.
On the dorsal side of the brain and completely concealed in the higher vertebrates by the dominating growth of other parts, is a small stalklike evagination of the brain wall called the epiphysis, or pineal body. In fact one or another of three different structures, the paraphysis, parietal organ, and epiphysis (Fig. 418), all of neural origin and easily confused as to their probable homologies, may be located in this part of the brain.
The most anterior of these structures is the paraphysis, which lies directly in front of the velum transversum at the junction between the cerebral hemispheres and the diencephalon. The parietal organ is sensory in character rather than glandular, and in Sphenodon and some other lizards it becomes a median eye-stalk of more or less efficiency, reaching the top of the head under a window-like foramen that pierces the skull. The epiphysis and the parietal body, although arranged in tandem fashion whenever both are present, as in cyclostomes, were probably once paired structures lying side by side. The epiphysis is the more constant of the two structures, being almost universally present in vertebrates. Originally probably a sense organ, it has become glandular and even fibrous in character.
The pineal body in mammals is the persisting proximal or basal portion of the epiphysis. Its shape resembles somewhat that of a pine cone, from which circumstance it takes its name. Relatively larger in children than in adults, and attaining its best development in man at about seven years of age when involutionary changes begin to appear, it measures only about 8 mm. in length.
The pineal body has been known to anatomists since early times and has always been a source of much curiosity and speculation. Repeated attempts have been made experimentally to discover its activity and to ascertain significant structural modifications associated with abnormal conditions in the development of this organ, but the results have been conflicting and disappointing. The fact that it is a highly vascular structure, frequently lobulated like a gland and containing secreted pigment, has raised the suspicion that it may have some endocrine function, but its extirpation does not seem to be followed by unmistakable physiological consequences, except that sexual maturity is somewhat hastened when it is destroyed. While it is no doubt a degenerate structure, it may possibly be of physiological importance with some obscure endocrine function, yet hardly more has been proved with regard to it today than when Descartes (1596-1650) three centuries ago guessed that it was the “seat of the soul.”