Henry Gray (18251861). Anatomy of the Human Body. 1918.
4. The Ductless Glands
There are certain organs which are very similar to secreting glands, but differ from them in one essential particular, viz., they do not possess any ducts by which their secretion is discharged. These organs are known as ductless glands. They are capable of internal secretionthat is to say, of forming, from materials brought to them in the blood, substances which have a certain influence upon the nutritive and other changes going on in the body. This secretion is carried into the blood stream, either directly by the veins or indirectly through the medium of the lymphatics.
These glands include the thyroid, the parathyroids and the thymus; the pituitary body and the pineal body; the chromaphil and cortical systems to which belong the suprarenals, the paraganglia and aortic glands, the glomus caroticum and perhaps the glomus coccygeum. The spleen is usually included in this list and sometimes the lymph and hemolymph nodes described with the lymphatic system. Other glands as the liver, pancreas and sexual glands give off internal secretions, as do the gastric and intestinal mucous membranes.
a. The Thyroid Gland (Glandula Thyreiodea; Thyroid Body)(Fig. 1174).The thyroid gland is a highly vascular organ, situated at the front and sides of the neck; it consists of right and left lobes connected across the middle line by a narrow portion, the isthmus. Its weight is somewhat variable, but is usually about 30 grams. It is slightly heavier in the female, in whom it becomes enlarged during menstruation and pregnancy.
The lobes (lobuli gl. thyreoideæ) are conical in shape, the apex of each being directed upward and lateralward as far as the junction of the middle with the lower