36 INTERNAL SECRETION 



quently a second larger body. In every case, however, there is 

 a larger or smaller number of small and minute accessory bodies, 

 and these are found distributed over the space between the 

 principal parathyroid body and the tip of the thymus gland, in 

 and on the thyroid, below the lower pole of the thyroid, and about 

 and within the tip of the thymus. 



In apes, it frequently happens that both the parathyroid 

 glands are situated outside the capsule upon the lateral surfaces 

 of the thyroid gland. In Macacas rhesus an external parathyroid 

 gland is frequently found at or about the middle of the lateral 

 surface of the thyroid ; or it may occur, though more rarely, at 

 the upper or lower pole. The other gland is found embedded 

 in the substance of the thyroid, near the internal surface, and at 

 about the middle of the organ. This second gland is usually 

 the larger. 



In birds, the entire thyroid apparatus lies in the cavity of 

 the thorax, on both sides of the trachea, and consists of the 

 thyroid and 1-2 parathyroids, which are placed at the upper or 

 lower poles of the thyroid. 



In the tortoise, Doyon and Kareff found at the base of the 

 neck a parathyroid body of a yellow colour on each side of the 

 thvmus. 



/ 



In fish, the only portion of the thyroid apparatus which is 

 known is the thyroid gland, and this is situated at the fork of the 

 branchial artery. 



PHYSIOLOGY OF THE THYROID APPARATUS. 



HISTORICAL. 



From very early times the thyroid gland has been regarded as 

 an organ possessing specific physiological functions. We gather 

 from the older physiologists, Horsley (1891), Hellin (1893), Langen- 

 dorff (1899), that before any definite information as to its structure 

 was forthcoming, the most extraordinary notions regarding the 

 nature of its activity prevailed. It was believed to enhance the 

 beauty of the form and contour of the neck; to be a kind of 

 cushion for the larynx ; to act as a sounding-board for the 

 voice, &c. 



An early theory, based on the anatomical aspects of the 

 thyroid gland and, more particularly, upon its extreme 

 vascularity, held that the function of this organ was the regulation 

 of the cerebral circulation. As a matter of fact, the thyroid 

 glands, with their exceptionally numerous blood-vessels which 

 are in direct communication with the great vessels of the brain 

 and their enormously developed capillary system, do, to a certain 

 extent, influence the blood which circulates through the brain. 

 These conditions might seem to suggest that the thyroid apparatus 



