THE THYMUS GLAND 



1273 



Structure. Microscopically the parathyroids consist of intercommunicating columns of 

 cells supported by connective tissue containing a rich supply of blood capillaries. Most of the 

 cells are clear, but some, larger in size, contain oxyphil granules. Vesicles containing colloid 

 have been described as occurring in the parathyroid, but the observation has not been confirmed. 



No doubt the parathyroid glands produce an internal secretion essential to the well-being 

 of the human economy; but it is still a matter of dispute what symptoms of disease are pro- 

 duced by their removal and suppression of their secretion. Pepere believes that they show 

 signs of exceptional activity during pregnancy, and that parathyroid insufficiency is a main 

 factor in the production of tetany in infants and adults, of eclampsia, and of certain sorts of 

 fits. It is probable that the tetany following parathyroidectomy is due to the accumulation of 

 ammonium carbonate and Kendall has suggested that the function of the parathyroid is to con- 

 vert ammonium carbonate into urea. 





THE THYMUS GLAND (Fig. 1178). 



The thymus is a temporary organ, attaining its largest size at the time of 

 puberty (Hammar), when it ceases to grow, gradually dwindles, and almost 

 disappears. If examined when its growth is most active, it will be found to con- 

 sist of two lateral lobes placed in close contact along the middle line, situated 

 partly in the thorax, partly in the neck, .and extending from the fourth costal 

 cartilage upward, as high as the lower border of the thyroid gland. It is covered 

 iy the sternum, and by the origins of the Sternohyoidei and Sternothyreoidei. 



Thyroid gland 



Left common 

 carotid artery 

 Left internal 

 jugular vein 



Left subclavian vessels 



Trachea , 



Thyroid veins 



Right vagus 



Superior vena cava, 



FIG. 117S 



mus of a full-time fetus, exposed in situ. 



Below, it rests upon the pericardium, being separated from the aortic arch and 

 great vessels by a layer of fascia. In the neck it lies on the front and sides of the 

 trachea, behind the Sternohyoidei and Sternothyreoidei. The tw r o lobes generally 

 differ in size; they are occasionally united, so as to form a single mass; and some- 

 times separated by an intermediate lobe. The thymus is of a pinkish-gray color, 

 soft, and lobulated on its surfaces. It is about 5 cm. in length, 4 cm. in breadth 

 below, and about 6 mm. in thickness. At birth it weighs about 15 grams, at puberty 

 it weighs about 35 grams; after this it gradually decreases to 25 grams at twenty- 

 five years, less than 15 grams at sixty, and about 6 grams at seventy years. 



Development. The thymus appears in the form of two flask-shaped entodermal 

 diverticula, which arise, one on either side, from the third branchial pouch (Fig. 

 1175), and extend lateral ward and backward into the surrounding mesoderm in 

 front of the ventral aortae. Here they meet and become joined to one another by 

 connective tissue, but there is never any fusion of the thymus tissue proper. The 

 pharyngeal opening of each diverticulum is soon obliterated, but the neck of the 

 flask persists for some time as a cellular cord. By further proliferation of the cells 

 lining the flask, buds of cells are formed, which become surrounded and isolated 

 by the invading mesoderm. In the latter, numerous lymphoid cells make their 



