THE THYROID AND PARATHYROID BODIES 963 



extracts of parathyroid tissue. While the exact significance of the 

 symptoms just enumerated is not known, it appears that this tetany 

 is the outcome of some profound metabohc change resulting in an in- 

 toxication. In other words, in the absence of this gland certain toxic 

 substances escape reduction, and finally attack the tissues. This ex- 

 planation finds substantiation in the experiments of MacuUum,^ 

 which show that bleeding and infusion of saline solution causes the 

 tetany to disappear, and that the injection of the blood-serum of 

 animals in tetany produces these symptoms in other animals. 



The specific hypothesis suggested by these experiments, is that the 

 parathyroids possess the power of detoxication by preventing the ac- 

 cumulation of certain products of metabolism. This conclusion, how- 

 ever, is not fully justified, because it may also be true that these poisons 

 are not formed in the normal body and develop only in the absence of 

 the parathyroids. More recently, Paton^ has brought forth the 

 hypothesis that this gland regulates the metabolism of guanidin and 

 thereby exerts a controlMng influence upon the activity of the muscles. 

 Upon its removal, the guanidin accumulates and gives rise to a fatal 

 tetany. This contention finds support in the fact that the guanidin 

 compounds in the blood and urine are markedly increased after 

 parathyroidectomy and are also present in excessive amounts in the 

 urine of children suffering from idiopathic tetany. Furthermore, it 

 is possible to evoke the symptoms of parathyreopriva by the injec- 

 tion of salts of guanidin. 



B. THE THYMUS GLAND 



Position and Structure of the Thymus. — This glandular mass is 

 situated in the anterosuperior recess of the mediastinal space, and 

 covers the great vessels. By origin it is a bilateral organ, consisting of a 

 right and left lobe with corresponding prolongations upward. This 

 division is also in evidence in the adult organ, because although they 

 overlap, its two portions may be separated from one another without 

 much difficulty by following the line of the intervening connective 

 tissue. The size of this organ differs greatly in accordance with the 

 age of the individual. In infants, for example, its average weight is 12 

 grams, at puberty 35 grams, at sixty years less than 15 grams, and at 

 seventy years less than 6 grams. ^ When fully developed, it extends 

 across the upper portion of the pericardial sac and reaches upward 

 very nearly to the thyroid gland. 



It is invested by a thin capsule of areolar tissue which also divides 

 its substance into lobules. The different follicles entering into the 

 formation of the latter, are made up of a central portion or medulla and 



1 Jour. Exp. Med., xi, 1909, 118, also: Jour, of Pharm. and Exp. Therap., ii, 

 1911, 421. 



2 Quart. Jour, of Exp. Physiol., x, 1917, 203. 

 ' Hammar, Archiv fiir Anat., 1906. 



