408 PHYSIOLOGY. 



increases their power. The thyroid acts mechanically as a safeguard 

 of the brain against engorgement. In a sudden increase of blood- 

 pressure, whether from increased activity of the heart or from 

 increased capillary resistance, the thyroid is capable of passing 

 through its vessels a large amount of blood within a very short time, 

 so as to take it back directly from the arterial into the venous sys- 

 tem and thus prevent its entrance into the cerebral circulation. 



PARATHYROIDS. 



Sandstroem, a Swedish anatomist, first discovered the para- 

 thyroids. Vassale and Gley first established their function. In man 

 the internal or superior parathyroids are located on the posterior 



Fig. 1476. The Four Parathyroids in Man, Posterior View. 



( ZUCKERKANDL. ) 



surface of the thyroid gland, at the junction of the upper and middle 

 third of the gland. The external or inferior are seated near the lower 

 margin of the thyroid on its posterior surface. There are four para- 

 thyroids in man, two on each side of the median line. In the lower 

 animals and in man there are accessory parathyroids, some located, 

 according to Pepere, on the posterior surface of the thymus, in its 

 capsule, or in its superficial interlobular grooves. In man they are 

 often associated with epithelial, tubular or cystic formations. The 

 structure of the parathyroids is different from that of the thyroids, both 

 histologically and embryologically. The cells of the parathyroids have 

 a large nucleus surrounded by a small amount of protoplasm in which 



