The Endocrine System 233 



In an animal which has been experimentally parathyroidectomized, 

 tetany is prevented by the administration of extracts containing the 

 parathyroid hormone, a reduction in the intake of phosphorus, an in- 

 crease in the intake of calcium, plus other measures. In such animals, 

 the administration of vitamin D facilitates the absorption of calcium 

 from the intestine. In the normal animal, this vitamin is important in 

 the utilization of calcium and phosphorus for the prevention of rickets. 

 It has been suggested that there is some relationship between this vita- 

 min and the parathyroids, possibly with the vitamin being important 

 in regulating the absorption of these elements from the intestine. 



The Effects of Hyperactivity. — This condition is produced either 

 by spontaneous oversecretion of the gland or by experimental injec- 

 tion of extracts of the gland. At times, tumors may stimulate the 

 gland to excessive secretion. In such a condition, the calcium level 

 of the blood immediately rises while the phosphorus level falls. The 

 calcium is withdrawn from the bones to raise this level. In extreme 

 cases, the bones actually become soft due to this removal, and the 

 marrow, as well as the bony regions, is replaced by fibrous tissue. The 

 calcium thus withdrawn from the bones is deposited in muscle tissue, 

 in the kidney tubules, in the walls of the intestine, and in the heart, 

 lungs, liver, etc. Some calcium also is lost through the urine and feces. 

 The final result of extreme hyperparathyroidism is, of course, death. 



THE ADRENAL GLAND 



In mammals, just anterior to each kidney is a small flattened gland- 

 ular mass, the adrenal gland or suprarenal body. The exact position 

 of this gland varies in different species of the vertebrates, but in man 

 it forms a caplike structure over the anterior end of the kidney. 



Like the thyroid gland, this gland has an exceptionally rich blood 

 supply with the blood entering directly from the dorsal aorta by means 

 of the suprarenal artery. Around each gland is a heavy capsule of 

 connective tissue. 



Both histologically and functionally the adrenal gland consists of 

 two completely separate parts. The outer portion, the cortex, has its 

 cells arranged in three poorly defined zones. The inner medulla is 

 more homogeneous in appearance, having irregular strands and masses 

 of cells separated by elongate sinuses. 



Experimental removal of this gland and of the individual parts 

 has demonstrated that while the medulla is not essential for life, the 

 cortex is. 



