232 The Vertebrate Organ Systems 



Both are constantly being added to and removed from bony tissue. In 

 the normal individual, the calcium level of the blood is relatively 

 constant. 



Although the calcium level in the blood is not very high (9 to 12 

 mg. per cent) its presence is absolutely essential for all types of cellu- 

 lar functions. As it is important in the maintenance of cell permeability, 

 it controls the rate of nerve transmission, muscular contraction, and 

 countless other important activities. The proper blood calcium level 

 is maintained through the dynamic equilibrium with the calcium of the 

 bones, mediated through the action of the hormone of these important 

 glands. 



The mechanism by which parathormone functions is still contro- 

 versial. Some maintain that the primary efifect of the parathyroids on 

 calcium is the regulation of the activity of the bone-building and bone- 

 destroying cells. Others believe that the primary effect is upon the 

 secretion of phosphorus by the kidneys with the calcium metabolism 

 being of only secondary importance. Possibly the true explanation 

 is a combination of these two theories. 



The Secretion of the Parathyroids. — Extracts of the para- 

 thyroid gland have been prepared and studied experimentally. This 

 extract, known as parathormone, has not been purified, nor have its 

 active principles been identified. Apparently the hormone is a protein 

 since its chemical reactions are typical of the reactions of this class of 

 compounds. 



This extract is not used extensively in treatment of parathyroid 

 deficiency, chiefly due to the fact that the body appears to build up a 

 tolerance against it. 



The Effect of Hypoactivity. — Hypoparathyroidism occurs in 

 human beings only when portions of the glands are inadvertently re- 

 moved during thyroidectomy. In such individuals there is an imme- 

 diate fall in the calcium level of the blood with an accompanying rise 

 in the phosphorus level. The most apparent manifestations of this 

 hypofunctioning occur in muscle twitchings, cramps, and even convul- 

 sions. These symptoms appear to be correlated with the low blood 

 calcium level. 



The twitchings of the muscles and the tetany may be due to inter- 

 ference with the liberation of acetylcholine from the nerve endings, 

 which is then followed by an impairment of synaptic transmission. At 

 any rate, the irritability of the neuromuscular system is increased. 



