GLYCERYL TRINITRATE — NITROGLYCERIN 107 



The arteries of the splanchnic area appear to be most affected by 

 nitrites but the dilatation is also pronounced in the arteries of the 

 extremities and in the cerebral, coronary and pulmonary arteries. 

 The veins are also somewhat relaxed, but this is not important. 



Heart. The heart's action is accelerated on account of depres- 

 sion of the vagus! That the depression is due to this action is shown 

 by the fact that if the vagus endings are first paralyzed by atropine, 

 there is no additional increased rate of the organ. But, there is still 

 some controversy whether the depression of the vagus is due to direct 

 action upon the vagus center, or whether it is the regular reflex de- 

 pression which accompanies lowered arterial pressure. 



Summary. 



1. Depression of the arterial muscles resulting in dilatation of 

 their arteries. 2. Increased rate of the heart. 3. Perhaps some 

 increased tone and strength of the heart. 



Blood. When mixed with a nitrite the oxyhemoglobin of the 

 blood is converted into methemoglobin. This compound is more 

 stable than the oxyhemoglobin and cannot serve as a carrier of oxy- 

 gen, but harmful effects rarely follow the administration of medicinal 

 doses. The nitrites differ somewhat from the other methemoglobin 

 formers in that they do not destroy the red corpuscles and after a 

 time the methemoglobin is reduced in the tissues to oxyhemoglobin. 

 If present in the blood in large amounts, methemoglobin causes dysp- 

 nea and cyanosis ; but if placed under a jar containing oxygen under 

 pressure, animals usually recover from this poisoning by the ni- 

 trites. 



Respiratory System. The respirations are quickened and 

 deepened but this is probably due to changes in the circulation {in- 

 creased supply of carbon dioxide to the medullary centers). From 

 very large doses there is later a depression of the center and asphyxia. 



Nervous System. There is no direct action upon the brain but 

 the cerebral arteries are dilated along with the others, and on account 

 of this or the rapid fall of pressure, there may be dizziness and 

 momentary throbbing of the head and faintness. The respiratory 

 center is stimulated, the vagus center depressed. 



Muscles. Other forms of plain muscles (ureters, bronchioles 

 and intestines) are relaxed, but not to the same extent as are those 

 of the blood vessels. With the exception of the heart, striated 

 muscle is not affected. 



Temperature. This may be lowered on account of dilatation of 

 the vessels of the skin and increased secretion of sweat but this 

 action is not marked. 



Excretion. Excretion of the nitrites is largely by the kidneys 

 as the nitrates but any increase in the secretion of urine is secondary 

 to the action of the drug upon the circulation and depends upon the 

 relative dilatation of the renal arterioles and those of the general cir- 



