CIRCULATION 277 



Nitrites cause the muscular coat of the arterioles to relax, 

 and thus, by diminishing peripheral resistance, permit blood 

 to flow freely from the arteries into the capillaries. 



Salts of barium have precisely the opposite effect, caus- 

 ing the skin to become pale from imperfect filling of the 

 capillaries, and producing a marked rise in the arterial 

 pressure. Contraction of the muscles of the arterioles is 

 produced, and the flow of blood from arteries to capillaries is 

 retarded. 



Not only is the state of the arterioles influenced thus by 

 drugs, but it is also affected by the internal secretions (see 

 p. 416) from certain organs. A powerful vaso-constrictor 

 adrenalin is produced in the medulla of the suprarenals. 



The condition of the arterioles may be studied in many 

 different ways 



1st. By direct observation. 1. With the naked eye. A red 

 engorged appearance of any part of the body may be due to 

 dilatation of the arteriole leading to it. It may, however, be 

 due to some obstruction to the outflow of blood from the part. 

 2. With the microscope. In certain transparent structures, 

 such as the web of the frog's foot, or the wing of the bat, or 

 the mesentery, it is possible to measure the diameter of the 

 arterioles by means of an eye-piece micrometer, and to study 

 their dilatation and contraction. 



2nd. The engorgement of the capillaries brought about by 

 dilatation of the arterioles manifests itself also in an increased 

 size of the part. Everyone knows how on a hot day, when 

 the arterioles of the skin are dilated, it is difficult to pull on 

 a glove which, on a cold day, when the cutaneous arterioles 

 are contracted, feels loose. By enclosing a part of the body 

 in a case with rigid walls filled with fluid or with air which is 

 connected with some form of recording tambour, an increase 

 or decrease in the size of the part due to the state of its 

 vessels may be registered. Such an instrument is called a 

 plethysmograph. 



3rd. When the arterioles to a part are dilated and the 

 blood is flowing freely into the capillaries, the part becomes 

 warmer, and by fixing a thermometer to the surface, conclu- 

 sions as to the condition of the arterioles may be drawn. 



4th. By streaming blood through the vessels and observing 



