452 VETERINARY PHYSIOLOGY 



generally compensated for by contraction in other parts of 

 the body (p. 460). 



(3) The viscosity of the blood. — The friction between 

 the walls of the blood-vessels and the blood is modified 

 by the viscosity of the latter (p. 475). After severe 

 haemorrhage this is markedly decreased, and the resistance 

 to the flow in the small vessels is correspondingly decreased, 

 and hence the arterial pressure tends to fall. 



(1) Methods of Studying the Condition of the Arterioles and 

 Capillaries. 



l.S'^. 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 arterioles leading to it. But the 

 capillaries may be more particularly dilated, when, as a 

 result of partial stagnation and the more complete removal of 

 oxygen from the blood, the part may have a bluish colour. 

 The engorgement may, however, be due to some obstruction 

 to the outfioiv of blood from the part. 2. With the micro- 

 scope. 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 and the 

 capillaries by means of an eye-piece micrometer, and to 

 study their dilatation and contraction. 



2nd. Engorgement of the capillaries brought about either 

 by dilatation of the arterioles, of the capillaries, or of both, 

 or simplv bv increased force of the heart raisin^ the arterial 

 pressure, manifests itself also in an increased size of the jxirt. 

 Every one 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 vessels are contracted, 

 feels loose. By enclosing a part of the body in a case with 

 rigid walls, filled with fluid or with air, and 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 

 bulk-measurer (plethysmograph or oncograph). 



2rd. When the arterioles in 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, 



