CHAPTER XXV 

 THE CAPILLARY CIRCULATION 



THE blood is brought into contact with the tissues through the 

 endothelial wall of the capillaries; this is therefore of the greatest 

 tenuity. Here takes place that exchange of material which maintains 

 the combustion of the body the fire of life. The aim of the circu- 

 lation is attained when the arterialized blood laden with food material 

 and oxygen is driven into the capillaries of the body. 



In size, the capillaries vary in different organs. In the brain the 

 length has been estimated to be 0-709 millimetre (pons) and 042 milli- 

 metre (optic thalamus); in the mucosa of the stomach 0-6 millimetre; 

 and in the liver 0-5 to 1-1 millimetres. The diameter of the capillaries 

 varies from 0-007 to 0-013 millimetre. 



Malpighi (1661) first observed the capillary circulation under the 

 microscope. He examined the lung, the mesentery, and bladder of the 

 frog. It has since been seen in many other transparent or translucent 

 parts of animals. 



The Microscopical Examination of the Circulation. By using a 

 low power it is possible to examine simultaneously arteries, capillaries, 

 and veins in the same field. The first thing which strikes the observer 

 is the different direction of the stream in the arteries and in the veins. 

 On account of the reticular arrangement of the capillaries, the direc- 

 tion of the stream through them is by no means constant. There 

 may be a complete cessation of the flow for a period in a capillary 

 channel, or the direction of the current may even be reversed for a 

 longer or shorter time. The flow through the arteries is by far the 

 most rapid. In the veins, also, the stream is so rapid that it is diffi- 

 cult to catch the contour of the corpuscles. The stream is slower in 

 the small veins, and in the capillaries the movement is, as a rule, so 

 tardy that the individual corpuscles can be determined without any 

 difficulty. The inconstancy of the capillary stream is generally ap- 

 parent. If a group of capillaries be kept for some time under observa- 

 tion, the blood is occasionally seen to hurry suddenly through a number 

 of these with increased rapidity. This continues for a while, and then 

 the stream becomes again slower and slower, till after an interval it 

 resumes the quiet rate of flow which has been maintained without 

 interruption in the neighbouring capillaries. These variations depend 

 on alterations in the lumen of the afferent arteries. 



The arterial stream is pulsative, and each systole may be recognized 

 even in very small arteries by the rhythmical acceleration and re- 



218 



