COUKSE OF BLOOD IX THE CAPILLAEIES. 283 



these estimates as mere suppositions ; and they are given for 

 what they are worth. 



With the older physiologists, the contractility of the capil- 

 laries was a subject of discussion. Some went so far as to 

 suppose that these little vessels were the seat of rhythmical 

 contractions which materially assisted the flow of the blood. In 

 microscopic examinations, irritation or stimulation is seen to 

 produce contraction of the smallest arteries ; but there is no 

 evidence that the capillaries, which have a single amorphous 

 coat, have any such property. They undergo, while under 

 observation, considerable alterations in caliber; but this is 

 due, in all probability, to differences in the pressure of blood 

 in their interior. The capillaries can only be considered as 

 endowed with elasticity, which enables them to react upon 

 their contents, when there is any diminution in pressure. In 

 the vascular system, contractility disappears with the muscu- 

 lar fibre- cells which form the middle coat of the arterioles. 



Course of the Blood in the Capillaries. 



The phenomena of the capillary circulation are only ob- 

 servable with the aid of the microscope. It was not granted 

 to the discoverer of the circulation to see the blood moving 

 through the capillaries, and he never knew the exact mode 

 of communication- between the arteries and veins. After it 

 was pretty generally acknowledged that the blood did pass 

 from the arteries to the veins, it was disputed whether it 

 passed in an intermediate system of vessels, or became dif- 

 fused in the substance of the tissues, like a river flowing 

 between numberless little islands, to be collected by the ve- 

 nous radicles and conveyed to the heart. Accurate micro- 

 scopic investigations have now demonstrated the existence, 

 and given us a clear idea of the anatomy, of the interme- 

 diate vessels. In 1661, the celebrated anatomist, Malpighi, 

 first saw the movement of the blood in the capillaries, in the 

 lungs of a frog. Since that time, physiologists have studied 



