232 THE VASCULAR MECHANISM. 



nels, in a stream which though more or less irregular is steady and even, 

 not broken by pulsations, and slower than that in either the arteries or the 

 veins. We have further seen that the capillaries vary very much in width 

 from time to time ; and there can be no doubt that the changes in their 

 calibre are chiefly of a passive nature. They are expanded when a large 

 supply of blood reaches them through the supplying arteries, and, by virtue 

 of their elasticity, shrink again when the supply is lessened or withdrawn ; 

 they may also become expanded by an obstacle to the venous outflow. 



On the other hand, as we have also stated, there is a certain amount of 

 evidence that, in young animals at all events, the calibre of a capillary 

 canal may vary, quite independently of the arterial supply or the venous 

 outflow, in consequence of changes in the form of the epithelioid cells, allied 

 to the changes which in a muscle-fibre or muscle-cell constitute a contrac- 

 tion ; and though the matter requires further investigation, it is possible that 

 these active changes play an important part in determining the quantity of 

 blood passing through a capillary area ; but there is as yet no satisfactory 

 evidence that they, like the corresponding changes in the arteries, are 

 governed by the nervous system. 



Over and above these changes of form, the capillaries and minute vessels 

 are subject to changes and exert influences by virtue of which they play an 

 important part in the work of the circulation. Their condition determines 

 the amount of resistance offered by their channels to the flow of blood 

 through those channels, and determines the amount and character of that 

 interchange between the blood and the tissues which is the main fact of the 

 circulation. 



If the web of the frog's foot, or, better still, if some transparent tissue of 

 a mammal be watched under the microscope, it will be observed that, while 

 in the small capillaries the corpuscles are pressed through the channel in 

 single file, one after the other, each corpuscle as it passes occupying the 

 whole bore of the capillary, in the larger capillaries (of the mammal), and 

 especially in the small arteries and veins which permit the passage of more 

 than one corpuscle abreast, the red corpuscles run in the middle of the 

 channel, forming a colored core, between which and the sides of the vessels 

 all around is a colorless layer, containing no red corpuscles, called the 

 " plasmatic layer " or " peripheral zone." This division into a peripheral 

 zone and an axial stream is due to the fact that in any stream passing through 

 a closed channel the friction is greatest at the sides, and diminishes toward 

 the axis. The corpuscles pass where the friction is least, in the axis. A 

 quite similar axial core is seen when any fine particles are driven with a 

 sufficient velocity in a stream of fluid through a narrow tube. As the veloc- 

 ity is diminished the axial core becomes less marked and disappears. 



In the peripheral zone, especially in that of the veins, are frequently 

 seen white corpuscles, sometimes clinging to the sides of the vessel, some- 

 times rolling slowly along, and in general moving irregularly, stopping for 

 a while and then suddenly moving on. The greater the velocity of the flow 

 of blood, the fewer the white corpuscles in the peripheral zone, and with a 

 very rapid flow they, as well as the red corpuscles, may be all confined to 

 the axial stream. The presence of the white corpuscles in the peripheral 

 zone has been attributed to their being specially lighter than the red corpus- 

 cles, since when fine particles of two kinds, one lighter than the other, are 

 driven through a narrow tube, the heavier particles flow in the axis and the 

 lighter in the more peripheral portions of the stream. But, besides this, the 

 white corpuscles have a greater tendency to adhere to surfaces than have 

 the red, as is seen by the manner in which the former become fixed to the 

 glass slide and cover-slip when a drop of blood is mounted for microscopical 



