THE CAPILLARY CIRCULATION. 675 



tween the blood and the tissues, or air, outside the capillary walls. The 

 enlargement of the capillaries which accompanies such stagnation of 

 the blood in them, and also the shrinking of these vessels, as the part 

 recovers, imply an exercise of elasticity by their walls; but this can- 

 not be, under any circustances, a moving force in the circulation, but 

 rather a means of adapting the size of the capillaries, to variations in 

 their contents. 



The motion of the blood in the capillaries, when observed under the 

 microscope, in animals not too much disturbed in the experiment, is 

 constant, equable, and regular ; but the character of the movement 

 may be modified by the dilatation or contraction of the neighboring 

 small arteries under the action of cold or other stimuli, by obstruc- 

 tions in the veins, and by the condition of the heart itself. When, as 

 already mentioned, the heart's systole is weak, the motion of the blood 

 in the capillaries may, from the non-development of a perfect recoil 

 in the arteries, become pulsatory ; and when the heart is still more 

 enfeebled, the blood in the capillaries may merely oscillate, or be com- 

 pletely arrested, or a backward current may even take place in it. 

 These and many similar disturbances, even under the microscope, have 

 been often erroneously referred to active influences in the coats of the 

 capillaries, or in the surrounding tissues. 



The motion of the blood in the capillaries is more rapid in the centre 

 of each little stream, and slower at its surface, near the walls of the 

 vessel. The existence of corpuscles in the living blood, affords the 

 means of determining this fact ; for the red corpuscles may be seen 

 to move, comparatively swiftly, along the centre of the vessel, whilst 

 the white corpuscles travel much more leisurely along the sides, the 

 ratio of their respective movements being as 9, or even 17, to 1 (We- 

 ber). The outer thin, more slowly moving film, in contact with the 

 inner surface of the capillaries, measures, under different circum- 

 stances, from Jth to only Jth of the diameter of the vessel. It forms 

 the still layer or space of Poiseuille, in which the white corpuscles 

 move slowly along, as if some special attraction retained them against 

 the sides of the vessel, whilst the red corpuscles are hurried along the 

 centre. This striking phenomenon may have, in part, a physical ex- 

 planation ; for a retardation always occurs in the movement of that 

 portion of a fluid which is in contact with the walls of a tube, as com- 

 pared with the rate of motion along its axis, this effect being due to 

 friction in large tubes, and also to capillary attraction in small tubes. 

 In the living animal economy, the retardation of the circumferential 

 layer of blood in the capillaries, must have an important influence on 

 the nutritive, secretive, and respiratory processes, all of which are ac- 

 complished within a certain range of the capillary circulation ; it may 

 merely facilitate the withdrawal from the blood, and the escape through 

 the capillary walls, of certain necessary materials ; or it may be itself 

 an indication of nutritive, or other attractions from without, operating 

 on the stratum of fluid lying next to the thin capillary walls. Some 

 such attraction may prevail between the pale corpuscles and the walls 

 of the vessels themselves, but the existence of this has not been estab- 

 lished. These corpuscles, however, appear to be naturally much more 



