368 THE CIRCULATION OF THE BLOOD. [CHAP. XXVII I. 



velocity, which is uniform when not affected by any extraneous 

 influence; the course and rate of the stream are indicated by 

 blood-particles which are carried along in it, and which seem to 

 chase each other through the various channels and among the little 

 islands of the solid particles of the tissue which the blood is destined 

 to nourish. These particles most frequently pass in a single row, 

 with a variable interval between them : sometimes, however, they 

 seem to rush in pairs, or in threes, according to the size of the 

 capillary channels through which they flow. Secondly, it will be 

 noticed that the greatest velocity of the stream is in its centre, a 

 fact which comports with what is observed in rivers and other chan- 

 nels through which water flows, while towards the circumference 

 the stream becomes much slower, so that the layer of fluid which is 

 in immediate contact with the capillary wall is almost or completely 

 still. The particles are carried along in the centre or rapid part of 

 the stream, and but occasionally a solitary particle seems attracted 

 towards the circumference. This is most frequently a colourless 

 corpuscle, so that sometimes several colourless corpuscles are seen 

 at intervals in contact with the wall of the capillary, as if drawn to 

 it by some special attractive force, or moving much more slowly 

 around the central mass of red particles. The sudden change from 

 a rapid to a slow movement, or to perfect stillness, when one of these 

 particles is thus drawn from the centre to the circumference of the 

 stream, serves to display, in a very satisfactory manner, the peculiar 

 feature of this portion of the contents of the capillary, which, from 

 its apparent stillness and from the paucity of blood-particles in it, 

 has been called the still layer of the liquor sanguinis. The exist- 

 ence of this still layer is doubtless a purely physical phenomenon, 

 identical with that which is known to take place when fluids pass 

 through inorganic capillary tubes, in which the circumferential layer 

 seems to adhere or to be attracted to the wall of the tube ; and it 

 would clearly favour the transmission of nutrient or other material 

 dissolved in the liquor sanguinis, through the wall of the vessel, in 

 obedience to a force of attraction between the blood and the tissue. 

 This still layer forms but a small portion of the whole capillary 

 stream perhaps about -th or ^th its breadth ; it is greater when 

 the circulation is slower : it is also broader, and therefore more 

 visible when a vessel makes a bend. The application of cold to the 

 capillaries increases the breadth of this layer, whilst heat produces 

 an opposite change. 



The capillary vessels dilate or contract, under particular circum- 

 stances. Their dilatation is passive, and due either to an increased 



