VARYING SIZE OF THE CAPILLARIES. 335 



these vessels. The capillaries, then, must not be regarded as altogether 

 distinct in their endowments, from the vessels with which they are con- 

 nected on either side ; but merely as intended, by their minute sub- 

 division and inosculation, to bring the blood into sufficiently close rela- 

 tion with the tissues they are to nourish, and to allow a greater degree 

 of transudation of its elements by the comparative thinness of their 

 walls. 



595. When the flow of blood through the Capillaries of a trans- 

 parent part, such as the web of a Frog's foot, is observed with the 

 microscope, it appears at first to take place with great evenness and 

 regularity. The influence of the contractions of the heart may be seen 

 to extend itself into the smaller arteries ; the blood moving onwards 

 in them with a somewhat jerking motion. But this influence alto- 

 gether disappears in the capillary network ; the flow of blood through 

 this being even and continuous, except when the action of the heart is 

 becoming weak and irregular, or when its influence is impeded by 

 obstruction in the vessels leading to the part, the blood being then 

 impelled by a succession of jerks, with intervals of complete repose. 

 But on watching the movement for some time, various changes may 

 be observed, which cannot be attributed to the heart's influence, and 

 which show that a certain regulating or distributive power exists in 

 the walls of the capillaries, or in the tissues which they traverse. Not 

 only do we occasionally perceive some of the tubes enlarging, so as to 

 admit several files of blood-discs instead of one, whilst others that 

 previously received several now only admit one; but we also see 

 vessels coming into view, which were not previously noticed, whilst 

 other vessels seem to become obliterated. This apparently new forma- 

 tion and obliteration of vessels, however, does not really take place ; 

 for a more close examination shows, that the former of these appear- 

 ances is due to the entrance of red corpuscles into passages which 

 existed before, but which were in such a state of contraction as enabled 

 them only to admit the fluid portion of the blood ; whilst, by a con- 

 verse change in certain capillaries, from the dilated to the contracted 

 state, the appearance of obliteration is produced, the red corpuscles 

 being excluded, and the transparent fluid of the blood being alone 

 transmitted by them. 



596. But these are by no means all the irregularities which may 

 be detected by a close scrutiny of the Capillary circulation. The 

 velocity of the current is liable to great and sudden variations, which 

 cannot be accounted for by any change in the heart's action, or in the 

 supply of blood afforded by the arteries ; and this change may manifest 

 itself, either in the whole capillary network of a part, or in a portion 

 of it; the circulation taking place with diminished rapidity in one 

 part, and with increased energy in another, though both are supplied 

 by the same trunk. These variations are sometimes manifested by the 

 complete change in the direction of the movement, in certain of the 

 transverse or communicating branches; this movement taking place, of 

 course, from the stronger towards the weaker current. Not unfrequently 

 an entire stagnation, of longer or shorter duration, precedes the 

 reversal of the direction. Irregularities of this kind are most frequent 



