1 76 THE CIRCULATION. 



Some remarkable observations of Cohnheim, however, 

 to which attention has been lately drawn by Dr. Chaiifcon 

 Bastian, apparently prove without doubt, that under cer- 

 tain circumstances of congestion or inflammation, the red 

 blood corpuscles may pass bodily through the walls of the 

 capillaries ; the process consisting of a gradual protrusion 

 of the capillary -wall by the corpuscle, until at length the 

 latter slowly advances through it and out of it into the 

 tissues around. 



Various theories have been advanced in explanation of 

 these remarkable phenomena ; but Dr. Bastian's idea 

 appears the most probable, that the perforation of the 

 capillary-wall is caused primarily by certain changes in the 

 shape of the red corpuscles, attended with protrusion of 

 their walls, analogous to the amoeboid, movements of the 

 white corpuscles before referred to (p. 86). By means of 

 such movements it appears that the white corpuscles also 

 may, under certain inflammatory conditions, pass through 

 the walls of the minute veins. 



Much diversity of opinion has long prevailed respecting 

 the possession by capillaries of any power to aid the pro- 

 gressive motion of the blood. It may be stated, with 

 tolerable certainty, that the capillaries themselves possess 

 no such power, and that the influence which they seem to 

 exercise on the movement of their contained blood, is 

 referable in part to the action of the small arteries, 

 and in part to the results of the relation which exists 

 between the tissues without, and the blood within, the 

 capillaries. 



Thus, the capillaries contract on the application of cold : 

 but this is due not to any contraction similar to that of 

 muscular tissue, but to their elasticity, and to that of the 

 surrounding tissues, which close in, when by the contraction 

 of the small arteries (which, as already stated, can be 

 made to contract by cold), the flow of blood into the 

 capillaries is diminished. 



