32 THE TONER LECTLUES. 



reversed. During such a reversal of the current, tbe fork of one 

 of the large arteries was brought into the field. Instead of the 

 backward-flowing blood columns intermingling with each other 

 at the fork where the smaller branches joined the larger trunk, 

 and then travelling toward the heart in one solid round and 

 homogeneous cylinder, it was observed that as far as the lield 

 of view extended the blood on the proximal side of the fork 

 continued to flow backward in two distinct streams. Some- 

 times indeed the two currents travelled with different velocities. 

 These two separate currents appeared to preserve their indi- 

 viduality, and as nearly as possible the shape which character- 

 ized them while within the smaller branches. They Avere, in 

 fact, two separate and distinct cylinders of flowing blood con- 

 tained within the lumen of the larger arterial trunk, still pre- 

 serving by their inherent tendencies, or by the viscosity of 

 their elements, the relative positions in which these elements 

 had previously travelled. So far as the corpuscles of a column 

 of blood moving in a vessel are concerned, we know their rela- 

 tive position ; the mass of red corpuscles generally occupies 

 the centre, while the greatest number of white blood-cells are 

 near the periphery of the column. Thus the capability of 

 arterial blood, when flowing sluggishly, of receiving and for 

 some time retaining forms impressed by a narrow mould, 

 received ocular demonstration. 



The tongue of a frog was next operated lapon. It was 

 drawn out and fixed convenientl}' for observation. One of 

 the medium-size arteries of the organ, at a point where the 

 vessel gave off a branch about half the size of the main 

 trunk, was arranged for study by i:)lacing it in the field of the 

 microscope. By means of a delicate s^erre-fine the main 

 trunk of the selected vessel was com])resscd at a position a 

 little below the branch in such a manner that the point of 

 compression, the collateral branch, and the intervening portion 

 of the main trunk were all in the field and well seen at the 



