784 EEPOET — 1887. 



inquiries to impart a lateral impulse to discs, so tliat a processional movement might 

 become visible when the simple rotation was not so. 



To have been translated without rotation, a body, such as an arterial disc, must 

 have been subjected either to a force passing solely through its centre of gravity — 

 to suppose which is absurd — or it must have been subjected to forces (also in its 

 line of projection) about its centre of gravity and on either side of its line of motion 

 strictly equal and parallel to otie another, the which, in the presence of the fact that 

 the arteries are so largely curved — a condition necessitating unbalanced impulses — 

 seems to the author equally an impossibility. 



Any other direction of impulse during translation necessitates also rotation ; 

 thus impingement on the side of the vessel is a tangential force not passing through 

 the centre of gravity of the disc, and such would give rotation ; also the other 

 tangential force described in the volume for 1881 of the British Association Report 

 by the author as evidenced by the experiment of Dr. Plateau with a rotating 

 globule of oil. Both this cause of rotation and that just before mentioned are in 

 harmony with the natural law above referred to ; and when to these causes of 

 rotation is superadded the fact of the very oblate form of the disc, coupled with 

 the important nature of the functions assumed to be by such means fulfilled, the 

 author cannot but believe that the cycloidal rotation of arterial discs would long 

 since have been manifested by the microscope but for the extreme dilHculty and 

 delicacy attending its recognition. 



