200 ON THE NATURE AND PRINCIPLES OF 



stand why we should deny the possibility of the 

 latter occasionally acting as well as the former cause. 

 Whichever view we adopt, there can be little doubt 

 that the principle determining that undue supply of 

 blood to a particular artery which acts as the cause 

 of various pathological phenomena, is, in the nature 

 of the mechanism or contrivance by which it operates, 

 identical with that regulating those variations in the 

 distribution of k the blood which arc necessary for 

 numerous physiological purposes. Before, then, we 

 can clearly understand how a pathological determi- 

 nation of blood is induced, we must possess a precise 

 knowledge of the nature and mode of operation of 

 the immediate causes of the natural irregularities in 

 the distribution of the circulating mass. And it is 

 scarcely necessary to remark, that we have no positive 

 information on this point. Dr. Billing is indeed the 

 only writer who has even attempted a rational ex- 

 planation of this action. lie supposes that the coats 

 of a particular artery, being weakened by the with- 

 drawal of that nervous influence to which they owe 

 their contractile power, oppose less resistance than 

 natural to the distending pressure of the mass of 

 aortic blood, an additional quantity of which is thus 

 driven into that particular set of vessels. And though 

 believing the contractility of the arterial tunics to be 

 innate, and, therefore, not dependent upon a supply 

 of nervous influence, I am by no means disposed to 

 deny the possibility of their diminished contractile 

 power, however induced, being occasionally the im- 

 mediate cause of determination of blood. This view, 

 it will be seen, refers the production of the local 



