PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 227 



the psychical plienomeua in the series we have mentioned, we shall find 

 ourselves at ouce encoimteriug difficulties far beyond our power to over- 

 come. If it be true, as has been said, that all investigation, when reduced 

 to its ultimate element, is measurement, how can that be iuvestigated 

 which is beyond the reach of all measurement? What unit will serve to 

 express in figures the phenomena of intelligence, icill, and sensibility f 

 These, although similar in some respects, and belonging to the same gen- 

 eral class of phenomena, are so heterogeneous to others with which they 

 are associated as to admit of no comparison. Physiologists, therefore, 

 address themselves, as we have said, at first to the study of the phenom- 

 ena which ofler the most easy study, and these are almost always the 

 last terms of a series of acts such as those which I have mentioned. 

 Each successive discovery, then, facilitates further advance, and enables 

 the investigator to rise toward results which at first appear unattainable, 

 and to elevate himself almost to the level of those questions on which 

 speculative philosophy has spent itself in fruitless efforts. We, therefore, 

 commence with the study of muscular action. The first step in this 

 was made when the unattainable inflnence of the will was replaced 

 by the electrical stimulus; muscular contraction then commenced to be 

 studied by itself, separate from all extraneous influences. 



You know of what assistance in studies of this kind is the application 

 of the graphic method with the registering apparatus. It was by means 

 of this method, and the instrument called the myograph, that Helm- 

 holtz, in 1850, prosecuted his admirable researches on muscular action, 

 and I was enabled to add my contributions to the theory of muscular 

 contraction. The myograph enables us to note the exact instant when 

 the phenomenon begins, its duration, and extent ; while the curve traced 

 on the cylinder makes known all the circumstances of its production. 

 Xovr, the possibility of noting the precise instant of the muscular con- 

 traction furnishes us with the means of the examination of the second 

 of the acts which form the object of our researches, namely, the trans- 

 mission of the nervous force. 



Up to a very recent date, even as late as 1815, it was thought that 

 sensitive impressions were transmitted to the brain from the extremi- 

 ties, and that the impulse of the will returned with the rapidity of 

 lightning, the time necessary for the transmission being regarded as 

 infinitely small; and, indeed, some physiologists, Miiller among others, 

 contended that this point never could l3e settled by science. Tlie honor 

 of falsifying this prediction belongs to Helmholtz, who carried out, in 

 1850, a programme of experiments, suggested by Du Bois-Raymond 

 five years before. He proved that the time taken ,by nervous force to 

 traverse the length of a specified nerve could be accurately measured. 

 After him, Valentine, Du Bois-Raymond, Bonders, and Marey repeated 

 the experiments, and considerably simplified the method of operation. 



In all the researches the plan which was followed consisted, first, in ex- 

 citing a nerve in the neighborhood of the muscle to which it belonged, 

 and determining the interval which elapsed between this irritation and the 

 contraction which resulted from it ; secondly, in stimulating the nerve at 

 a greater distance from the muscle, and determining how much longer con- 

 traction was retarded than in the former case. This delay necessarily 

 follows from the greater distance which the nerve-impulse has to traverse 

 in the second case, and thus indicates the rapidity of this agent along 

 the nerve which has been operated upon, or, in other words, furnishes the 

 means for the deduction of the absolute velocity of the nervous im- 

 pulse. Helmholtz found that about 0,00175 of a second was occupied 

 by a nervous impulse in traversing a distance of forty-three millimeters, 



