718 



and easily digs lioles, (casting out (lie earth with its hind-legs); it 

 lias about the same rate of conduction in the nerve as the Frog. 

 The same thing is the case with the Tree-Frog (Hyla arborea), which 

 pounces upon winged insects as its pi-ey. 



Now, for a slow American Toad (Bufo lentiginosus), which has 

 the same size as our Common Toad, the mean diameter of the fibers 

 of the neivus isciiiadicus is, according to Donaldson and Hokk, 11. 2 

 niicia, as against 14.7 for a frog specimen of about the same 

 weight, belonging to liana virescens. In a common American 

 Lizard (Sceloporus undnlatns) of a body weight of 8.2 grains, the 

 mean diameter of the nerve libers in the |)lexus brachialis was 

 9.8 micra, on the other hand only H.2 micra in the Horned Toad 

 (Phrynosoma cornutum), which is at least six times heavier, and 

 owes its name to the fact that it moves more like a toad than as 

 the proverbially «piick lizard, to the family of which it belongs. 

 For an equally great rapidity as the said Lizard the nerve liber of 

 the so much heavier Hoi-ned Toad would have to double its diameter 

 (to 12.7 micra). The American Turtle Chrysemys marginata, though 

 probably weighing scarcely less than a kilogram, i.e. certainly as 

 much as a hundred times more than the said small Lizard, has 

 nerve libeis of a mean diameter of no more than 12.4 micra, in its 

 |)lexus brachialis. ') It would have to amount to 18.7 micra for 

 equal rapidity as that Lizard. 



GöTHMN also points out that Shrimps of the genera Crangon, 

 Palaemon and others, which are among the quickest animal species, 

 possess thick nerve fibers provided with medullary sheaths. ') L. 

 and M. Lapicquk found the greatest rapidity of nerves and muscles of 

 all the Invertebrates which they examined, in the tail of Palaemon. ') 



Alcock was the first to inquire into the possible influence of the 

 size of the body on the rate of propagation of the influxion in 

 nerves, by experiments on the jiervus ischiadicus of the Frog, and, 

 externally, in the nervus medianus of Man. ") He finds for Man, as 



1) Gf. these diameters of nerve fibers in H.H. Donaldson and G. W. Hoke, On 

 the Areas of the Axis Cylinder and Medullary Sheath as seen in Cross Sections of 

 the Spinal Nerves in Vertebrates. Journal of Comparative Neurology and Psychology. 

 Vol. 15, Philadelphia 1905, p. 9-lL 



') G. F. GöTHLiN, Die doppelbrechenden Eigenschaften des Nervengewebes. 

 Kungl. Svenska Vetenskapsakademiens Handlingar, Ny Följd, Bd. 51 (1913), p. 84. 



3) Louis et Marcelle E.apicque, Quelques chronaxies chez les MoUusques et 

 Crustacés marins. Comptes rendus de la Sociëté de Biologie. Année 1910. Vol. 2 

 (69). Paris 1910, p. 280. 



1) N. H. Alcock, On the Rapidity of the Nervous Impulse in Tall and Short 

 Individuals. Proceed. Roy. Society, Vol. 72 (190H). London 1904, p. 414—418. 



