725 



mass with great velocity, and skilfiiily steers it. Tlie flipper-iike hind- 

 limbs of the Seals (Phoca), which form a kind of tail, move the body 

 along through pretty intricate but nimble movements; in other cases 

 (Otaria) very large pectoral tins render such services to the ajiimal, 

 the feet, directed backwards here too, acting more like a helm, worked 

 by powerful muscles. For the Fishes it is again the tail which 

 derives its propelling force from the muscles of the hind part of 

 the body. The (Crocodiles have a strong propelling tail. For all these 

 vertebrate aquatic mammals the locomotoi- muscles must not only 

 be quick, but also particularly voluminous and strong, because of 

 the great density of the medium, in which the movements take 

 place; hence they must consist of thick, but also of numerous muscle 

 fibers. Hence very voluminous and at t'.ie same time very numerous 

 neurones. In fact Lkgkndre demonstrated that the nerve fibers of 

 the medulla and the roots of the Dolphin (Delphinus delphis L) are 

 among others much thicker than in Man, the Stag, the Dog, the 

 Rabbit, and the Mouse. He partly accounts for the high cephali- 

 sation of this aquatic Mammal by the thickness of its nerve fibers ^). 



Though 1 do not enter moi-e fully into the discussion of the 

 mutual relation of the dimensions of muscle- and nerve fibers 

 here, 1 will, however, point out that between, homoneuric species 

 the variations of the dimensions of the neurones and their parts, in 

 function of the body weight, entirely account for the variations of 

 the weight of the brain, in function of the body weight; hence that 

 the number of the neurones (differently from what I thought possible 

 at first) remains the same. This must also apply to the sarco-neurones, 

 hence to the muscle fibers. 



That the cell-body of the neurone must become more voluminous, 

 for homoneuric species, in ratio to the arithmetical longitudinal 

 dimension of the body, is a consequence of the already long known 

 dynamic proportion that the weight (the mass) of similar animals aug- 

 ments according to the cube of the homologous longitudinal dimen- 

 sions, the muscular force, on the other hand, proportional to the area 

 of the section of homologous muscles or the ó-(^Wfï>'f' of the homologous 

 longitudinal dimensions. Already ninety years ago STKAUS-DtJRCKHEiM ') 

 set forth clearly that thus, for similar animals, consequently having 

 the same organization, but of different sizes, the time of every 

 movement must be nearly proportionate to the longitudinal dimen- 



1) R. Legendre, Notes sur Ie système nerveux central d'un Dauphin (Delphinus 

 delphis). Bulletin du Muséum d'histoire naturelle, 1912, W. 1, p. 6—7. PI. I. 



^) H. Straus DüRCKHEiM, Considerations générales sur ranatomie comparée des 

 animaux articulés. F^aris 1828, p. 189 et seq. 



