245 



From the still little known, but very important measurements of 

 muscle fibres by von dkr Malsbuhg ^j the same may be derived for 

 individuals of unequal size of different Mammalia. 



In Table III some individuals of five species, being in a fairly 

 good condition, and of a body weight as different as possible, are 

 compared as regards the relation between the latter and the cross- 

 section of homologous muscle fibres. This cross-section appears to 

 increase in direct ratio to the surface of the body, hence to the 

 cross-section of homologous muscles, which means that the number 

 of muscle fibres within a species does not change with increasing 

 size of the body. Also the number of nerve fibres and that of tlie 

 nerve cells of the brain may then be admitted as being the same 

 within a species. 



This is certainly not the case between different species, for when 

 the specific differences of calibei- are taken into account, through 

 which homologous muscle fibres of different species of animals (jnst as 

 not homologous muscles of the same species) are distinguished, not 

 much remains of a direct influence of the size of the body on tliis 

 caliber of the muscle fibre. The number of the muscle fibres must, 

 therefore, greatly increase with the size of the animal species. 

 According to von dkr Malsburg the average diameter of the muscle 

 fibres in the rectus abdominis and the gastrocnemius is for the Ox 

 45.88 micra (in its different breeds 35.35 to 63.37 micra), for the 

 Horse 39.20 micra (breeds 33.26 to 48.60 micra), for the Pig 42 

 micra, for the Sheep 22.61 micra (breeds from 18.50 to 30.85 micra), 

 for the Groat 18.90 micra. For tlie average diameter of the muscle 

 fibres in the gastrocnemius of the Dog (calculated from v. d. M.'s 

 records, applied to the mean body weight of this species) 21 micra 

 may be assumed, the average of four hares is 19.20 micra, and of 

 five mice 17.40 micra, the body weights of these two last species 

 being to each other as 200 : J . 



With not inconsiderable specific differences (but much smaller 

 than between the different breeds and individuals), only small differ- 

 ences between these species are to be ascribed to the influence of 

 the size of the body. Thus also Mayeda and Schwalbe *) found in 



I 



^) Karol von der Malsburg, Die Zellengrösse als Form- und Leistungsfaktor der 

 landwirtschaftlichen Nutzliere. Arbeiten der Deutschen Gesellschaft fur Züchtungs- 

 kunde. Heft 10. 367 pag. Hannover 1911. 



') R. Mayeda, Ueber die Kaliberverhaltnisse der quergestreiften Muskelfasern. 

 Zeitschrift fur Biologic. (Kühne und Voit). N.F. Bd. 9, der ganzen Serie Bd. 27, 

 p. 129. München und Leipzig 1890. — G. Schwalbe und K. Mayeda, Ueber die 

 Kaliberverhaltnisse der quergestreiften Muskelfasern des Menschen.lbid,p. 487,489,515. 



16 



Proceedings Royal Acad. Amsterdam. Vol. XXV. 



