2 



ANIMAL MECHANICS. 



From To 



Birds, .... T ^ 5S . . ^ g . . average, . . . . ¥ i T 



4. Reptiles, . . . . ^ . . ^ . . „ . . . . ^ 



5. Fishes, Tfc ... ^ • '■ » • • • • rf T 



6. Insects, . . . . ^ . . ^ . , „ . . . . ^ 



Professor Donders, of Utrecht, and Dr. Buys Ballot,* esti- 

 mate the cross section of the fibres of the m. biceps humeri 

 and m. brachialis anticus, in Man, as j^th of a square milli- 

 metre. This estimate, converted into English measures, gives 

 for the diameter 



<T= \/-^-x 3 -^-= 0.001986=— in. 



K 5OO 7T IOOO ^ 503 



In this formula, n denotes the ratio of the circumference to 

 the diameter of a circle. 



The same authors found for the cross sections of these 

 muscles the following measurements : — 



m. biceps (long head) . . . 530 mra- square. 



„ (short head) ... 452 „ 

 m. brachialis anticus ... 614 „ 



Total . . . i5 9 6 mm - 



From these results it follows that the total number of ele- 

 mentary fibres in these muscles amounts to 798,000. These 

 important muscles are the only ones concerned in holding the 

 forearm flexed at right angles to the arm, and we shall have 

 occasion hereafter to make use of the results just given on the 

 authority of Donders and Ballot. 



The elementary fibres are not circular in their cross sec- 

 tion, but polygonal, in consequence of their mutual pressure ; 

 and microscopical examination shows thatthe blood vessels which 

 supply the fibres are interposed at the angles of the polygons : 

 from this it follows that the muscles which have the smallest 

 fibres are best provided with blood, and are therefore ca- 



* "Over de Elasticiteit der Spiren," p. 47, 8vo., Utrecht, 1863. 



