274 



OF MUSCULAR CONTRACTILITY. 



REPTILES 



FISH 

 INSECTS 



Frog 



Lizard 



Boa - 



Skate 



Cod - 



Sprat 



Staghorn Beetle 



Blue-bottle Fly 



to ik 

 <U 

 to jiff 



t0 & 

 t0 *V 

 t0 20<J 



to 



It is interesting to remark, upon this table, that the Muscular Fibre of Reptiles 

 and Fishes is upon the whole much larger than that of other Vertebrata, and 

 that its dimensions present the greatest extremes of variation ; whilst in Birds 

 it is much smaller than in all other Vertebrata, and its dimensions are also less 

 variable. Further, the size of the fibres bears no proportion to that of the 

 animal; for we observe that in the Chaffinch they are larger than in the Owl, 

 in the Cat larger than in the Horse, and in the Frog often larger than in the 

 Boa. Moreover in Insects, the diameter of the fibres is even greater than it 

 is in Mammalia. Some difference of opinion exists as to whether the arrange- 

 ment of the fibrillae within the sarcolemma is such as to form a hollow or a 

 solid bundle. It frequently happens that, when a fibre is torn across, the 

 appearance of the brush-like tuft of fibrillas at the broken extremity is such as 

 to indicate that they form a hollow cylinder ; but this may be accounted for 

 by the fact that the outer layer of fibrillae is evidently adherent in some degree 

 to the Sarcolemma, and will consequently be extended with it, at the moment 

 of its rupture, beyond the deeper layers. The appearance presented by trans- 

 verse sections shows, that the bundle of fibrillae contains no central cavity ; the 

 extremities of the cut fibrillae, however, cannot always be distinguished in 

 Mammalia, in consequence, as it would seem, of their close and intimate lateral 

 union ; but they are very evident in Birds, Reptiles and Fishes (Fig. 64). 

 The addition of an acid increases the distinctness of the fibrillae, by widening 

 the interstices between them. 



Fig. 64. 



Transverse Section of Muscular Fibres from Pectoral Muscle of Teal: showing the irregular form of the 

 fibres, and the aggregation of circular particles, with which they are completely filled. After Bowman. 



370. When the fibrillae are separately examined, they are found to present 

 an alternation of dark and light spaces ; and these points are capable of being 



