196 



ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



greatest proportion between the maximum arid minimum being, except in In- 

 sects, as 2 to 1. 



Human 



Other Mammalia 



Birds 



Reptiles 



Fish 



Insects 



Mean. 



9-4 

 10-9 

 10-4 

 U-7 

 11-1 



9-5 



229. It has been maintained by some, that each Muscular Fibre is a hol- 

 low bundle of fibrillae ; but the appearance presented by transverse sections 

 proves that this is not the case, the whole area of the tube being occupied by 

 nbrillre, without any trace of central cavity. The extremities of the cutfibrillae, 

 however, cannot always be distinguished in Mammalia, in consequence, as it 

 would seem, of their close and intimate lateral union ; but they are very evi- 



Fig. 97. 



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. 



dent in Birds, Reptiles, and Fishes (Fig. 97). The addition of an acid in- 

 creases the distinctness of the fibrilla?, by widening the interstices between 

 them. 



230. When the fibrillse are separately examined, they are found to present 

 an alternation of dark and light spaces, corresponding with the transverse 

 stria; of the fibre, and the lighter intervals between them. It is this alterna- 

 tion, which gives to the fibrillas the beaded appearance they present, when 

 their outline is not perfectly seen. When good specimens, however, are 

 carefully examined under a sufficient magnifying and good defining power, it 

 is seen that the border of the fibrillac is straight or nearly so ; so that the 

 beaded appearance is an optical illusion. Moreover, each of the light spaces 

 is seen to be crossed by a delicate but distinct line, separating it into two 

 equal parts ; and upon attentive examination it is seen, that a transparent 

 border, equal in breadth to either of these parts, is seen at the sides, as well 

 as between the ends, of the dark spaces. Thus each dark space is completely 

 surrounded by this pellucid border ; and it can scarcely be doubted that the 

 whole constitutes a complete though minute cell, and that the entire librilla 



