STRUCTURE OF MUSCULAR TISSUE. 



305 



ing with the state of contraction or relaxation of the fibre. 1 It has been main- 

 tained by some that each Muscular fibre is a hollow 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 fibrillae, without any trace of central 



Fig. 93. 



Fig. 94. 



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. 



cavity. The extremities of the cut fibrillae, however, cannot always be distin- 

 guished in Mammalia, in consequence, as it would seem, of their close and inti- 

 mate lateral union ; but they are very evident in Birds, Reptiles, and Fishes 

 (Fig. 93). The addition of an acid increases the distinctness of the fibrillae, by 

 widening the interstices between them. 



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

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

 of the fibre, and the lighter intervals between them. 

 It is this alternation which gives to the fibrillae the 

 beaded appearance thej*present, when their outline 

 is not perfectly seen (Fig. 94). When well-prepared 

 specimens, however, are carefully examined under a 

 sufficient magnifying and good defining power, it is 

 seen that the border of the fibrillae is straight or nearly 

 so; the beaded appearance being 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, exists at 

 the sides, as well as between the ends, of the dark spaces (Fig. 95). 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 fibrilla is made up of a linear aggregation of such cells. 3 When the 



1 It is curious that whilst the diameter of the fibres varies considerably in different 

 animals, being generally much greater in the cold-blooded than in the warm-blooded tribes 

 (corresponding with the larger size of the blood-disks of the former), the diameter of the 

 fibrillce, and the distance of the striae from each other, vary extremely little. 



2 This account of the ultimate structure of Muscular Fibre was first published simul- 

 taneously (March, 1846), by the Author of this Treatise, in his "Elements of Physiology," 



20 



Fragment of Muscular Fibre 

 from macerated heart of Ox, 

 showing formation of strife by 

 the aggregation of fibrillae. 



