160 LOCOMOTION. [CHAP. vn. 



commonly take a parallel course together, the bulgings occasioned by 

 the corpuscles give rise to partial longitudinal shadows, extending 

 for some way beyond the corpuscles in the intervals of the fibres. 

 As these irregular longitudinal shadows occur pretty uniformly 

 throughout a bundle of fibres, and as some of them are necessarily 

 out of focus, while others are in focus, the whole mass commonly 

 presents a confused reticulated appearance, which has given rise to 

 an almost universal notion, that the fibres interlace one with 

 another. This idea, however, is, in most cases, erroneous. It is 

 doubtful whether these fibres are invested by a sarcolemma : none 

 has hitherto been detected in an unequivocal manner. It is also 

 still a matter of speculation how they terminate, or whether they in 

 all instances have a termination. In the case of the transverse 

 fibres of the intestine, for example, it is uncertain whether each fibre 

 surrounds the canal once, returning unto itself as a ring, or more 

 than once, as a spiral; or whether it passes only partially round it, 

 the circle being completed by others. Whether the areolar tissue 

 (the representative of the fibrous), that is found in connection 

 with these fibres, serves to give them an attachment, by union 

 with their extremities, or by involving them in its meshes, is also 

 altogether unknown. In the gizzard of the bird, the ends of the 

 fibres are united to white fibrous tissue, thus making an approxi- 

 mation to the striped fibre, as they do in colour. But we have not 

 been able, after diligent search, to detect the true transverse stripes, 

 which Ficinus describes to exist in this organ. 



Of the Distribution of the two Varieties of Fibre in the Body. The 

 striped fibre is met with in all muscles of the body whose action 

 can be directly influenced by the will, and also in those of the 

 pharynx, the oesophagus, and the heart. In the oesophagus it 

 seems to be mingled with the other variety to a somewhat uncer- 

 tain extent. In some specimens from the human subject we have 

 failed in detecting any in the lower half of that tube, either in the 

 circular or longitudinal layer; but in other examples we have 

 found them to within an inch of the stomach.* It is still unknown 

 in what manner the two kinds of fibre are arranged at their 

 point of junction: some supposing them to be intermingled; others, 

 that they pass into one another by imperceptible gradations of struc- 

 ture. The former of these views is the more accordant with our 

 own observations ; and Mr. Mayo mentions a fact, which seems to 



* Among the lower animals, Mr. Gulliver has pointed out similar varieties. 

 (Proceed, of Zool. Soc., No. 81.) (See also Lancet, Aug., 1842.) 



