MUSCLES OF ANIMAL LIFE. 275 



reversed by an alteration of the focus of the microscope ; so that the appear- 

 ance is evidently due to the mode in which the light passing through them is 

 refracted, and not to any difference of colour in the two series of points. It is 

 in fact precisely that which is given by a glass rod with beaded enlargements, 

 when held to the light ; the beads will appear bright, and the intervening 

 spaces dark. When the fibrillae are separately examined, however, under the 

 highest magnifying power, and the most perfect definition, it is perceived that 

 they are perfectly cylindrical ; and that the dark spaces are slender lines, run- 

 ning directly across them. Hence the beaded appearance must be regarded 

 as an optical illusion ; the light points being the centres of highly-refracting 

 particles, which act as lenses ; and the dark lines the intervals between them, 

 formed of a less powerfully refracting substance. The size of the ultimate 

 fibrils is stated by Wagner to be nearly the same in all Vertebrata, as well as 

 in Insects, and in the Craw-fish ; its usual extremes being from about Jj F to 

 TT&T* f an i* 10 * 1 ' but Muller has observed them in the Frog to be .sometimes 

 as much as jg^j of an inch, and in the Parrot to be 4-^3-5 of an inch in 

 diameter. Taking the average stated by Wagner, and comparing it with the 

 average dimensions of the fibre in the Human Species, which is estimated by 

 Mr. Bowman at about TT ^- of an inch, each Muscular fibre in Man may be 

 regarded as composed of from five to eight hundred fibrillae. The transverse 

 strise, which are peculiarly characteristic of the Muscular Fibre of Animal 

 life, have been supposed, by several observers, to be due to the pressure of 

 circular bands or girths, surrounding the fibrillce ; and some have even imagined 

 that they might be attributed to a continuous spiral coil. The existence of 

 such bands, is not, however, indicated in any other way. As already stated, 

 the fibrillae are only bound together by the Sarcolemma or enveloping tube ; 

 and this is composed of a simple transparent membrane, destitute of any ap- 

 pearance of bands. The spaces between the light and dark striae on the fibre, 

 exactly correspond with those between the light and dark points in individual 

 fibrillae ; and this is the case in all conditions of the muscle ; so that it cannot 

 be doubted, that the appearance must be the result 

 of the same cause in each condition. The average Fig. 65. 



distance of the striae in the muscular fibre of differ- 

 ent animals, is very nearly uniform ; as will be seen 

 from 'the following table. Between the extremes, 



** 

 however, there is considerable variation; and this 



will be presently shown to depend upon the condi- 

 tion of the muscle at the time of examination. The Fragment of Muscular Fibre 

 distance is not only often different in the same mus- from macerated Heart of Ox > 

 cle and the same fasciculus, but even in the same ' bowing St?S 7 



~, . ,.~, r -I i mi n tlie aggregation or bead-like n- 



fibre m different parts of its length. The figures brilte . After Bowman, 

 indicate the number of striae in TTJ ! jr3 . of an inch. 



Maximum. Minimum. Mean. 



Human 15-0 6-0 9-4 



Other Mammalia 15-0 6-7 10-9 



Birds 14-0 7-0 10-4 



Reptiles 20-0 6-7 11-5 



Fish 18-0 7-5 11-1 



Insects 16-0 4-5 9-5 



The extremes in the same specimen, however, were in no instance so widely 

 apart as the table indicates for the Class ; the greatest proportion between the 

 maximum and minimum being, except in Insects, as 2 to 1. 



371; The general opinion as to the disposition of the fibres during the con- 



