132 MANUAL OF HISTOLOGY. 



100 200), hydrochloric acid (1 in 50 200), and among the latter 

 a solution of chromic acid (1 in 200). It is very probable that 

 the amount of longitudinal striation varies in different muscles, 

 being related, perhaps, to physiological properties, or possibly 

 the result of mechanical causes. It is certain that both kinds 

 of striation may be found in great perfection in fibres treated 

 with almost any reagent that does not destroy them. Some- 

 times muscle is seen to be split into fibrillae, each of which 

 shows the transverse stripe, though the shreds are so fine that 

 each disk is represented by a dot merely. 1 This may be de- 

 tected very well in the muscle of the lobster after it has been 

 picked to pieces in glycerine. 



Returning to the transverse stripes in vertebrates, the striae 

 are very near together in the frog, and thus this useful animal 

 is not specially desirable. The muscle of the rabbit is much 

 better, and human muscle is, perhaps, better still. The muscle 

 of the human embryo in the last months of pregnancy is par- 

 ticularly good. A very high power will often show the narrow 

 black line in the midst of the white band. Sometimes one edge 

 of each black stripe will be very sharply marked against the 

 glaring white, while the other side will present a less marked 

 contrast. If the stage of the microscope admits of rotation," in- 

 structive effects can also be obtained. As the field turns round, 

 the brightness at the sharp border of the black stripe gradually 

 decreases, to return on the other side. Again, .this change may 

 not occur. Sometimes, when the upper edge of the fibre is pre- 

 cisely in focus, the black and white stripes may be made to ap- 

 parently exchange places, if the lens is slightly depressed. This 

 is probably to be accounted for as follows : First, we may for 

 the present assume that the black and white bands are caused 

 by disks of different nature. Take a series of such disks and 

 imagine them somewhat inclined to one side, like a roll of 

 coins on their edges leaning against a support. A vertical line, 

 representing the line of vision, that passes through a black disk 

 at the upper border of the roll will strike a light one at a 

 deeper level. A peculiar effect may be obtained by removing 

 the diaphragm and employing very oblique light. The black 



1 The fact that muscle removed from the body can be reduced to nbrillse does not 

 prove that these are pre-existing elements. 



' It is to be regretted that this movement is not more common. 



