86 HISTOLOGY 



bodies. These are of black staining anisotropic substance and are 

 called Z. They mark the limit of the sarcous element or muscle unit 

 of the fibril, and where two elements touch, the dot is double, although 

 the two parts are molded together and do not usually appear as twin 

 bodies. The upper segment in our figure shows both a double Z body 

 (where it touches the segment next lower) and a single or half Z body at 

 its upper end, where it is attached to the connective-tissue cells. 



The Z bodies, then, form the intermediate line and are connected 

 with one another by a transparent or non-staining membrane that ex- 

 tends through the cell from side to side, even in such parts of it as contain 

 no myo-fibrils (see Fig. 83, m.). This can be well seen in the portion of 

 the fiber figured, especially where the larger and smaller bundles of 

 fibrils have separated to make room for the two large nuclei and their 

 surrounding sarcoplasm. 



The second muscle segment shows no great difference from the first, 

 although it is a trifle shorter. In the third, however, a change has 

 occurred and Q has become thinner in the middle. The whole segment 

 is about one twentieth shorter than the second. In the fourth segment 

 the Q area is not only thinned out in its middle, but its substance 

 does not stain as black in this thinned middle part. 



The remaining five segments are alike in the fact that the Q areas 

 are elongated until their black ends have met the corresponding ends of 

 the Q areas of the neighboring segments. In doing this they hide the Z 

 dots and make a much darker and wider band across the muscle fiber 

 in its place. Their middle parts have become clear and non-staining 

 except that an edge appears to be left on each side. This is probably 

 a refraction line. 



In the middle of this Q area, now light and non-staining, appears a 

 small black dot that resembles the Z body of the relaxed stage except 

 that it is smaller and is not connected with its neighbors by a mem- 

 brane. This dot with its neighbors forms the M stripe. In Figure 84 

 the M stripe is the same width as the two halves of the Q stripe. 



The usual appearance of the fiber is now entirely changed, and upon 

 a careless examination appears to have been reversed, as though the Z 

 bands and Q bands had changed places. It takes a careful eye, even in 

 such a favorable specimen as the one from which this figure was drawn, 

 to run from band to band and note the real change. This difficulty is a 

 weakness of the eye muscles and can be demonstrated by an attempt 

 to count the pickets in a distant fence, which can be seen clearly 

 if the eye remains still, but are lost count of if the eye moves to follow 

 them. 



Many other forms of striated muscle have a more complicated pattern 

 of sarcous element than the ones we have been studying, as in some in- 



