Sl^RIATED MUSCLE 



plates appear homogeneous in section, although at one point it appears 

 that the fibrils are separately and distinctly shown in several of the 

 plates. 



A closer study of the structure and 

 relations of the fibrils should now be 

 made in some muscle whose elements 

 are more favorable for a detailed ob- 

 servation. A suitable muscle for this 

 purpose can be found in longitudinal 

 sections of the muscle in a lobster's limb 

 joint. Figure 83 is from the basal joint 

 of the antenna in a lobster that was on 

 the point of emerging from its old shell. 



This figure represents a part of a 

 fiber, the upper end of which was at- 

 tached to the shell. The two smaller 

 nuclei are probably the first edges of 

 nuclei that are, in reality, as large as the 

 two larger ones. 



The fortunate condition is that the 

 part of the muscle next to the shell was 

 at rest and in a relaxed condition, while 

 all the other segments showed successive 

 stages of contraction. The first segment 

 measured, in one of its magnifications, 

 17^ mm. Each succeeding one meas- 

 ured a little less, until the last is only 12 

 mm. long. This alone shows us that we 

 have a carefully graded series of contrac- 

 tion stages, and an examination of the 

 appearance of the successive stages con- 

 firms this view. 



In the first upper segment, of the 

 eight sarcous segments shown in any one 

 of the sixteen myo-fibrils that compose this fibril bundle, we can notice 

 that the anisotropic material is deposited in a long rod-like area in the 

 middle of the segment. This area we will designate as Q, and it is 

 usually so called in works on muscle structure. 



On each side of Q is an area of non-staining isotropic substance not 

 more than a quarter as long (or wide, if we consider the whole band that 

 they form) as Q. Conforming with usage, we shall call this ;'. There 

 are two of these. 



Bordering each ; on its side farthest from Q is a row of dots or round 



FIG. 83. Bit of a muscle fiber from a 

 lobster's antenna. Partly contracted. 

 Lettering of unit regions same as in 

 Fig. 81. m., delicate membrane ex- 

 tending through cytoplasm from all 

 Z planes. Apparent difference in 

 size of muscle nuclei due to tangen- 

 tal sectioning of the smaller ones. 

 conn.t., connective-tissue fibrils which 

 attach the muscle fibrils to the hypo- 

 dermal cell fibrils. 



