OBSERVATIONS ON STRUCTURE OF CELLS AND NUCLEI. 331 



terminating freely in the ground-substance of the mesen- 

 tery. Whether the bundles be large or small, the indi- 

 vidual fibres are sufficiently separate from each other as to 

 allow their examination in all parts. Of great beauty are 

 those parts of our specimens in which the minute bundles 

 are composed of a limited number of fibres (two — five), and 

 when they freely intercommunicate with each other. 



Most of the muscle-fibres remain undivided, and terminate 

 at each extremity in one single pointed end; there are, how- 

 ever, such as divide close at the nucleus or at a part not 

 distant from it into two, seldom into three, branches. 



The nucleus of the muscle-fibres is oval and slightly 

 flattened; when seen in profile it measures in the mean 

 0*027 mm. in the long, 0*008 mm. in the transverse, 

 diameter. The nucleus lies not exactly in the middle of the 

 long diameter of the muscle-fibre, but not very far from it, 

 as the following measurements show: 



In muscle-fibre (a), 0"306 mm. from middle of nucleus to the corre- 

 sponding extremity of the muscle-fibre. 

 0'294 mm. the rest. 

 In muscle-fibre {0), 0"282 mm. from middle of nucleus to the corre- 

 sponding extremity of the muscle-fibre. 

 0354; mm. the rest. 



Tiie length of the thin examples is about 0"C1 mm. ; but 

 there are others which are shorter, being much thicker. 



As I have mentioned above, the muscle-fibres are, in speci- 

 mens that have been well spread out, so much sejiarated from 

 each other that tliey can be easily followed from one end 

 to the other, and it is thus a matter of great facility in 

 seeing that most fibres are more or less wavy and curved, 

 and that some possess from place to place varicose sioellings. 

 Examining these varicosities with a high power we observe 

 that they are composed of two substances: — (1) a sheath 

 containing transversely arranged thickenings in the form 

 of annular markings, such as represented in fig. IT e, and 

 (2) a central bundle of fibrils;^ the thickness of this bundle 

 varies naturally according to the thickness of the point 

 examined, i.e. whether a part near the extremity or the 

 nucleus («.e. near the thick part) of the muscle-fibre, but is 

 always slightly thicker corresponding to a varicosity than in 

 the parts between. 



' That the substance of unstriped muscle-fibres in general is longitudinally 

 striated, i.e. probably composed of longitudinal fibrils, has been known to 

 liistologists for some time {Arnold, ' Strieker's Manual of Histology,' chapter 

 on unstriped muscle ; E. Eleiv, ' Handbook of the Piiysiol. Liiburatory/ 

 edited l)y Dr. Burdon-Saiidcrson, ciiapter on unstriped muscle tissue, fj'f. 

 07 of plate xxv ; W. Ylemming, 1. c, p. 714). 



