130 MANUAL OF HISTOLOGY. 



cells. (Fig. 50.) The nucleus is at about the middle. The cell 

 swells out around the nucleus, and quickly contracts again be- 

 yond it. A small cell, such as is found in the wall of a small 

 blood-vessel, is consequently spindle-shaped, but we find many 

 in the frog's bladder that run out into fine threads of indefinite 

 length. Sometimes one end of a cell divides into two fibres. 

 (See Figs. 51 and 52.) 



The nucleus sometimes appears to be homogeneous, though 

 it usually contains one or more granules, sometimes considered 

 to be nucleoli. When using a high power the writer has some- 

 times found that the nucleus contained many granules, so ar- 

 ranged as to suggest very strongly a transverse striation. A 

 row of granules at each end of the nucleus is sometimes found. 

 Muscular fibres in the walls of small, transparent blood-vessels 

 are very instructive objects, because by changing the focus we 

 can observe them as they curve round the vessel, both in longi- 

 tudinal and in transverse section. At those places where a 

 transverse section of one end of the cell is in focus we see what 

 appears to be a granule merely. If another part near the nu- 

 cleus is brought into focus, it shows as a small circle, while if 

 the nucleus happens to be cut transversely, it gives the effect 

 of a dark spot inside a circle. 



VOLUNTARY MUSCULAR FIBRE. 



lN"o tissue-is more easily recognized than striped muscular 

 'fibre, yet none is more difficult to understand. 1 



The fibres are cylinders or irregular prisms of varying length. 

 Their diameter in the human body varies, according to Frey, 

 from .0113 to .0563 mm. Each fibre is tightly inclosed in a struc- 

 tureless elastic membrane, called the sarcolemma. This sheath 

 is not very easily demonstrated ; but if fresh muscle be roughly 

 picked to pieces in water, shreds of it may be seen at the torn 

 ends of fibres, and sometimes it can be made out where the 

 muscular substance has been injured in the course of a fibre. 



1 Any attempt at an account of the many views that have been and are held, 

 would make this article far too long. A few only will be mentioned, and these inci- 

 dentally. It is hoped that this defect, if it be one, will be compensated for by the 

 fulness of the bibliography. 



