6o 



NORMAL HISTOLOGY. 



FIG. 69. 



of over 500 /* and a breadth of 20 /*. Occasional cells with bi- 

 furcated ends are encountered, especially among the lower verte- 

 brates. 



The spindle muscle-cell is invested with a very delicate, homo- 

 geneous, hyaline sheath, closely resembling elastic tissue, and 

 corresponding to the sarcolemma of the striated fibre ; within this 

 envelope lies the soft, semi-fluid, contractile protoplasm, embedded 

 in which, near the centre of the cell, lies a characteristic, narrow, 

 rod-shaped nucleus. Delicate longitudinal fibrillas sometimes can 

 be made out extending the entire length of the cell ; these are re- 

 garded by many histologists as representing the actively contractile 

 parts of the cell, the surrounding protoplasm being largely passive. 

 Transverse markings are also often seen ; these correspond in posi- 

 tion to local variations in the diameter 

 of the cell, and are probably due to 

 corrugations in the enveloping mem- 

 brane. 



The individual spindle-cells are closely 

 fitted together and united by an albu- 

 minous cement-substance ; they are dis- 

 posed in groups or bundles, which, on 

 cross-section, are made up of rounded 

 polygonal areas of varying size, the 

 larger possessing round nuclei, while the 

 smaller have none. Since these areas 

 are the sections of nucleated spindle- 

 cells, the large nucleated fields corre- 

 spond to sections passing through the 



nucleus of the cell, while the small ones are sections of the cell fall- 

 ing near the pointed ends. The bundles of muscle-cells are arranged 

 to form layers or sheets, as in the digestive tract, or net-works, 



as in the eye, pleura, etc. 



Involuntary muscle in transverse 

 section : portions of three bundles are 

 represented, separated by areolar 

 tissue (<t) : the nucleated areas are 

 sections of the muscle-cells through 

 their nuclei ; the smaller figures repre- 

 sent sections of the cells cut nearer the 

 ends. 



FlG - 7- 



Examined in longitudinal sec- 

 tion, or in considerable masses, 

 it is difficult to distinguish the 

 individual component fibre- 

 cells, the involuntary muscle 

 in such cases closely resembling 



Involuntary muscle in longitudinal section : the fibrOUS Connective tisSUC \ 

 muscle-cells are often cut obliquely, and hence appear i 



shorter than when isolated. ever, the numerous more or 



less regularly disposed rod- 



shaped nuclei, and the absence of the delicate wavy fibres, together 

 with the impression of greater density, usually suffice to establish 

 the identity of the muscle. 



