6o 



NORMAL HISTOLOGY. 





FIG. 69. 





of over 500 it 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 fibrillae usually 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. 



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 

 fibrous connective tissue ; how- 

 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. 



Involuntary muscle in transverse 

 section : portions of three bundles are 

 represented, separated by areolar 

 tissue (a): 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. 



Involuntary muscle in longitudinal section : the 

 muscle-cells are often cut obliquely, and hence appear 

 shorter than when isolated. 



