IOO 



HISTOLOGY 



that is unicellular and spindle-shaped, and has all the characteristics of 

 smooth muscle except one (Fig. 98). When contracted, it shows, not a 

 transverse striation, but a peculiar series of oblique striations, little groups 

 of which pass each other at an angle. The exact nature of this striation 



FIG. 97. A, longitudinal sections of several muscle fibers from the squid, Loligo; B, trans- 

 verse sections of the same at different levels. X 700. 



has not been determined upon, and so it cannot be compared with that 

 of ordinary striated muscle. So, also, the cell itself cannot be accurately 

 compared with other unicellular and spindle-formed muscle cells. Both 

 this last form and that of the squid are "voluntary fibers." 



FIG. 98. Portions of two longitudinal sections of fibers from the closing muscle of Venus. 

 ., nucleus. X 870. 



The involuntary muscle cell of the vertebrates will form our next and 

 last example of this class of muscle fiber. This resembles the squid's 

 fiber in that its myo-fibrils are laid down in the peripheral sarcoplasm. 

 It shows more of them, however, so many, that but little sarcoplasm can 



FIG. 99. A, longitudinal section of several smooth muscle fibers in the bladder of a calf; 

 conng.fi., connecting fibrils between the cells; B, transverse section of similar fibers. 



be found. What can be seen lies at either end of the rather elongate 

 nucleus. This cell has its own variations in the different forms of verte- 

 brates, and our example is taken from a section of the bladder of a calf 

 (Fig. 99). In some other forms the nucleus is much longer and thinner, 

 and can be seen to twist and curl when the muscle contracts. The 



