HISTOLOGY 



141 



the adult, lie between these two layers consist of tissues which do not 

 retain the epithelial character of the embryonic tissues from which they are 

 derived, but give rise to more or less bulky and solid masses of material. 

 Non-epithelial tissues also play an important part in providing structures 

 accessory to the primary epithelia, as when connective tissue forms a 

 dermal layer of the skin, or muscular and connective tissues become associ- 

 ated with the enteric epithelium to form the wall of the digestive tube. 



The important types of adult non-epithelial tissues are the following: 

 (i) muscular; (2) nervous, exclusive of neuro-epithelial structures; (3) 

 tissues serving for mechanical support — the connective and skeletal tissues; 

 (4) adipose tissue or fat; (5) blood. 



Muscular Tissue 



Locomotion in some protozoans is effected by beating of cilia. The 

 movements of large animals depend on contractile mechanisms. Con- 

 tractility is inherent in protoplasm. The least specialized protoplasm is 



Fig. 103. — A, unstriated ("smooth") muscle cell with single nucleus; B, shows a 

 small portion of the length of a multinucleate striated fiber. (From Kingsley.) 



apparently able to contract in the direction of any of its axes. When 

 protoplasmic mechanism for effecting vigorous, quick or long continued 

 contracting is established, the abihty to contract becomes restricted to one 

 axis. The protoplasmic structures which seem to be somehow imme- 

 diately concerned with contraction are exceedingly fine fibrils, the myofi- 

 brils, which extend through the cell parallel to the axis of contraction. 

 Some unicellular animals exhibit fibrils which are apparently of the nature 

 of myofibrils and serve as intracellular motor elements. In metazoa cer- 

 tain cells become more or less elongated and differentiate myofibrils 

 extending lengthwise of the cell — a muscle cell (Fig. 103). In the simple 

 small coelenterate Hydra, however, the contractile fibers which effect the 

 movements of the animal are not independent muscle cells but are merely 

 long processes from the basal ends of the epithelial cells. It is significant 

 that, in the absence of a mesoderm, the primary epithelia are able to 

 provide a motor mechanism. Such epithelial motor processes may have 

 been the forerunners of muscle cells. 



Among invertebrates the usual type of muscle element is a much 

 elongated cell having a single nucleus, more or less numerous myofibrils 



