HISTOLOGY 



NON-EPITHELIAL TISSUES 



93 



The primarily essential parts of a metazoan animal are the epidermal 

 epithelium and the enteric epithelium. Certain of the organs which, 

 in 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. 



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 ciHa. The 

 movements of large animals depend on contractile mechanisms. Con- 

 tractility is inherent in protoplasm. The least specialized protoplasm 

 is 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 ability to contract becomes restricted to 

 one axis. The protoplasmic structures which seem to be somehow 

 immediately concerned with contraction are exceedingly fine fibrils, 

 the myofibrils, which extend through the cell parallel to the axis of 

 contraction. 



Fig. 87. — A, unstriated ("smooth") muscle cell with single nucleus; B shows a small 

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



Among invertebrates the usual type of muscle element is a much 

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

 extending through the protoplasm lengthwise of the cell, and having the 

 usual cell-wall devoid of any special membranous covering. Such cells, 

 associated together to form layers, bundles or masses, constitute the 

 muscles of the body-wall and the viscera. Certain invertebrates, how- 

 ever, whose muscles are, in one way or another, especially efficient have 

 muscle cells or more complex sort. The myofibrils become strongly 

 developed and each fibril exhibits an alternation of darker and lighter 

 zones. The zones of either type lie exactly alongside one another on 

 adjacent fibrils so that they give the impression of transverse bands or 



