THE FROG AND VERTEBRATES IN GENERAL 169 



lium. Examples of this are found generally distributed in the 

 skin epithelium covering the body. In Man, sensitive epithelial 

 cells respond to cold, heat, pressure, etc. The sense of taste in the 

 tongue and the amazingly keen olfactory sense found in many 

 Vertebrates are due to sensory epithelial cells. (W. f. 146.) 



2. Muscular Tissue 



The specialized contractile muscle tissue which, responding 

 to the control of the nervous system, brings about through its own 

 contractions the almost innumerable and continuous movements 

 present in higher animals, is necessarily very widely distributed. 

 Muscle tissue may be classified as (a) the unstriated, or involun- 

 tary, muscle tissue which is not under direct control of the will, 

 and (b) the striated, or voluntary, muscle tissue, the move- 

 ments of which are under voluntary control. The unstriated 

 muscle tissue, examples of which may be found in the various 

 organs, is under the general control of the autonomic nervous sys- 

 tem, (c) Cardiac muscle tissue in the heart is also involuntary 

 in its functioning but structurally is regarded as a separate type 

 of muscle tissue. (W. fs. 98 ; 99.) 



The unstriated muscle tissue which for the most part forms the 

 muscular layers in the walls of a number of the important organs, 

 such as those of the alimentary canal, blood vessels, and urinary 

 bladder, is regarded as a simpler type than the striated. A micro- 

 scopic examination shows that it consists of elongated, spindle- 

 shaped cells, each with a nucleus which is also elongated in the 

 same direction as the cell. The muscle cells are frequently branched 

 at the ends. Also there is a considerable variation in their length. 

 For example, in the walls of the blood vessels the cells are short 

 and correspondingly thick, whereas in the walls of the bladder 

 they are long and thin. The cytoplasm shows a fine longitudinal 

 striation which is very different from the marked, transverse stria- 

 tions of the voluntary type of muscle tissue. The spindle-shaped 

 cells lie close together with a certain amount of cement substance 

 and connective tissue elements intermingled. Thus they are 

 closely held together and work as a unit. The rate of contraction 

 in unstriated muscle tissue is much slower than that of striated 

 muscle tissue, but the movements can be continued for a longer 

 time. (W. fs. 7, B; 34.) 



The striated muscle tissue of the body is largely centered in the 



