Embryonic Development; Differentiation of Tissues - 287 



VISCERAL 



nuclei 



nuclei 



CARDIAC 



KELETAL 



JACQUES PwteER 



Fig. 15-14. The three kinds of muscle tissue. 



muscle is very rapid — such that contraction 

 and relaxation are completed in less than 

 0.1 second, compared to an average of 1 and 

 10 seconds, respectively, for cardiac and 

 visceral muscles. Also the contractions of 

 skeletal muscle are finely graded and pre- 

 cisely controlled as to their force and ampli- 

 tude. Essentially, each separate skeletal mus- 

 cle is a bundle containing thousands of paral- 

 lel fibers stretching between two parts of the 

 skeleton. Each fiber receives an individual 

 nerve supply. Accordingly the demands of 

 the nervous system can be varied in such a 

 way that all or only some of the fibers con- 

 tract, adding their strength to the whole con- 

 traction. Cardiac muscle, in contrast, tends 

 to contract in an all-or-none fashion, and a 

 single excitation tends to spread throughout 

 the whole syncytium. In lesser degree such 



generalized contractions also occur in vis- 

 ceral muscle; but in this tissue the excitation 

 seems to be passed from cell to cell. 



Nerve Tissue. The cells of the nervous sys- 

 tem are called neurons (Fig. 15-15). Each 

 neuron consists of a cell body, or centron — 

 that is. the nucleus and the cytoplasm imme- 

 diately surrounding the nucleus — and of deli- 

 cate threadlike nerve fibers, which extend 

 out from the cell body. Some of these fibers 

 normally conduct impulses toward the cen- 

 tron, and these fibers are called dendrons; 

 but other fibers, the axons, normally convey 

 impulses away from the centron. The many 

 neurons that compose the nervous system 

 form a coordinating system of fibers connect- 

 ing the many sensory and motor structures 

 of the body. 



The centron part of a nerve cell main- 



