GENERAL STRUCTURE OF ANIMALS. 



'9 



pearance of cell structure visible; but its essential cell 

 structure is seen in the embryonic development of muscle. 

 Fig. 10 shows the fibers of 

 the muscle of the embryonic 

 heart of a monkey. The for- 

 mation of fibers by fusion of 

 nucleated cells is evident. 

 We shall discuss this again in 

 connection with the physiol- 

 ogy of muscle as an organ. 



5. Nerve Tissue. — This 

 is the highest and most won- 

 derful tissue of the animal 

 body ; not, however, so much 

 in structure (for it is perhaps 

 less specialized than muscle), 

 but in its function. With it, 

 in some way imperfectly un- 

 derstood, is connected the 

 transmission of impressions 

 from the external world to 



the consciousness, and from the will back again to the 

 external world. With it also is connected sensation, 

 consciousness, will, thought, and all the higher faculties 

 of the mind. 



Varieties. — There are, again, two kinds of tissue in 

 nerve tissue, viz., gray granular and white fibrous. The 

 gray granular consists of nucleated' cells of different 

 forms and sizes, apparently connected with one another 

 by interlacing fibers (Fig. n). The white fibrous con- 

 sists of very slender parallel fibers, of great length, con- 

 nected with gray-matter cells. The characteristic func- 

 tion of the gray granular matter is the origination of 

 nerve force ; the function of the white fibrous matter 

 is the transmission of the same. The one may be com- 



Fig. 10. — Muscular fibers of the 

 heart of the embryo monkey. 

 After Kent. 



