94 GENERAL PRINCIPLES OF ZOOLOGY. 



other. Each fibril is formed of singly and doubly refractive 

 l)arts, whicli alternate with one another in more or less compli- 

 cated arrangement. Since now the doubly refracting parts of the 

 fibrils within a bundle lie at about the same level, there is caused 

 a cross-striation extending through the whole bundle. Finally, 

 scattered here and there between the muscle-fibrils are the muscle- 

 corpuscles, spindle-shaped protoplasmic bodies with a nucleus, the 

 remnants of the cells which have formed the musculature. 



4. Nervous Tissue. 



Function of Nervous Tissue. — As the muscular tissue brings 

 about motion, so the nervous tissue serves for the transmission of 

 stimuli. It communicates the stimulations of the sense-organs at 

 the periphery to the central nervous system, the seat of conscious- 

 ness, and here brings about percej^tion (centripetal nerve tracts); 

 further, it transmits the voluntary impulses to the periphery,', par- 

 ticularly to the musculature (centrifugal nerve tracts). By the 

 nervous system, finally, the stimuli arising in various places are 

 co-ordinated, thus furnishing the elements for that which we call 

 independent psychic activity. 



Elements of Nervous Tissue. — The agent of the transmission 

 of stimuli is undoubtedly a specific nerve-substance different from 

 protoplasm. Hence we speak of nerve fibrillfe as of muscle fibrills, 

 the product of the special nerve-cells, but the relations iuvolved 

 are not sufficiently understood. 



The elements of the nervous system are divided into ganglion 

 cells and nerve-filres, but it must be remembered that these are 

 not independent of each other, but that the fibres are enormously 

 elongated processes of the ganglion cells. In the vertebrates the 

 ganglion cells vary greatly in size; besides small elements there 

 are large cells, only exceeded by the eggs in size which correspond- 

 ingly have large nuclei recalling the germinal vesicles. Unipolar, 

 bipolar, and multipolar ganglion cells are recognized, the differ- 

 ences depending upon the number of processes (nerve-fibres) which 

 arise. In multipolar cells the number is very large (fig. 51) and 

 are of two kinds, dendrites and axons or neurites. Dendrites are 

 so called because they branch again and again, not far from their 

 origin from the cell. The axons (of which there is usually but 

 one to a ganglion cell) can be followed to a long distance with- 

 out giving off branches, except here and tliere lateral side twigs 

 (collaterals) which arise at right angles to the main fibre; they 



