GENERAL HISTOLOGY 83 



substance is formed in a limited space (see figs. 193, 240, 241 and their 

 legends). 



In vertebrates and arthropods the contractile fibre-cells occur in the 

 vegetative organs as elements of the 'organic musculature'; on the other 

 hand, we find here the epithelial musculature in the cross-striated primary 

 bundles, separated from the epithelium, and only developmentally refer- 

 able to the epithelium of the body cavity (fig. 51). A primary bundle is 

 cylindrical, bounded externally by a structureless envelope, the sarco- 

 lemma. Its contents consist of fine fibrils, which, closely parallel to one 

 another and pressed closely together, run from one end of the mass to the 

 other. Each fibril is formed of singly and doubly refractive parts, 

 which alternate with one another in more or less complicated 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 (fig. 51). 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. Although the primitive bundles retain no epithelial 

 characters, their epithelial nature is shown in their origin from the epithe- 

 lium of the primitive body cavity (from that part of its wall known as the 

 protovertebrce). 



4. Nervous Tissue. 



Function of Nervous Tissue. — As muscular tissue causes motion, 

 so nervous tissue serves for the transmission of stimuli. It conveys the 

 stimulations of the sense-organs at the periphery to the central nervous 

 system, and here brings about perception {centripetal nerve tracts) ; further, 

 it transmits the voluntary and reflex impulses to the periphery, particu- 

 larly to the musculature and glands {centrifugal nerve tracts). By the 

 nervous system, finally, the stimuli arising in various places are co-ordi- 

 nated, 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 proto- 

 plasm. The prevailing view is as follows: The elements of the nervous 

 system are divided into ganglion cells and nerve-fibres, but it must be re- 

 membered that these are not independent of each other, but that the fibres 

 are enormously elongated processes of the ganglion cells. In both gan- 

 glion cells and nerve fibres there are extremely fine fibrilke which are 

 connected with each other and are to be compared to the wires of a 



