FUNCTIONS OF NERVE CELLS. 515 



networks in the wall of the intestine. In many instances, how- 

 ever, the exact mode of connection between the nerve and the 

 protoplasm of the tissue elements, to which it bears impulses, has 

 not been satisfactorily made out. In the remaining class of nerve 

 terminals the cells are grouped together so as to form larger and 

 smaller colonies, and more definitely deserve the name of nerve 

 or ganglion cells. These are the central terminals, and are placed 

 either in the cerebro-spinal axis, or in swellings of the nerves 

 called sporadic ganglia. 



Of these nerve cells there are many varieties, all of which have 

 the following characteristics : The cells are of considerable size, 

 and have processes branching off from them, by means of which 

 they communicate with the nerve fibres. These processes may 

 be single or many, hence they are spoken of as uni-, bi-, or mul- 

 tipolar cells, etc. The nucleus is commonly very distinct, and 

 contains a well-marked nucleolus. The abundant protoplasm, 

 which is usually contained in a delicate cell wall, is in direct con- 

 nection with the axis cylinder of the nerve fibres, with which it 

 communicates by means of thin strands of protoplasm that pass 

 out from the cell by the processes. A delicate striation of the 

 protoplasm may sometimes be recognized, indicating the course 

 of the nerve fibrils as they run into the cells from the processes. 



THE FUNCTIONS OF NERVE CELLS. 



Any mass of living protoplasm, such as an amoeba, can receive 

 extrinsic impulses, which affect directly its conditions, and though 

 the impression may be very localized in its application, yet all 

 the parts of the cell participate in the sensation, and probably 

 take part in the resulting movement. 



Besides those acts of which we can recognize the cause, many 

 others occur in an amoeba which we are not able to trace to any 

 definite cause other than the energies derived from its special 

 powers of assimilation. We say, then, that not only can an 

 amoeba feel local stimulation, transmit the impulse to remoter 

 parts of its body, and respond by movement to the stimulus, but, 

 moreover, as the result of intrinsic processes of a chemical nature, 

 it can initiate impulses which appear as motions, etc. We may 



