UNIVERSITY' 



GENERAL STRUCTURE OF THE 



These facts show beyond question the direct anatomical comn 

 in many instances, of the axis cylinder of nerve fibres with the processes 

 of nerve cells. In the gray substance of the brain, medulla oblongata, 

 and spinal cord, in man and mammalians, the multipolar nerve cells 

 often present certain processes which assume the appearance of axis 

 cylinders, and which join the bundles of fibres running in the direction 

 of nerve roots. It is presumable, therefore, that they become after 

 a time nerve fibres ; although none of them, in these last named situ- 

 ations, have been seen invested with a medullary layer. According to 

 Gerlach, on the other hand, there is a tract of gray substance in the 

 spinal cord, throughout its dorsal portion, where the nerve cells, though 

 provided with branching prolongations, do not present any process 

 resembling an axis cylinder ; and in the sympathetic ganglia of man, 

 the dog, and the cat, Key and Retzius* have been able to follow the 

 branched cell-processes for considerable distances among the neighbor- 

 ing tissues without ever seeing one of them converted into a inedullated 

 nerve fibre. It is possible that this may still have taken place beyond 

 the point of observation ; but it must also be considered as doubtful 

 whether some nerve cells have not a different anatomical connection 

 than that by cell-processes and axis cylinders. 



Physiological Properties of the Nerve Cells. The nerve cells, and 

 the gray substance of which the}" form part, act as centres, in which 

 nervous impressions are received through the sensitive fibres from the 

 periphery, and from which a stimulus is sent out through the motor 

 fibres to the muscles. Every such collection of gray substance is 

 called a " nervous centre." While the nerve fibres accordingly are 

 organs of transmission, the gray substance and its nerve cells are an 

 apparatus in which the nervous influence is changed from one form to 

 another. The nervous centre receives the impressions conveyed to it, 

 and converts them into impulses to be transmitted elsewhere. How 

 this change is effected in the nerve cells is unknown ; but it is evidently 

 essential to the physiological operation of the nervous system, since 

 neither sensation nor movement is ever excited, in the normal condition, 

 through the nerve fibres, unless they are in communication with a 

 nervous centre. 



In the action of the nervous system, therefore, the communication 

 established between different parts of the body is always circuitous. 

 It passes through a nervous centre, in which the impression coming 

 from one organ is replaced by a stimulus which excites the other. This 

 is called the "reflex action " of the nervous system, because it is first 

 sent inward to the nervous centre and then returned or reflected in the 

 opposite direction. In this process, the intermediate act between the ; 

 inward and outward passage of the nervous current is accomplished in 

 the gray substance. 



* Anatomie des Nervensystems und des Bindegewebes. Stockholm, 1876. Zweite 

 Halfte, pp. 125, 137. 



