OF THE NERVOUS SYSTEM. 415 



longations. Cells of this description, provided both with ramified pro- 

 longations and with a single unbranched axis cylinder process, are 

 found abundantly in the spinal cord, the medulla oblongata, the cerebel- 

 lum, the corpora striata, and the optic thalami. The axis cylinder pro- 

 cess, in the spinal cord, passes into the bundles of medullated nerve 

 fibres forming the roots of the nerves and the columns of the cord ; and 

 in the convolutions of the cerebellum and the cerebral ganglia, the nerve 

 fibres which penetrate the gray substance lose their medullary layer 

 and become reduced to the condition of a naked axis cylinder, similar in 

 appearance to the prolongations of the nerve cells. For these reasons 

 it is considered probable that the nerve fibres are connected by con- 

 tinuity of substance with the axis cylinder process of the nerve cells. 

 But, according to the most careful observers, 1 this connection is more or 

 less hypothetical, and is not positively shown by direct observation. It 

 is evident that there is a physiological communication between the nerve 

 fibres and the nerve cells-; but it is possible that such a communication 

 may take place by other methods than an immediate continuity of their 

 substance. 



Finally, it is certain that there are nerve cells in the gray matter 

 which are not directly connected with nerve fibres. According to the 

 observations of Gerlach, there is a tract throughout the dorsal portion 

 of the spinal cord, near the central part of its gray substance, where all 

 the nerve cells are provided with branching prolongations, but do not 

 possess any undivided process resembling a cylinder axis. It is not 

 known whether such cells exist also in other portions of the nervous 

 system. 



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

 the gray substance of which they form a part, act as centres, in which 

 the nervous impressions are received through the sensitive nerve fibres 

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

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

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

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

 constitute an apparatus in which the nervous influence is modified in 

 character, and changed from one form to another. Their function is 

 to receive impressions conveyed to them by the nerve fibres, and to 

 convert these impressions into impulses which are transmitted to dis- 

 tant organs. The nature of the process by which this change is effected, 

 and the action which goes on in the nerve cells during its accomplish- 

 ment, are entirely unknown to us ; but it is evidently essential to the 

 physiological operation of the nervous system, since neither sensation 

 nor movement is ever excited, in the natural condition, through the 

 nerve fibres, unless they are in communication with the gray substance 

 of a nervous centre. 



1 Kolliker, Elements d'Histologie Humaine, 5me Edition. Paris, 1868, pp. 361, 

 363, 365, 399. 





