FUNCTIONS OF DIFFERENT PARTS OF THE NERVOUS SYSTEM. 259 



non-inedullated, is not well known ; but it is supposed that they end, in 

 part at least, in the muscular coat of the small arteries. Even in the 

 substance of certain organs, as for example, in the heart and lung, 

 innumerable minute visceral sympathetic ganglia are met with ; and 

 beneath the mucous membrane of the alimentary canal, microscopic 

 structures, resembling gray nerve-cells, are also found, and are sup- 

 posed to belong to the sympathetic system. 



On examining the course of the fibres forming the connecting cords 

 between the trunk of the sympathetic and the cranial and spinal 

 nerves, which are sometimes regarded as the roots of the sympathetic 

 system, it is found that they consist of two sets of fibres, passing each 

 from one system to the other : the cerebro-spinal white medullated 

 fibres pass through the ganglia of the sympathetic, and so onwards 

 into the longitudinal cords which form the trunks of the sympathetic 

 nerve, and thence into the branches given oflf to the prevertebral plex- 

 uses ; the proper sympathetic fibres, always small and usually non- 

 medullated, pass to the anterior branches of the corresponding spinal 

 nerve. The posterior roots receive fibres from their own spinal gan- 

 glion. From these facts, it follows that the fibres of the cerebro- 

 spinal and sympathetic systems are here intermingled ; it is also ap- 

 parent why the branches of the sympathetic nerves, although more or 

 less white in the first part of their course, become more pinkish as they 

 get nearer to their distribution. As ail the sympathetic nerves prob- 

 ably contain a few fibres derived from the cerebro-spinal axis, so all 

 the cranial and spinal nerves probably contain, in their branches of 

 distribution, some sympathetic nervous fibres. It must further be con- 

 cluded that the sympathetic nervous system is not to be regarded as a 

 mere offset from the cerebro-spinal system, nor yet entirely indepen- 

 dent of it ; but rather that it is a special nervous apparatus, having 

 numerous gray nervous centres of its own, though intimately connected 

 with, and therefore influenced by, the cerebro-spinal system. 



FUNCTIONS OF DIFFERENT PARTS OF THE NERVOUS SYSTEM. 



The nerves, whether cerebro-spinal or sympathetic, being composed 

 entirely of nerve fibres, either white or gelatinous, are considered to 

 act merely as conductors of impressions, or of the effects of impres- 

 sions. The white parts of the substance of the spinal cord, the me- 

 dulla oblongata, the pons Varolii, the cerebral peduncles and hemi- 

 spheres, and the cerebellum, must also likewise be limited, functionally, 

 to conducting properties. The gray matter of the sympathetic gan- 

 glia, and of the several parts of the cerebro-spinal axis, must not only 

 conduct, but also reflect, diffuse, and transfer impressions, and must 

 even originate changes which stimulate the excitability of the nerve- 

 fibres. The ganglionic masses, whether of the cerebro-spinal or sym- 

 pathetic system, are therefore said to be centres of activity. The con- 

 nections of the gray matter, and its greater relative vascularity, are 

 conditions which also favor this view. For the continued activity of 

 both the white and gray matter, it is necessary that they should be 

 adequately supplied with healthy arterial blood, and be maintained 

 within the limits of a certain range of temperature. Imperfectly 



