CENTRAL NERVOUS SYSTEM. 639 



repair as is made occurs in the peripheral system only, while a cell once 

 damaged by injury to its nucleated portion is not to be replaced. 



PART II. THE PHYSIOLOGY OF GROUPS OF NERVE-CELLS. 



A. ORGANIZATION AND ARCHITECTURE OF THE CENTRAL, NERVOUS 



SYSTEM. 



THE reactions of groups of associated nerve-cells have usually furnished 

 the largest mass of facts presented under the title of the physiology of the 

 central nervous system. When it was recognized that the nerves formed 

 pathways by which the sensory surfaces of the body were put into connection 

 with the central system, and also the pathways by which this system was in 

 turn rendered capable of controlling the tissues of expression, it became at 

 once important to determine over what nerves the impulses arrived at the 

 central organ, how they travelled through that organ, and by what other 

 nerves they were again delivered at the periphery. 



Both anatomical and physiological research have been directed to this end. 



The arrangement of these paths as found in the adult human nervous system 

 is our principal object ; at the same time it should not be forgotten that the 

 reactions of simpler mammalian systems have furnished the greater number of 

 facts, and if the pitfalls surrounding the assumption that the reactions found 

 in the nervous system of a rabbit or monkey hold true in all detail for that 

 of man can be avoided, no danger and much gain will follow from the use of 

 the facts of comparative physiology. 



Physiological Unity of the Central Nervous System. So far as its 

 physiology is concerned, the nervous system of any mammal must be regarded 

 as a unit. Custom, however, sanctions a division into a central and peripheral 

 nervous system. The central system is usually taken as that enclosed within 

 the bony cavities of the cranium and vertebral canal, excluding the dorsal 

 root-ganglia ; the peripheral, that formed by the spinal and cranial nerves and 

 the ganglia associated with them. Neither of these parts has an independent 

 significance, and furthermore the central system is largely penetrated by nerve- 

 fibres from the dorsal spinal roots, fibres which have an origin outside of those 

 cells which form the walls of the medullary tube and constitute the central 

 system in the strict morphological sense. On the other hand, the retina, which 

 is in large measure morphologically a part of the medullary system, is, as a 

 rule, not counted as belonging to this system, but is put down as a peripheral 

 sense-organ. These facts are here mentioned solely to emphasize the point that 

 gross anatomy has found convenient certain methods of division which, if 

 strictly followed, confuse the morphological relations. Yet, for many purposes, 

 the subdivision into central and peripheral portions is advantageous. 



General Arrangement of the Central Nervous System. The general 

 architecture of the central system is best understood by means of schemas 

 (Figs. 162 and 163). As the typical arrangement is found in the spinal cord, 

 a cross section through this part will most readily express the facts. 



