PHYSIOLOGY OF THE NERVE-CENTRES. 625 



Cesalpinus (1571) remarks that interruption of the blood-stream makes the parts insensible. 

 Thomas Willis described the chief ganglia (1664). In Des Cartes there is the first indication of 

 reflex movements ; Stephen Hales and Robert Whytt showed that the spinal cord was 

 necessary for such acts. Prochaska described the reflex channels, [while Marshall Hall 

 established the doctrine of reflex, or, as he called them, "diastaltic" actions]. Duverney 

 (1761) discovered the ciliary ganglion. Gall traced more carefully the course of the 3rd and 

 6th nerves, and also the spinal nerves into the grey matter. Hitherto only nine nerves of 

 the brain had been enumerated ; Sommerring (1791) separated the facial from the auditory 

 nerve, Andersch (1797) the 9th, 10th, and 11th nerves. 



Physiology of the Nerve-Centres. 



358. GENERAL. [The nerve-fibres and nerve-cells constitute the elements 

 out of which nerve-centres are formed, being held together by connective-tissue. 

 In the process of evolution, groups of nerve-cells with connecting fibres are arranged 

 to constitute nervous masses, whereby there is a corresponding integration of 

 function. Thus, with structural integration there is a functional integration. 

 When the structure suffers so also does the function, and those parts which are 

 most evolved, as well as those actions which have to be learned by practice, are the 

 first to suffer during the dissolution of the nervous system.] 



General Functions. The central organs of the nervous system are in general 

 characterised by the following properties : 



1. They contain nerve J cells, which are either arranged in groups in the interior 

 of the central organs of the nervous system, or embedded in the peripheral branches 

 of the nerves. [Nerve-cells are centres of activity, originate impulses and conduct 

 impulses as well, while nerve-fibres are chiefly conductors.] 



2. The nerve-centres are capable of discharging reflexes, e.g., reflex-motor, reflex- 

 secretory, and reflex-inhibitory acts. 



3. The centres may be the seat of automatic excitement, i.e., they may 

 manifest phenomena, without the application of any apparent external stimulus. 

 The energy so liberated may be transferred to act upon other organs. This 

 automatic state of excitement or stimulation may be continuous, i.e., may be 

 continued without interruption, when it is called tonic automatic or tonus ; or it 

 may be intermittent, and occur with a certain rhythm (rhythmical automatic). 



4. The central organs are trophic centres for the nerves proceeding from them ; 

 they may also perform similar functions for the tissues innervated by them. 



5. The psychical activities are dependentjipen an intact condition of the gangli- 

 onic central organs. These various functions are distributed over different centres. 



[The term "centre" is merely applied to an aggregation of nerve-cells so related to each 

 other as to subserve a certain function, but, inasmuch as these cells are connected to each other 

 and with other cells in many ways, various combinations of them may result ; again, we have 

 also to take into account the greater or less resistance in some paths than in others, so that 

 the variety of combinations which these cells may subserve is enormous. These cells give off 

 processes which branch, and anastomose with processes from other cells. Thus, innumerable 

 ways are opened up to nervous impulses by these combinations, so that ,in a certain way 

 we may regard a cell as a junction of these conducting fibres, or a " shunt" whereby an impulse 

 may be shunted on to one or other branch in the direction of least resistance, or in the best 

 beaten path as it were, while there may be a " block " in other directions.] ; 



[In connection with the histology of the central nervous system we have to study: 

 A. The nervous constituents. B. Non-nervous constituents. 



(1) Nerve-fibres.- (1) Vessels (blood and lymph). 



(2) Nerve-cells. (2) Epithelium. 



(3) Sustentacular tissue. 



(a) Connective-tissue. 



(b) Neuroglia.] 



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