SPINAL, CORD. 613 



kind of transmission of an impulse in a direction differing from 

 that by which it arrived at the nerve cell is called reflection, and 

 motions such as that just alluded to are called reflex acts. Col- 

 lections of cells, whose duty seems to be habitually to receive 

 impulses from the periphery and to change their direction, are 

 called reflex centres. 



Some groups of nerve cells send forth impulses, either con- 

 stantly or periodically, without receiving any nerve impulse from 

 the surface. Such centres are called auto- 

 matic, since they appear to act indepen- 

 dently of influences from without. The 

 only source of energy these cells have is 

 the warmth and nutrient material carried 

 to them by the blood flowing in their im- 

 mediate neighborhood. The vasomotor 

 centres are good examples of automatic 



Centres, in which the Constant Or tonic Sensory receiving organ 

 _ . _, with attached afferent nerve 



character of action predominates. The fibre . G . central organs- 



respiratory centre is one from which auto- ganglion ceils. M. 



. . , . . . ,, j. , , eral organ and efferent 



matic impulses are rhythmically discharged nerve . 

 by a special regulating apparatus. 



Besides having the power of conducting, reflecting, and origi- 

 nating impulses, we must attribute to the activity of the nerve 

 cells of the brain the various mental phenomena, such as feeling, 

 thought, volition, memory, etc., which form of activity may be 

 excited either by impulses arriving from without, or from the 

 spontaneous (automatic) action of the cells of the cerebral 

 cortex. 



THE SPINAL CORD. 



Being the great bond of connection between the brain and the 

 majority of the peripheral nerves, the spinal cord is obviously a 

 conducting apparatus of the very first importance, and from the 

 quantity of nerve cells lying in its gray matter, it must also enjoy 

 the function of a governing organ or nerve centre. 



These two great duties of the spinal marrow had better be 

 considered separately : 



