SPINAL CORD. 613 



causes immediate and involuntary winking of the eyelid. This 

 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 au- 

 tomatic, since they appear to act inde- 

 pendently of influences from without. 

 The only source of energy these cells 

 have, is the warmth and nutrient mate- 

 rial carried to them by the blood flowing 

 in their immediate neighborhood. The 

 vaso-motor centres are good examples of 

 automatic centres, in which the constant s. Sensory receiving or- 

 or tonic character of action predomi- 8 with attached affer- 

 mi f ent nerve fibre. G. Cen- 



nates. ihe respiratory centre is one from 



\ . J . tral organs ganglion 



which automatic impulses are rhythmi- cellg M Peripheral or- 

 cally discharged by a special regulating gan and efferent nerve, 

 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. 



