DOUBLENESS OP THE NERVOUS SYSTEM. 327 



the brain of one person may be more energetic than that of another, even 

 though its weight may be less. It is not to be denied, however, that there 

 is a connection between mental power and the quantity of cerebral matter, 

 when individuals of the same kind are compared, or that in the animal 

 series the psychical powers decline as the cerebrum diminishes in size. 



Few topics are more worthy of the attention of the physiologist than 

 that of the variable psychical powers of man, and yet few have Ilg variable 

 been more overlooked. By variable psychical powers I mean psychical . 

 those periodicities of increase and diminution in our intellect- p<rv 

 ual efficiency, which may be noticed not only in diseased, but abo in 

 healthy states. On the principles we have presented, these find their 

 explanation in the temporary physical states of the organ, such as its 

 condition of repair, its existing facility for oxidation, and the constitution 

 of the blood as respects a proper arterialization. 



The most striking structural characteristic of the nervous system is its 

 symmetrical doubleness, the cranial and spinal nerves com- Symmetrical 

 ing forth by pairs to their distribution on the right and left ^l { 

 sides of the body. The manner of development from the tem. 

 spinal axis laterally implies such a construction, and, indeed, gives ori- 

 gin to two halves so equal and alike that it has often been said each 

 person consists of two separate individuals. Examining those organs 

 which, by reason of the elaborateness of their mechanism and principles 

 of action, enable us to determine with satisfactory precision Fun ction of 

 the function discharged by each one of the members of the each lateral 

 pair, as in the case of the eye or the ear, we may corne to the organ 

 following conclusions : Each is a distinct organ in itself, capable of its 

 meeting the requirements of the economy in a sufficiently satisfactory 

 manner, and therefore forms a distinct whole ; but the pair can likewise 

 act simultaneously, re-enforcing, to a certain degree, each oth- Con . ointl 

 er's power, though in this double action there by no means double organs 

 arises a double intensity of effect. The closure of one ear jjj^j J 

 to a sound does not diminish the loudness by one half, nor crease 'their 

 does the shutting of one eye reduce to one half the bright- pre ' 

 ness of a light ; but, though there is not such a doubling of effect when 

 both eyes or both ears are employed, there is a degree of precision in the 

 resulting indication which is not to be gained by the use of one of these 

 organs alone In such a double organ, then, the result is not so much 

 a heightening of the final impression as the giving to it of a greater de- 

 gree of precision. 



Moreover, each organ seems to exert a compensating influence over its 

 fellow in any deficiencies or imperfections it may possess. Compensation 

 Tims it is rare that both eyes are of an equal optical good- of defects - 

 ness, as most individuals wiH find on making a personal examination; 



