OF THE NERVOUS SYSTEM. 367 



contact with its mucous membrane. The lachrymal gland is excited 

 to increased activity by anything which causes irritation of the 

 conjunctiva. In all such instances, the physiological connection 

 between two different organs is established through the medium of 

 the nervous system. 



The function of the nervous system may therefore be defined, in 

 the simplest terms, as follows : It is intended to associate the different 1 

 parts of the body in such a manner, that stimulus. applied to one organ ^ 

 may excite the activity of another. 



The instances of this mode of action are exceedingly numerous. 

 Thus, the light which falls upon the retina produces a contraction 

 of the pupil. The presence of food in the stomach causes the gall- 

 bladder to discharge its contents into the duodenum. The expul- 

 sive efforts of coughing are excited by a foreign body entangled in 

 the glottis. 



It is easy to understand the great importance of this function, 

 particularly in the higher animals and in man, whose organization 

 is an exceedingly complicated one. For the different organs of 

 the body, in order to preserve the integrity of the whole frame, 

 must not only act and perform their functions, but they must act in 

 harmony with each other, and at the right time, and in the right 

 direction. The functions of circulation, of respiration, and of 

 digestion, are so mutually dependent, that if their actions do not 

 take place harmoniously, and in proper order, a serious disturb- 

 ance must inevitably follow. When the muscular system is ex- 

 cited by unusual exertion, the circulation is also quickened. The 

 blood arrives more rapidly at the heart, and is sent in greater 

 quantity to the lungs. If the movements of respiration were not 

 accelerated at the same time, through the connections of the nerv- 

 ous system, there would immediately follow deficiency of aeration, 

 vascular congestion, and derangement of the circulation. If the 

 iris were not stimulated to contract by the influence of the light 

 falling- on the retina,, the delicate expansion of the optic nerve 

 would be dazzled by any. unusual brilliancy, and vision would be 

 obscured or confused. In all the higher animals, therefore, where 

 the different functions of the body are performed by distinct organs, 

 situated ia- different parts of the frame, it is necessary that their 

 action should be thus regulated and harmonized by the operation 

 of the nervous system. 



