THE STRUCTURE AND FUNCTIONS OF NERVE-TISSUE. 27 



to its destination in the muscles, but passes first to the 

 ganglion, and is thence reflected from the sensory surface to 

 the muscles.* This, which at first sight appears a round- 

 about or cumbrous sort of process, is really the most economic 

 that is available. For we must remember the enormous 

 number and complexity of the stimuli to which all the higher 

 animals are perpetually exposed, and the consequent neces- 

 sity that arises for there being some system of co-ordination 

 whereby these innumerable stimuli shall be suitably responded 

 to. And such a system of co-ordination is rendered possible, 

 and actually realized, through this principle of reflex action. 

 For the animal body is so arranged that the innumerable 

 nerve-centres, or ganglia, are all more or less in communica- 

 tion one with another, and so receive messages from all parts 

 of the body, to which they respond by sending appropriate 

 messages down the nerve-trunks supplying the particular 

 groups of muscles which under the given circumstances it is 

 desirable to throw into contraction. In other words, when a 

 stimulus falls upon the external surface of an animal, it is 

 not diffused in a general way throughout the whole body of 

 the animal, so causing general and aimless contractions of all 

 the muscles ; but it passes at once to a nerve-centre, and is 

 there centralized; the stimulus is dealt with in a manner 

 which leads to an appropriate response of the organism to 

 that stimulus. For the nerve-centres which receive the 

 stimulus only reflect it to those particular muscle-groups 

 which it is desirable for the organism, under the circumstances, 

 to throw into action. Thus, to take an example, when a 

 small foreign body, such, as a crumb of bread, lodges in the 

 windpipe, the stimulus which it there causes is immediately 

 conveyed to a nerve-centre in the spinal cord, and this nerve- 

 centre then originates, by reflex action, a highly complicated 

 scries of muscular movements which we call coughing, and 

 which clearly have for their very special object the expul- 

 sion of the foreign body from a position of danger to the 

 organism. Now it is obvious that so complicated a series 

 of muscular movements could not be performed in the 

 absence of a centralizing mechanism ; and this is only one. 

 instance among hundreds of others that might lie adduced of 



• The term, however, ii nol a happy one, because tin- process is some- 

 thing more than the reflection of the original stimulus or moleoulai disturb- 

 UOe ; tin' ganglion adds a new disturbance. 



