242 



PHYSIOLOGY FOR DENTAL STUDENTS. 



highly specialized for the purpose of receiv- 

 ing impressions from a distance. Many of 

 the efferent fibers which arise from the cells 

 of the head ganglia go to the muscles which 

 move the head end of the animal, others, how- 

 ever, do not run directly to effectors, but they 

 run down the nerve chain to make synaptic 

 connection with the cells of some of the seg- 

 mental ganglia. This connection of the cells 

 of the head ganglia with those supplying the 

 segments enables the former to exercise a dom- 

 inating influence over the activities of the lat- 

 ter, the purpose being that approaching dan- 

 gers may have a greater influence in deter- 

 mining the response of the animal than stim- 

 uli that are merely local. When, for example- 

 some sight or sound of an approaching enemy 

 is received by the head ganglia, these will 

 transmit impulses down the ganglion chain 

 which so influence the various nerve cells as to 

 produce, in all of them, a co-ordinated action 

 for the purpose of getting the animal out of 

 danger. Even should some local stimulus be 

 acting on one or more of the segments, the 

 stimulus which is received through the head 

 ganglia will obtain the upper hand and annul 

 or inhibit the local influence. The part will 

 become subservient to the whole. This illus- 

 trates the integration of the nervous system, 

 which, as we pass to higher animals, we shall 

 find to become more and more developed and 

 intricate. 



So far, however, the nervous reaction is 

 purely of the nature of a reflex; but in the 

 higher animals other factors, namely, memory 

 and volition, come to exercise a dominating in- 

 fluence on the nature of the response. The 



Fig. 40. Dia- 

 gram of nervous 

 system of segment- 

 ed invertebrate ; a, 

 sup raoesophageal 

 ganglion ; b, sub- 

 oesophageal gang- 

 lion ; oe, oasopha- 

 gus or gullet. 



