152 ORGANS AND ORGAN SYSTEMS 



of larger ganglia, known as the sub-oesophageal ganglia. The 

 commissures from them pass around the oesophagus, one on 

 each side, and connect with a pair of ganglia in the peris- 

 tomium, dorsal to the mouth. Because of their course around 

 the oesophagus, these commissures are called the circum- 

 oesophageal commissures, and the two dorsal ganglia are called 

 the cerebral ganglia (c.c. and e.g.). This pair represents the only 

 morphological element comparable with a brain of higher ani- 

 mals, but it is probable that they have no functions different 

 from those of the ordinary ganglia of the ventral chain. As 

 there are more sensory cells in the anterior region of the worm, 

 it is probable, however, that their functions are more fre- 

 quently called into play, so that it is a more active organ than 

 any of the other ganglia. 



A Reflex Action. Consciousness, as we understand it in 

 human beings, probably does not exist in the earthworm, but 

 the relation between nervous impulse and muscular response 

 is so delicately adjusted that movements are produced, which 

 in human beings we would interpret as conscious acts. The 

 complicated movements of a worm, in its efforts to free itself 

 from some irritating environment, may all be traced back to a 

 relatively simple series of processes termed a reflex action (Fig. 

 61). Each reflex action involves five distinct elements of the 

 nervous system: (i) A sensory cell which receives the stimulus 

 from the outside; (2) a nerve fiber bearing the sensory or affer- 

 ent impulse transmitted from the sensory cell; (3) a central 

 nerve cell in the ganglion which receives the sensory impulse, 

 and transforms it into a motor or efferent impulse, which now 

 travels over (4) an efferent or motor nerve to (5) a muscle cell. 

 The stimulus, thus conveyed, starts contraction in the muscle. 

 It is probable that other centers of activity are stimulated, and 

 that, by nerve cells and their processes, the nerve fibers trans- 

 mit impulses from one ganglion to another along the entire 

 course of the central nervous system. This is probable be- 

 cause of the presence in the ventral nerve chain of different 

 kinds of nerve fibers, some of which originate in the ventral 

 ganglia and send processes in both directions along the ventral 



