THE NERVOUS SYSTEM. 779 



anterior retractor, the visceral the posterior. The closing of 

 the shell is active, and is caused by the passage of impulses 

 to the muscles along the motor nerves. The opening is 

 more passive, as the elastic ligament causes the valves to 

 gape when the muscles relax. This relaxation is caused by 

 inhibitory nerves, which inhibit the action of the motor 

 nerves, and the muscles in consequence return to their 

 former condition. The inhibitory nerves to both muscles 

 pass out from the cerebral ganglia, but there is no evidence 

 to justify the assumption that these have any "brain" 

 function. The motor cells of the cerebral and visceral 

 ganglia can be stimulated through many peripheral sensory 

 nerves. The heart is innervated from the visceral ganglia, 

 but some physiologists who minimise the importance of the 

 innervation maintain that the heart's activity is largely pro- 

 toplasmic, and that the nerves have chiefly or wholly an 

 inhibitory or trophic function. 



In the Cephalopoda the supra-cesophageal mass cor- 

 responds physiologically to the brain of Vertebrates. 

 When it is destroyed, the ordinary vital functions, such as 

 respiration, circulation, etc., are unaltered ; the animal con- 

 tinues to respond to external stimuli, but the power of 

 " volition " is gone : if left to itself, it remains in one posi- 

 tion until death ensues. From this fact we see that the 

 centres, or presiding nerve cells for all the automatic 

 functions, are placed elsewhere than in the brain, but that 

 this originates all the " voluntary ' muscular movements. 

 Of the various centres, the respiratory is located in the 

 pleural ganglia ; from it nerves pass out which end in the 

 stellate ganglia, and are both motor and sensory for the 

 mantle. This centre is not self-acting, that is, not auto- 

 matic, as are the corresponding centres in Vertebrates, 

 but is only reflexly stimulated into activity by impulses 

 borne by afferent nerves from some part of the body. 

 It seems most reasonable to suppose that this condition 

 is primitive, and that the automatic form of activity is 

 derived. The centre for the movement of the chromato- 

 phores is in the sub-cesophageal mass. The activity of the 

 heart is said by some to be purely "protoplasmic," but 

 co-ordination of the parts of the heart, the branchial hearts, 

 etc., is effected by means of the ganglia placed in the 



