THE HEART. SUMMARY. 209 



4. We may picture to ourselves, for the sake of an initial working hypothesis, 

 that the elements have some such arrangement as shown in Fig 117. It is 

 assumed; a. that the giant bipolar cells are the inhibitory agents and the centers 

 from which the rhythmic impulses radiate to all parts of the heart; b. they are 

 probably connected with the adjacent dorsal and lateral integument through 

 branches of the segmental nerves, and with the ventral cord, via the five or more 

 segmental cardiacs. The latter probably consist, in the main, of axones from 

 the giant cells, and possibly of axones from cells located in the branchial neuro- 

 meres; c. the dendrites of the giant cells are probably confined to the median cord; 

 they do not appear to extend into the plexus on the haemal surface of the heart, 

 or into the lateral nerves; d. the small multipolar cells are probably motor cells, 

 distributing their fibers through the main branches of the plexus to the lateral 

 nerves, and then forward (and backward ?) to their terminals in the muscle cells 

 of the heart; e. afferent sensory fibers probably run from their paccinian corpus- 

 cle-like terminals either directly to the median cord, or to it via the lateral nerves. 



On these assumptions, \ve may explain the experimental results as follows: 



Placing the electrodes on the lateral nerves, or on the median surface of the 

 median one, or on the main transverse branches of the plexus, inhibits the heart- 

 beat to a varying extent because of the variable number of accelerator or inhibitory 

 fibers that are contained in a given nerve branch, or of the number of small 

 ganglion cells overlying the larger ones in the median cord. The automatism of 

 the branchial section of the heart is due to the presence of the giant bipolar cells, 

 from which the rhythmic impulses pass in an axial direction along the giant nerve 

 tubes the whole length of the heart. From these tubes numerous collaterals are 

 given off right and left, apparently connecting with the motor cells which are found 

 along the entire length of the heart, but less abundantly at the anterior than at 

 the posterior end. The giant cells are covered on the haemal and lateral sides by 

 thick layers of motor neurones, hence they can be reached only from the under 

 side of the median cord. When the cord is stimulated from that side complete 

 inhibition of the heart-beat follows. 



It is difficult to understand why stimulation of the vagus and branchial 

 neuromeres and of the segmental cardiacs has so far produced no noticeable effects. 

 It is probable that a more careful investigation of this point will furnish important 

 results. 



THE NERVOUS SYSTEM AND SENSE ORGANS OF VERTEBRATES AND ARACHNIDS. 



General Summary of Chapters I-XII. 



i. The foundations of the nervous system of vertebrates are laid in the 

 trochosphere or ccelenterate stages, and the most important steps in its early 

 evolution were made in the rotifers, phyllopods, marine arachnids, and ostraco- 

 derms. 

 14 



