COMPARISON WITH VERTEBRATES. 193 



and spontaneity of movements between the gills cut off from the hindbrain and 

 those that are united directly with it by one or both cords. 



We may therefore conclude that each branchial neuromere contains only a 

 part of a respiratory reflex center; and that the inhibitory control and the coordi- 

 nation, or unification, of the respiratory and other related gill movements is 

 produced, in part, by the action of a special respiratory center located in the 

 vagus neuromeres, and in part by the hypobranchial nerve and muscle. 



The coordination of right and left sides is affected via the cross commissures. 



Comparison with Vertebrates. — The results obtained from an experi- 

 mental study of the respiratory centers of Limulus by Miss Hyde, Mr. Pearl and 

 myself are in essential agreement, and they harmonize with Miss Hyde's work 

 on other invertebrates and on the skate. In her admirable paper on the "Localiza- 

 tion of the Respiratory Center in the Skate,"' she makes the following statements: 



1 . " Students working in my laboratory have proved that the relative position 

 of the respiratory center in the central nervous system of the acrididae is practically 

 the same as in Limulus. 



2. "The respiratory movements of the skate are segmental processes. The 

 relationship of the respiratory organs and their segmental centers is not so obvious 

 as it is in the lower forms (i.e., Limulus). The developmental changes of shifting 

 and consolidation have begun to mask the segmental connections of the different 

 parts of the brain. 



3. "Each ganglion, through special fibers and cells, controls the activity of 

 the respiratory muscles with which it is segmentally related and is capable 

 of initiating impulses that produce coordinated rhythmical respiratory 

 movements. 



4. "The medulla may be severed both from the cord and the regions of the 

 brain anterior to it, or divided along its median suture, into two bilateral halves 

 without impairing the functions of the respiratory center. Each half is capable 

 of sustaining coordinated respiratory movements which part of the time may be 

 different in rhythm on the two sides. 



5. "Not only may either the spiracle and first gill arch, innervated by the 

 seventh and ninth nerves, or the last four gill arches, innervated by the tenth, 

 when isolated from the rest of the respiratory mechanism by a median and trans- 

 verse section continue their movements, but all other than the special part of 

 the respiratory center that controls these divisions may be destroyed, and either 

 the four gill arches or the spiracle and first gill arch will still pursue their coordi- 

 nated respiratory activity. 



6. "The skate illustrates, in its type of respiratory center, an intermediate 

 stage, between the simple segmental arrangement of the neurons presiding over 

 the coordinated respiratory movements found among invertebrates, and the com- 

 plex, modified, and specialised centers existing in higher vertebrates." 



't* ^i^ *!* *!* ^ *!* ^p 't^ ^* 



^ Am. Journ. Physiol., 1904. 



13 



