48 E VOL UTION AND DISEASE. 



of vertebrata is certainly very remarkable, but a much 

 more extraordinary instance of change of function is 

 manifested in the nervous system now to be considered. 



The Central Nervous System. Vertebrate animals are 

 distinguished from invertebrate not only in the posses- 

 sion of a vertebral column, but also by the fact that they 

 are furnished with a central nervous system known as 

 the spinal cord and brain. Of late years it has been the 

 opinion of many biologists that the separation of the 

 invertebrata from vertebrata is unjustifiable, and many 

 attempts have been made to bridge the gulf supposed 

 to exist between these great divisions of the animal 

 kingdom. 



Those invertebrates which approach nearest the verte- 

 brates are the cephalopods (cuttles, octopods, &c.), and 

 in these forms the central nervous system is represented 

 by ganglionic masses collected around the oesophagus or 

 gullet and united by commisural fibres : this arrangement 

 is known as the cesophageal collar, so that in order to 

 bring this into harmony with the anatomical disposition 

 of the vertebrate gullet, some eminent biologists have 

 maintained that the vertebrate mouth is secondary, and 

 that the primitive gullet traversed the central nervous 

 system by way of the third cerebral ventricle and 

 infundibulum, a diverticulum from the original vesicles 

 out of which the brain is ultimately developed. This 

 hypothesis has not found much favour, but recently 

 some observations and speculations have been announced 

 which throw much new and important light on the 

 matter. 



The central nervous system is traversed by a canal 



