6 PBOr. OWEK ON CEREBRAL HOMOLOGIES 



to the Invertebrates in which a homologue of the vertebrate 

 hemispheres may not be so largely developed or superadded. 



Accordingly I conclude that the collective neural centres and 

 their intercommunicating tracts in Invertebrates are the homo- 

 logues of those centres and tracts called " brain and spinal cord " 

 in Vertebrates, and that such " neural axis " marks, in both 

 grades of the animal series, the same position in the body, and the 

 same local relations to the vascular centre, m, and the alimen- 

 tary canal, I. As a corollary, the neural axis, or " ganglionic cord " 

 in Arthropods {h o n) denotes the neural position, and supports 

 the inference that its foremost portion, «, is simply displaced by 

 the course of the gullet through the brain in order to open by a 

 mouth upon the neural aspect of the body. The suppression of 

 such transcerebral tract in Vertebrates allow^s the continuation 

 of the alimentary canal forwards to an oral opening on the haemal 

 aspect of the body. Here the oesophagus offers no obstacle to the 

 approximation of the main cerebral centres to each other — the fore 

 brain to the hind brain. Hence that juxtaposed allocation of the 

 primary encephalic divisions, associated with the progressive accu- 

 mulations of grey and white neurine, which the cerebrum and 

 cerebellum present, in relation to the centres subservient to the 

 ingoing conductors of sensations and the outgoing ones of motions, 

 as we pass in their contemplation from the fish to the ape and the 

 man. 



The so-called " brain " in the Locusts answers to a part only 

 of the brain of a fish ; moreover it is not a " supraoesophageal 

 ganglion," but a " sub " or " haemcesophageal " one. 



The next neural mass in the brain of the Locust (J) answers 

 to the epencephalon of the fish ; it is not a " suboesophageal gan- 

 glion," but a " supra-" or " neuroesophageal " one, and the fore- 

 most of that series of the neural centres or " ganglions." 



The homologue of the vertebrate myelon in Invertebrates is 

 not protected by a special bony case or " vertebral column." The 

 " ganglionic cord " is nevertheless the most precious, as it is the 

 most delicate and crushable of an insect's organs. Hence it has 

 been, so to speak, ordained that the part of the body's surface to 

 which the neural axis is nearest should not be, as in the beast, 

 along the part most exposed and liable to blows. By a modified 

 flexure of the limb-segments the trunk of a beetle or lobster is 

 turned so as to hold the same relative position to the ground as 

 does the part of the beast's body least exposed to injuries. 



