CEREBRAL HOMOLOGIES. 



35 



Tlie part of the vertebrate brain to which, therefore, the 

 so-called " suboesophageal ganglion " in Invertebrates is 

 analogous and, I conceive also, homologous, is the basis 

 of the epencephalon known as the " medulla oblongata " 

 (macromyelon), or so much of that myelencephalous tract as 

 may be in connexion with the trigeminal and hypoglossal 

 nerves — the neural machinery, to wit, for the sensations and 

 motions of the parts forming or being lodged within, or fur- 

 nishing secretions to, the vertebrate mouth. 



Through the different course of the gullet, in relation to 

 certain nerve-centres in Vertebrates and Invertebrates a 

 greater degree of juxtaposition and concentration of those 

 centres connected with the special senses, and the neural 

 mechanism relating to the reception of their impressions, is 

 possible in the group in which the " brain," or sum of such 

 centres, is not traversed by the alimentary canal. 



We are thus prepared for the conception that, as the oral 

 nerve-centres in Invertebrates are so far removed from the 

 narial nerve-centres, so the ear-organs and their centres may 

 be correspondingly remote from the oral ones. 



Johannes Miiller recognized a structure in the fore leg of 

 the Gryllus hieroglyphus, which von Siebold detected in other 

 Orthoptera ; and this structure was by both regarded as the 

 true seat of the auditory sense. The vesicle, in connexion 

 with a quasi tympanic membrane closing an orifice in the fore 

 leg, receives two unusually large nerves from the foremost 

 ''thoracic ganglion" (fig. 11, o) ; these nerves accompany the 

 tracheal branch of the vesicle ; the lesser nerve attaches itself 

 to the vesicular dilatation, and there expands into a flattened 

 tract, displaying a structure akin to that of the acoustic nerve 

 lining the semicircular canals in Vertebrates. This inter- 



* Ante, pp. 8 and 9, tigs. 2 and 3. 



D 2 



