IQ ON THE NERVOUS SYSTEM . IV. 



CEsophageal ring through which the oesophagus passes. The existence of separate 

 lateral halves of the spinal chord in the Vertebrate embryo has been adduced as 

 evidence of identity. Has there, however, ever been seen an embryo of a Ver- 

 tebrate in which there were developed on the lateral halves of the spinal chord 

 itself, distinct and separate ganglionic masses united with the masses above and 

 below them by nervous fibres only "? It is true, as already stated, that in many Ver- 

 tebrates distinct enlargements of the chord correspond with the attachments of the 

 different pairs of spinal nerves ; but still, in these instances, the chord never loses 

 its vertebrate type ; there is always at least, even in Amphioxiis, an uninterrupted 

 mass of cell substance occupying the entire length of its centre, and this substance 

 in all other Vertebrates is inclosed in two half-sheaths of longitudinal nerve tubes. 

 The likeness recognized between the suboesophageal ganglion and medulla oblon- 

 gata is phi/siolo()ical, but not anatomical ; and physiology, all philosophical observers 

 agree, does not teach us homology. The wings of Birds and Insects, physiologi- 

 cally considered, are corresponding parts ; but not so as anatomical structures. On 

 the other hand, the tongue of the Giraffe, of the Woodpecker, the Chameleon, and 

 the Lamprey, anatomically considered, are identical parts, are homologous with the 

 organ ordinarily subservient to taste, but, physiologically studied, are appropriated to 

 widely different uses. Thus the suboesophageal ganglion may be, as regards its func- 

 tions, the analogue of the medulla oblongata, an analogy, however, which the pure 

 anatomist, independently of experiment, would never discover. 



Professor Owen, in speaking of the interspace in Fishes " produced by the divarica- 

 tion of the main lateral columns of the encephalou," through which passes the mem- 

 branous tube (infundibulum) connected with the hypophysis, asks, " Is this vertical 

 slit homologous with the encephalic ring perforated by the oesophagus of the Inver- 

 tebrata ? " * The homology in this case appears to be opposed by the fact, that the 

 slit in question in the Fish opens in front of the optic lobes, which last, if there be 

 any homology between the nervous systems of Vertebrates and Invertebrates, would 

 be the homologues of the supraoesophageal ganglia which give off the optic nerves. 

 The slit should therefore be behind, and not in front of these lobes. 



It is contended that the homology between the nervous centres of Vertebrates 

 and Articulates is contraindicated by the following facts: — 1st. The brain and spi- 

 nal chord are enveloped in a common sheath, the vertebral column and its contained 

 membranes, and are never in the same cavity with the viscera, even in the embryo ; 

 while the ganglionic chain is lodged and developed in the common cavity with the 

 organs of organic life. 2d. In Vertebrates the spinal chord is always on the back, 

 and the ganglionic chain always on the abdominal side in the Articulates. 3d. One 

 consists of a continuous mass of both tubular and vesicular structure, while in the 

 other the vesicular structure is interrupted. 4th. The oesophageal ring, with the 

 oesophagus inclosed, never exists in Vertebrates, but is always present in the 

 Articulates, in common with nearly all other Invertebrates. 5th. The embryonic 

 conditions of the two systems do not at any period clearly assimilate each other. 



* Lectures on Comparative Anatomy, Vol. II., Fishes, p. 181. 



