1889.] Physiology. 513 
continuation of the corresponding cell-groups found in the 
spinal region. 
Having homologized the spinal and cranial nerves, Gaskell 
formulates a theory of the origin of the central nervous sys- 
tem of vertebrates, to explain the degeneration in the anterior 
groups of cranial nerves. The central nervous system of the 
vertebrate, considered anatomically and morphologically, sug- 
gests two modes of origin which are apparently antagonistic to 
each other. The segmental arrangement of the nerves and the 
cells, from which they arise, points to the conclusion that the 
nervous tissue of the animal, from which the vertebrate arose, 
was arranged in a distinctly segmental manner. On the other 
hand the evidence of embryology points to the fact that the 
formation is tubular. Any theory must then take both these 
into account. Schwalbe concludes that the evidence points to 
the origin of the spinal cord from a bilateral chain of ganglia 
connected together by means of transverse and longitudinal 
commissures. Gaskell adopts Schwalbe's view, with the addi- 
tion to this system of another system of higher function, /. e., 
the cerebrum, cerebellum, etc., connected with the spinal sys- 
tem through the pyramidal tracts, the direct cerebellar tracts 
and others. This system is not represented in the spinal cord, 
and does not give rise to any outgoing nerves except nerves 
of special sense. Beside the nervous structures of the cord, we 
have the supporting structures ; both of these arise from the 
medullary tube. As to the connection between these two 
structures, Gaskell holds that both phylogenetically and onto- 
genetically the evidence points to the fact that " the central 
nervous system of the higher vertebrate has been formed by 
the spreading and increase of nervous material over the walls 
of an originally non-nervous tube, the cellular elements of 
which tube, whatever may have been its original function, 
have been utilized as supporting structures or have undergone 
gelatinous degeneration. Tailwards this tube emerges free 
from the encasing mass of nervous matter as the neurenteric 
canal and its walls are continuous with those of the alimentary 
canal. Headwards this tube passes into the third ventricle 
and has apparently no anterior opening." The spinal system 
of vertebrates corresponds to the infra-oesophageal ganglia 
and ventral chain of invertebrates, while the crura cerebri, 
peduncles of the cerebellum and other tracts extrinsic to the 
level of the ventral ganglion chain form the oesophageal collar, 
the system of higher function corresponding to the supra- 
