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dary sensory tract from the retina, the tractus opticus, not only takes 

 a siniihxr course and enters a common center with the secondary cu- 

 taneous tracts, but that common center is still in part a primary 

 cutaneous center. When these facts are added to the argument pre- 

 viously given, the writer believes that the hypothesis that the retina 

 is a modified part of the general cutaneous system is raised to the 

 highest degree of probability. 



Finally, the establishment of the character of the tectum as a 

 general cutaneous center adds one more piece of direct evidence in 

 support of the division of the brain into longitudinal zones on the 

 basis of function (10, 11, 12). It is no longer a matter of theory 

 that the somatic sensory column primitively extended forward beyond 

 the medulla oblongata as the dorsal column of the brain; it is posi- 

 tively known in existing vertebrates as far forward as the posterior 

 commissure. This makes it less surprising to find a general cutaneous 

 nerve at the extreme anterior end of the brain in selachians (12, 18). 



Other relations in the region of the velum. 



The writer will add a summary of further observations in the 

 region of the cerebellum and tectum in Scyllium. 



Tractus tecto-cerebellaris. A small tract of medullated fibers 

 passes from the chief fiber zone of the tectum into the cerebellum. 

 The medullated fibers seem to be accompanied by a larger number of 

 non-medullated ones. The medullation of the tract may not have been 

 complete in the specimen, which was not large. The tract runs dorsal 

 to the decussatio veli. It has no relation to the trigeminus bundle, 

 which runs beneath the fiber zone in the tectum, but corresponds to 

 the tractus tecto-cerebellaris II of Acipenser. 



The homology of the ganglion isthmi. This nucleus which 

 Edinger has independently described under the name of "ganglion 

 isthmi" is the same as that described by Mayser (13) under the name 

 of the "Rindeuknoten". Mayser saw that this was the end-nucleus 

 of his "secondary vago-trigeminal" tract. Goronowitsch (14) after- 

 ward saw the same tract and nucleus in Acipenser, and the writer 

 (8, 9) gave a detailed description of its structure from Golgi pre- 

 parations. It was shown, as Strong (19) had shown for the tadpole, 

 that the column of gray matter from which the secondary tract arose, 

 although variously modified and hypertrophied in different fishes, was 

 the common center for the visceral sensory fibers of the VII., IX. and 

 X. nerves. It has come to be known by the name of the common 

 bundle of root fibers running through it as the fasciculus communis. 



