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Observations on the Spinal Cord of the Emu and its Segmen
tation. By IRVING HARDESTY. (From the Hearst Anat-
By S. J. HOLMES. (From the Zoological Laboratory of
the University of Michigan.)
in the Common Toad. By WALTER C. JONES, M.D.
Concerning the Genetic Relations of Types of Action.
havior of Infusoria.
A Review of Some Recent Literature on the Chemistry of the
Central Nervous System. By ISADOR H. CORIAT.
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Entered a second-claw mattar in bo postalo at Gnaville, 0.
HERBERT S. JENNINGS,
University of Pennsylvania
J. MARK BALDWIN, Johns Hopkins University
B. F. KINGSBURY, Cornell University
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Comparative Neurology and Psychology
THE MORPHOLOGY OF THE VERTEBRATE HEAD
FROM THE VIEWPOINT OF THE FUNCTIONAL
By J. B. JOHNSTON.
With Plates I to IV.
TABLE OF CONTENTS.
176 176 177 180 182 184 188 191 194 197 204 207
Nature of the unsettled problems
Functional divisions of the nervous system
Number and relations of mesodermic somites
the determination of the neuromeres concerned
The acustico-lateral system
tion of the vagus. Shifting of organs
relations to the gustatory system 16. The first four head segments
Segment 4 b. Segment 3
d. Segment i Prostomium 17. The dorsal commissures of the brain
Studies from the Zoological Laboratory of West Virginia University, No.
18. The sympathetic system 19. Relation
dorsal and ventral nerve roots to the myotomes 20. Comparison of head and trunk Summary List of papers cited Description of figures
a. Nature of the unsettled problems. Shall we consider vertebrates as animals possessing a high degree of cephalization from their first appearace? The structure of their near relatives, Amphioxus and Ascidians, is against this view. The structural relations of vertebrates and invertebrates indicate that the ancestors of the vertebrates were segmented invertebrates in which the process of cephalization had not gone very far. Even within typical vertebrates evidence is not lacking that the special sense organs of the head were late to appear; that the branchial apparatus was at one time more extensive, reaching into what is now the trunk; that the nerves of the branchial region once had a more simple segmental arrangement; and that in the brain itself the several regions were once less highly specialized than at present. If Amphioxus be considered, the presence of true nephridia (41) in the head and the slight specialization in the head region seem to relate this "lowest vertebrate with invertebrates rather far down the scale.
If, then, the ancestral vertebrate had only a slight head development, it is evident that the interpretation of the special organs of the head of typical vertebrates is to be reached by a study of their structure, function, and phylogenetic history, with a view to tracing them back to their unspecialized beginnings. When each organ has thus been followed back to its ancestral condition we shall have reduced the vertebrate head to terms-not of the trunk, but of a more simple condition which underlies both head and trunk. Such is the real problem of head morphology as the writer understands it.
The central difficulty in framing such a conception of the head is the matter of segmentation. Head specialization has