J. MARK BALDWIN, Johns Hopkins University S. I. FRANZ, McLean Hospital, Waverley, Mass. S. J. HOLMES, University of Michigan B. F. KINGSBURY, Cornell University ADOLF MEYER, N. Y. State Pathological Inst. C. LLOYD MORGAN, University College, Bristol A. D. MORRILL, Hamilton College Published bi-monthly DENISON UNIVERSITY, GRANVILLE, OHIO Comparative Neurology and Psychology Volume XV 1905 Number 3 THE MORPHOLOGY OF THE VERTEBRATE HEAD FROM THE VIEWPOINT OF THE FUNCTIONAL DIVISIONS OF THE NERVOUS SYSTEM.1 By J. B. JOHNSTON. With Plates I to IV. TABLE OF CONTENTS. 5. I. 2. 3. Introduction. a. Nature of the unsettled problems b. Nerve components C. d. Functional divisions of the nervous system Bearing upon the subject of head morphology Number and relations of mesodermic somites Branchial apparatus and lateral musculature 4. Segmentation of the central nervous system Typical sense organs of vertebrates 176 176 177 180 182 184 188 191 194 The absence of nerve roots from one or more hind brain segments and The occipital region and the posterior limit of the head. The constitu 1 Studies from the Zoological Laboratory of West Virginia University, No. 9. 252 253 19. Relation of dorsal and ventral nerve roots to the myotomes 257 258 260 261 265 273 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 |