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. 4. I. 2. Introduction. a. Nature of the unsettled problems b. Nerve components C. Functional divisions of the nervous system d. Bearing upon the subject of head morphology 3. Branchial apparatus and lateral musculature Segmentation of the central nervous system 176 176 177 180 182 184 188 191 194 197 204 207 9. 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. 18. The sympathetic system 19. 20. Relation of dorsal and ventral nerve roots to the myotomes Comparison of head and trunk Summary List of papers cited Description of figures 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 gone so far that in known vertebrates few or no segments contain, even in the form of embryonic vestiges, all of the structures which were typically present in the anterior segments of the ancestral vertebrates. On this account it has been extremely difficult to reconstruct complete segments, even where our information as to existing structures is apparently complete. Not only have certain organs with their nerves and nerve centers undergone reduction, or disappeared, but new organs have arisen and old ones have been modified so that new nerves and centers have replaced or overshadowed old ones. Furthermore it appears that as a result of these changes various organs and nerves have been displaced from their proper segments by the crowding of lately formed highly developed structures. We are met by a present condition which is the result of processes of reduction and disappearance, modification and growth, and shifting of position of organs whose real segmental relations are to be discovered only by tracing them back to their primitive condition. Thus the deciphering of the segmental relations, depending, as it does, on the proper interpretation of the homology and phylogenetic history of all the organs, will carry with it the solution of the major problems of head morphology. It must not be hoped that the last word upon any of these problems can be said at once. The data are yet too meager for the solution of this greatest problem of the evolution of animal structure, in spite of the efforts of a large number of workers directed to it during several decades. What the writer hopes to do in the present paper is: first to apply a new method for the interpretation of head segmentation which recent work on the nervous system has made possible; and second to point the way to some profitable lines of investigation. b. Nerve components. The function of the nervous system to coördinate and direct all the organs of the body requires a definite and constant relation of its constituent parts to the several tissues and organs. The structural and functional relationships within the organism become impressed upon the nervous system and the arrangement of its cells and fibers serves as a guide to these relationships. Further, by a study of its residual or vestigeal structures and its ontogeny in various classes of animals we gain hints of many of the past relationships between the other organs. On account of its function as a go-between, the nervous system becomes for us an interpreter. The truth of this has long been realized but it has become of practical value only since the work of OSBORN, ALLIS and EWART led the way to the development of the theory of nerve components by STRONG, HERRICK and others. According to this theory, those fibers in the cranial nerves which supply the same kind of end organs enter the same or comparable regions in the brain. For example, all general cutaneous fibers (free nerve endings in the skin) enter the spinal V tract in the brain and the dorsal tracts in the cord. It happens that a nerve trunk is commonly composed of two or more sets or kinds of fibers. All those fibers in the several nerves which have the same central and peripheral connections are said to belong to one and the same system of nerve components. Although this theory concerns itself directly with the analysis of peripheral nerve trunks, its value rests upon the existence of functionally distinct types of end organs on the one hand and distinct nerve centers in the brain on the other hand. The analysis of nerve trunks into nerve components necessarily implies the analysis of the nervous system as a whole from the same point of view, that is, on the basis of function. In this new way of looking at the nervous system the brain, which from the standpoint of structure has always been regarded as the most complex and obscure portion, becomes the most illuminating-and this just because of its complex relations. Three considerations are important in this connection. First, the reasoning on which the theory of nerve components rested could not be made good unless it was shown that the fibers of two components were independent in their central relations. For example, unless it were clearly shown that the centers in which the general cutaneous fibers end are structurally and hence functionally distinct from those into which sensory fibers |