THE CENTRAL GUSTATORY PATHS IN THE BRAINS

OF BONY FISHES.

By C. JUDSON HERRICK.

Studies from the Neurological Laboratory of Denison University. No. XVIII'1.

SECTION II. THE PERIPHERAL GUSTATORY SYSTEM IN FISHES.

SECTION III. THE CENTRAL GUSTATORY SYSTEM OF CYPRINOID FISHES.

[. Primary Gustatory Centers.

2. Secondary Gustatory Tracts.

(1) Descending Secondary Gustatory Tract.

(2) Ascending Secondary Gustatory Tract.

3. Superior Secondary Nucleus and its Connections.

SECTION IV. THE CENTRAL GUSTATORY SYSTEM OF SILUROID FISHES.
SECTION V. SUMMARY AND GENERAL CONCLUSIONS.

Table of the Gustatory Paths in Fishes.

Mammalian Homologies of the Gustatory Centers of Fishes.
General Morphology of the Gustatory System.

The conception of the nervous system as a mechanism for putting the organism into correspondence with the external environment and, in higher animals, for coordinating the reacting apparatus itself (internal environment) may be said to give the key to its evolutionary history. These two factors have given direction to the differentiation of the nervous system into somatic and visceral systems respectively and the further subdivision of each of these.

This study was awarded the Cartwright Prize for 1905 by the Alumni Association of the College of Physicians and Surgeons, Columbia University, New York. It is published simultaneously in the Journal of Comparative Neurology and Psychology and the Bulletin of the Scientific Laboratories of Denison University, pages 375 to 456 of volume XV of the Journal being severally identical with pages 35 to 116 of volume XIII of the Bulletin.

The labors of the students of nerve components have given us for the peripheral nervous system a paradigm or schema which seems to hold for all vertebrates, though with infinite variation in its details; and it now remains to correlate these peripheral components with the central conduction paths so as to give a detailed knowledge of the whole course of each reflex pathway.

In attacking this general problem there are obviously two general lines of procedure open to us :-(1) beginning with the simplest brains we may work out exhaustively for each critical species in the phylogenetic series the conduction paths as completely as possible by monographic treatment of types and thus in the end approximate to a reconstruction of the phyletic history of the nervous system. (2) Or we may take each sensori-motor reflex system as the unit and trace its phylogeny through the series of types. This second method has the obvious advantage that one can start with the system in question in some type where it attains maximum development and, having arrived at a thorough knowledge of its anatomy and physiology here, it will be easier to read this schema backwards to the more primitive animals, as well as forwards in its further evolutionary modifications. It is hardly necessary to call attention to the fact that the human nervous system is the least favorable starting point for this sort of a research except for the neo-pallium and its appendages.

Each method has its advantages. The monographic treatment of type brains is really far more difficult, even in the lowest vertebrates, because of the difficulty in interpreting such simple undifferentiated pictures and analyzing a complex where there are few salient features. But nature has effected the analysis for us in some of the more specialized types by the hypertrophy of isolated systems; and if, as sometimes happens, the other functional systems are in a primitive or reduced condition, we have a favorable point of approach for a monographic study of the exaggerated functional system (cf. JUDSON HERRICK, '03).

The purpose of this study is to make such a detailed analy

sis of a single functional system of neurones-the gustatory system-in types where it reaches its maximum development and is obscured as little as possible by a high development of other systems. These conditions are fulfilled perfectly in the cyprinoid and siluroid fishes, whose brains are uncomplicated by any pathways leading to or from the cortex of the fore-brain and are in the main merely reflex mechanisms, but whose peripheral gustatory pathways are more highly developed than in any other vertebrates.

SECTION II. THE PERIPHERAL GUSTATORY SYSTEM IN FISHES. As is well known, taste buds occur freely scattered over the mucous lining of the mouth and gills of nearly all fishes from the lips to the oesophagus. These are innervated by the VII, IX, and X cranial nerves. Similar taste buds, commonly called terminal buds, occur in the outer skin of some fishes and these also have been recently shown to be gustatory in function (JUDSON HERRICK, '04). They are, in all cases where the innervation is known, supplied by the facial nerve. taste buds in the pharynx and back part of the mouth, then, are supplied from the vagus and glossopharyngeus, those in the front part of the mouth, lips and outer skin from a root of the facialis which apparently corresponds with the portio intermedia of human anatomy.

All

Associated with each of these roots are unspecialized visceral sensory fibers ending by free arborizations in the mucous membrane of the mouth cavity, these being very numerous in the region of the lower vagal roots and diminishing cephalad. The central connections of these two elements have not as yet been clearly differentiated, and both are provisionally designated the "communis system" of nerves by students of nerve components. With the unspecialized fibers we are not here concerned. The specialized communis fibers related with taste buds, either within or outside the mouth, and the ganglion cells from which they are derived will be termed the peripheral gustatory system of neurones.

The peripheral gustatory system has recently been worked up both anatomically and physiologically in a number of fishes, most thoroughly in the siluroids, or cat fishes (JUDSON HERRICK, 'or and '04). From the paper last cited we copy the accompanying figure (Fig. 1), which gives the peripheral distribution of the cutaneous branches of the communis root of the facial nerve to supply the taste buds in the outer skin, in the horned pout, Ameiurus.

Fig. 1. A projection of the cutaneous branches of the communis root of the right facial nerve in Ameiurus melas, Jordan and Cope. Natural size.

The branches of this nerve which supply taste buds within the mouth are not drawn. All of the branches shown are gustatory in function. From the Bulletin of the U. S. Fish Commission for 1902 (JUDSON HERRICK, '04).

The general form of these buds of Ameiurus and their relations to the skin are shown in Figure 2. Practically all parts of the skin in these fishes are provided with taste buds supplied by this root, these same areas receiving also general cutaneous nerves for tactile sensation from other nerve roots, and the barblets being especially richly supplied with both sorts of nerve endings.

It has been determined by an experimental study of a series of both fresh water and marine fishes (JUDSON HERRICK, '04) that the cutaneous communis nerves are gustatory in function and that this function is absent from the skin in species where these nerves and their sense organs are not developed. When edible substances or sapid solutions were brought in contact with these cutaneous sense organs the fishes would react by the