accessory to the vagus, and the so-called "sensory root" of the vago-glosso-pharyngeal nerve as a terminal rather than initial

nucleus.

On the last point Kölliker 1 remarks: "The so-called sensory nucleus of the vago-glosso-pharyngeus is, as we now know, no place of origin of the fibres of these nerves, but a terminal nucleus (Endkern) in which they end free, while from its cells other fibres arise which bring about connections with other parts of the brain." These connections are still largely undetermined. As regards the apportionment of this nucleus between the glosso-pharyngeal and vagus nerves, Kölliker,2 like Alexander Bruce, whom I have the honour of following in this lectureship, allots the upper part to the former and the lower to the latter. The fasciculus solitarius, or, in transverse section, rotundus, regarded as the sensory root of the vago-glosso-pharyngeal nerve, has been traced downwards for a distance beyond the decussation of the pyramids not yet determined. Roller, quoted by Kölliker, regards this fasciculus and its grey matter as the sensory root of the glosso-pharyngeus alone, but Kölliker, judging from the proportion in size between the fasciculus and that nerve, inclines to the belief that it is likewise the source of the sensory fibres of the vagus. In this view he is supported on embryological grounds by His. The relative position of these centres to one another and to neighbouring structures may be seen in many specimens.

1 Op. cit., S. 259.

2

Op. cit., S. 240.

Illustrations of the Nerve-Tracts in the Mid and Hind Brain," p. 4. 4 Op. cit., S. 248.

LECTURE III.

THE PHYSIOLOGY OF VISCERAL INNERVATION.

IN considering the relation of the nervous system to secretion, visceral motion, and excretion, I desire to touch upon some points in the more recent conceptions of physiologists which are at variance with the views of many of their predecessors, and which are still open to discussion. As regards secretion, for example, it may be permitted to inquire what a nerve of secretion is?

Secretion, like every other form of distal motion, being a peripheral, outgoing, or efferent event in its active phase, its immediately regulative nerves belong necessarily to the outgoing or executive series. They are motor nerves. Secretion, however, like every other peripheral activity, is not solely caused by influences entering or leaving a gland and its associated territory. The secreting cell, like the muscle cell, comes into being before, so far as we know at present, it is in touch with nerve elements. There is doubtless, therefore, an essential secretory irritability, as there is an essential motor irritability. But the length of time for which either secretion or motion could continue in an independent organism, when a given organ is severed from its nerve supply, is admittedly a limited period, and the actual length of this appears to be shorter in an inverse ratio to the complexity of the organism as a whole. Thus the reptile survives longer than the mammal when the main innervation of viscera essential to life is divided.

Every peripheral movement in an ordered mechanism, whether that mechanism be an animal, a plant, or a universe, has a regulating centre-an initial or necessary source of motion in general, or of vital motion, that is life, in particular, by which these properties are conditioned. To solve this mystery would be to solve the mystery of life or even of abstract being, a problem which has defied so far not only the physicist but also his bolder

brother the metaphysician, and appears likely to do so, until the finite contains the infinite, a somewhat remote prospect.

Without aspiring to be defined, as a learned judge is said to have defined the metaphysician, namely, as a blind man looking for a black hat in a dark room, the hat in question not being there, it is permissible to consider with caution, and with a just sense of our limitations, the centre or centres of energy in the animal economy, without reference to the manner in which force is produced or reproduced.

As sensibility, in one phase or another, precedes motion, in one form or another in more complex and fully developed animals, it may be assumed that the initiating centres of energy are in the sensory portion of the nervous system, and that it flows thence down the efferent stream of innervation. This position is opposed to that maintained by Bain,1 but this is not the time to argue the point. The impulses which explode energy in such centres arise in the afferent system of sensory nerves in the case of peripheral actions, and the active factors in such explosions are stimuli applied to such nerves.

We have seen in the anatomical section that the afferent nerves pass, so far as can be ascertained, through the ganglia which intervene between the periphery and the centre, without the interruption of arborisation on intervening cells.

The exploded energy, on the other hand, travelling down the efferent nerves, encounters innumerable arborisations on intercepting ganglion cells, and, like the current of a stream flowing towards its outlet into the ocean, splits or may split into as many fresh channels as arise between the centre and periphery. These fresh channels arise, we have seen, from the innumerable ganglion cells which are formed on the trunk and branches of the efferent system of nerves, cells which transmit and are permeated by the outflowing energy. The question then is, What is the relation of these cells to this energy?

Of experiments, the object of which is to show the relation of the nervous system to peripheral action, whether secretory, viscero-motor, or vasomotor, we may take the well-known one dealing with the nervous mechanism of secretion in the submaxillary gland as a type. I base my description of it on Professor Foster's lucid narrative in the last edition of his text-book of Physiology.2

The submaxillary gland of an animal is exposed, a tube is 1 "Emotions and the Will," p. 328.

2 P. 420 et seq.

placed in the gland duct, and its nerve supply rendered accessible to stimulation. The efferent nerves which go to the gland are (1) a branch of the chorda tympani, in the course of which, as it approaches the gland, ganglion cells appear; and (2) branches of the sympathetic, coursing along the blood vessels. The afferent nerves are the lingual branch of the fifth and branches of the glosso-pharyngeal nerve. Stimulation of the lingual, or of the glosso-pharyngeal, causes a profuse secretion of saliva by reflex action. If the lingual nerve be divided at a point more central than that at which its companion chorda diverges from it, stimulation of the lingual very naturally produces no reflex secretion. If, however, the peripheral end of the divided chorda be stimulated, the efferent impulses thus generated produce a copious secretion for some time, accompanied by vaso-dilation. That vaso-dilation is not the essential cause of the secretion, moreover, is shown by the cessation of the latter, even in presence of the former, if the secretory function be placed in abeyance by the administration of atropine, and also when the dissevered head of an animal still retains the faculty of glandular secretion.

Similarly, stimulation of the sympathetic is associated with the production of a more viscid, scantier, but more potent secretion, accompanied by vaso-constriction. In this case, atropine is said to have less influence in restraining secretion.

My excuse for relating these well-known facts on this occasion is, that they are of interest in connection with a parallel which I intend to draw, and which Professor Foster likewise suggests, between the action of different efferent channels of glandular innervation and those regulating the action of the heart.

The rôle of the peripheral ganglionic cell in this process interests us most at this moment.

When Langley, by the general and local use of nicotine,1 showed that, after paralysis of ganglionic cells, stimulation of post-cellular fibres could still induce peripheral results, he in no way indicated all the functions of the paralysed cell. The question of a peripheral reflex from these cells may, it is true, be regarded as answered in the meantime in a negative sense by physiologists; but the question of a possible storage of peripheral energy in efferent ganglia is not settled by the disproval of a true nerve reflex at the periphery.

These cells, we have seen, are acknowledged to have the power of producing axis cylinders, that is nerve fibres, but are denied other

1 Loc. cit.

properties than a vague nutritive quality, and the function of distributing a nerve supply over a wider area than would be possible to the cerebro-spinal fibres alone, which enter the ganglion.

The eviscerated heart which, when pricked, contracts, does so in the light of the fully resuscitated Hallerian theory of nerveher veless rhythmicality, by the explosion of non-neural energy in the muscle fibre. But this is surely an absolute assumption. The assertion, moreover, that this phenomenon can be induced in muscle, assumed likewise to be devoid of nerve supply, must also, we have seen, be regarded with more than scepticism, because such nerveless territories have in more than one instance been shown by improved methods of staining to be well supplied with nerves. Krehl and Romberg, for example, quoted by Dogiel,1 thought there were no nerves in the apex of the heart of a frog, but Dogiel and others have proved the contrary. Even in the assumed absence of ganglionic cells, which would be a large assumption, it is possible that nerve plexuses, with distributing points, such as were shown when dealing with the anatomy of the subject (Figs. 2 and 3), may retain an irritability capable of farther provocation, by the various artificial expedients used to rouse the energy of the eviscerated heart, or of portions of it. There is naturally no question here of true reflex action, but merely of the possible stimulation of efferent nerves, regulating by an acquired automatism the contraction of the muscles which they supply.

The conclusion, therefore, seems warrantable, until indubitable proof to the contrary has been adduced, that at least in the fully developed organs of more complex animals, persistent rhythmicality has its proximate and always subordinate centres in the efferent stream of innervation. No one, it appears to me, who has seen the perfect consecutive contraction of the auricles and ventricles in a recently killed animal, can avoid the belief that no theory of mere inherent rhythmicality can fully account for this phenomenon. The same lesson appears to be taught by the disorders of rhythmical action observed by the clinician, and with which we shall be concerned in a subsequent lecture.

What I have said with regard to cardiac rhythmical action seems to me to apply equally to viscero-motor and vasomotor action elsewhere. The relation of peripheral ganglia to the efferent nerves in such a rhythmical organ as the spleen, for example, has been shown in the anatomical portion of these lectures to be such, that these nerve-producing centres may, with1 Arch. f. mikr. Anat., Bonn, 1894, Bd. xliii. S. 225.