lated, the heart beats more slowly, and in some animals may come to a standstill in a condition of relaxation.

(B) The AFFERENT FIBRES, still more widely spread, are important for the functions of the various viscera. They are:

1. Sensory fibres carrying impulses from the pharynx, œsophagus, stomach and intestine, and from the larynx, trachea, bronchi and lungs generally. The pneumonia which follows section of the vagi depends on-(1) the removal of sensibility, and the ease with which foreign matters can enter the air passages; or (2) the violent breathing necessary when the motor nerves of the larynx are cut; or (3) the injury of trophic or vasomotor fibres.

2. Excito-motor nerves.--'. --There is no nerve that can be compared with the vagus in the variety of reflex phenomena in which it participates. Afferent fibres in this nerve cause spasm of the muscles of the glottis and thorax, and govern the respiratory rhythm, preside over inhalation of air and excite the expiratory muscles. Thus, irritation of the mucous membrane at the root of the tongue, the folds of the epiglottis, larynx, trachea or bronchi, causes spasmodic fits of coughing. Irritation of the pharyngeal or the gastric fibres gives rise, by reflex stimulation, to the act of vomiting.

Stimulation of the proximal cut end of the trunk of the vagus causes inspiratory effort and cessation of breathing movements in the position of inspiration. Stimulation of the central cut end of the superior laryngeal branch causes reflex spasm of the muscles of the larynx and a fixation of the expiratory muscles in the position of expiration. The fibres which regulate the respiratory rhythm consist of two sets, probably passing from the lungs to the inspiratory and expiratory centres, and causing each to act before its ordinary automatism would transmit any discharge of impulse to the thoracic muscles.

In the laryngeal branches are fibres which bear centrifugal impulses to the vasomotor centres in the medulla, and excite them to action. These, which may be grouped with the excitomotor channels, are spoken of as "pressor" fibres, from the influence they exert upon the pressure of the blood in the arteries.

3. Excito-inhibitory fibres pass from the heart to the vasomotor centre. Stimulation of these fibres, which take somewhat different courses in different animals, checks the tonic action of the vasomotor centre, and greatly reduces the blood pressure. Hence these fibres form the depressor nerve. Its terminals in the heart are stimulated by distention of that organ; and the vasomotor centre is thereby inhibited, the arteries dilate and the blood pressure falls so that the over-filled heart can empty itself.

4. Excito-secretory fibres.-Stimulation of the gastric endings of the vagus causes not only gastric, but also salivary secretion, which occurs as a precursor of gastric vomiting.

Section of both vagi in the neck causes the death of the animal within a day or two after the operation, and the following changes may be observed while it lives: 1. The heart beat is much quicker, as shown by the increased pulse frequency. 2. The rate of breathing is very much slower. 3. Deglutition is difficult, the food easily passing into the air passages through the insensitive larynx.

Section of the superior laryngeal nerves is followed by slight slowness of breathing, loss of sensibility in the larynx, entrance of food into the air passages, chronic broncho-pneumonia and death.

Section of the inferior laryngeal nerves gives rise to the same final result, because the muscles of the larynx are paralyzed, and closure of the glottis is impossible. A change in voice follows the section or injury of even one inferior laryngeal, as may often be seen in man from the effect of the pressure of an aneurism.

IX.-HYPOGLOSSAL NERVE.

This nerve appears in the furrow between the olivary body and the anterior pyramid, on a line with the anterior roots of the spinal nerves. It corresponds with the anterior roots in function, being a purely motor nerve. It bears impulses to the muscles of the tongue, and others attached to the hyoid bone.

Some sensory fibres lie in its descending branch, but these are probably derived from the vagus or trifacial nerves, with which its branches inosculate.

It is also said to contain the vasomotor fibres of the tongue. Section of the nerves causes paralysis of the muscles of the tongue; when this is unilateral, the tongue inclines to the injured side, while being protruded from the mouth; but, while being drawn in, it passes to the sound side. This is easily understood when it is borne in mind that the two acts depend upon the intrinsic muscles of the tongue, bringing about an elongation or shortening of the organ respectively.

CHAPTER XXX.

SPECIAL SENSES.

It has been pointed out that the afferent or sensory nerves receive impressions at the surface of the body, and carry the impulses to nerve cells in the brain, where they give rise to sensation. The afferent nerves are the means by which the mind becomes acquainted with occurrences in the outer world, and also the channels along which the impulses pass to reflex nerve centres whence they are sent to different parts, without causing any sensation in the nerve cells of the sensorium.

The ordinary sensory nerves are in such relationship to the surface that they are affected by slight mechanical and thermal stimuli, which throw them into activity and send impulses to the brain. But we are capable of appreciating many other impressions besides those excited by the ordinary sensory nerves. We feel the character of a surface by touch, and we distinguish between degrees of heat and cold, when the difference is far too slight to act as a direct nerve stimulus. We can appreciate light, of which no degree of intensity is capable of exciting a nerve fibre to its active state, or of stimulating an ordinary nerve cell in the least degree. We recognize the delicate air vibrations called sound, which would have no effect on an ordinary nerve ending. We can also distinguish several tastes; and, finally, we are conscious of the presence of incomprehensibly small quantities of subtle odors floating in the air. When the amount of the substance is too small to be recognized even by spectrum analysis, which detects extraordinarily minute quantities, we can perceive an odor by our olfactory organs.

There must, then, be a special apparatus for the reception of each of these impressions, in order that the nervous system may be accessible to such slender influences. In fact, special mechanisms must exist by means of which the quality of a surface, heat, light, sound, taste and odor are enabled to act as nerve

stimuli. These nerve terminals are known as the special sense organs, the physiology of which is at the same time the most difficult and most interesting branch of study in Biological Science.

The nerve fibres which carry the impulses from the various organs of special sense do not differ from other nervous cords, so far as their structure and capabilities are concerned. The special peripheral end organs are connected with nerve cells in the brain, the sole duty of which is to receive impulses from a special sense organ and distribute them to the brain centres, so that they may cause a special sensation. By whatever means a nerve trunk from a special sense organ be stimulated, its impulse excites the special sensation usually arising from stimulation of the special organ to which it belongs. Thus, electric stimulation of nerves in the tongue causes a certain taste; mechanical or other stimulation of the optic nerve trunk gives rise to the sensation of flashes of light, and a distinct odor may be caused by the presence of a bony growth, pressing upon the olfactory nerve.

The capability of the nerve centres connected with the nerves of special sense to give rise to a special sensation, is called their specific energy. And the special influence, light, sound, etc., which alone suffices to excite the special peripheral terminal, and which the given terminal alone can convert into a nerve stimulus, may be called its specific or adequate stimulus.

Although we habitually think of the sensation as if coming from the surface where the stimulus is applied, it is really only developed in the centres in the brain. Thus we say we feel with our skin, hear with our ears, and see with our eyes, etc., whereas these are only the parts from which the nerve impulses, giving rise to the specific energy, pass to the feeling, hearing or seeing regions of our cerebral cortex. This is obvious from what has been already said of the nerve fibres of the special sense organs. If the nerve be cut, no sensation is excited, though adequate stimulus reach the organ of special sense; and if a stimulus be applied to the nerve trunk, a similar sensation is produced, as if the specific stimulation had operated on the special nerve terminal from which these fibres habitually carried impulses. This periph