in some measure at least, reflexly by impulses arriving from the ear at a special coördinating nervous centre, the education of which is in advance of that of the voluntary centres, and, therefore, can only be controlled by the latter in persons specially educated in singing. Some persons who can sing a given note with promptness and exactitude, without any effort, would find much difficulty in overcoming, by volition, the accuracy of this perfect reflex mechanism. In fact, a person with a naturally "good ear" finds it difficult to sing out of tune, even if he try.

Though we feel that we have command over the pitch of the sounds produced in the larynx, we owe much of our accuracy to the aid given by our sound-appreciating organs and the nerve centres in connection with them.

SPEECH.

The variations in vocal sounds which give rise to speech are not produced in the larynx, but in the throat, mouth and nose. When unaccompanied by any vocal sound, speech only gives rise to a whisper; but when a vocal tone is at the same time produced, we have the ordinary loud speaking. Since vocal tones can only be produced by expiration, so we can only speak aloud by means of an expiratory current of air; but an inspiratory current may be made to give rise to a kind of whisper.

Speech is composed of two kinds of sounds, in one of which the sounds must be accompanied by a vocal tone, and are, hence, called "vowels; in the other no vocal tone is necessary, but changes in shape take place in the resonating chambers, so as to give rise to noises called consonants. As the pronunciation of the consonants is always accompanied by some vowel sound, and as the difference between the vowels is brought about by changes in the shape of the mouth, the distinction between the two sets of sounds is rather artificial than real.

The production of the different vowel sounds depends upon such a change being brought about in the shape of the mouth cavity and aperture, that a resonator, with a different individual note, is formed for each particular word.

The sounds called consonants are caused by some check or

impediment being placed in the course of the blast of air issuing from the air passages. They may be classified, according to the part at which the obstruction occurs, as follows:

1. Labials, when the narrowing takes place at the lips, as in pronouncing b, p, ƒ, v.

2. Dentals, when the tongue causes the obstruction by being pushed against the hard palate or the teeth, as in t, d, s, l.

3. Gutturals, when the posterior part of the tongue moves toward the soft palate or pharynx, as in saying k, g, gh, ch, r.

Consonants may also be divided into different groups, according to the kind of movements which give rise to them.

1. Explosives are produced by the sudden removal of the obstruction, as with p, d, k.

2. Aspirates are continuous sounds caused by the passage of a current of air through a narrow opening, which may be at the lips, as in ƒ, at the teeth as with s, or at the throat as in ch.

3. Resonants are the sounds requiring some resonance of the vocal cords, and the air current is suddenly checked by closure of the lips, as in m, or the dental aperture as in n or ng.

4. Vibratory, of which is the example, requires a peculiar vibration of the vocal cords, while either the dental or the guttural aperture is partially closed.

CHAPTER XXVIII.

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM.

ANATOMICAL SKETCH.

The nervous system includes the various mechanisms by which the distant parts of the body are kept in functional relationship with one another. By it the condition of the surroundings and the various parts of the body are communicated to a central department (cerebro-spinal axis) which in turn regulates and controls the activities of the various organs.

It is made up of two varieties of tissue, both of which possess special vital properties. The one, nerve fibres, composed of thread-like strands of protoplasm, connects the elements of the other, nerve corpuscles, which form the peripheral or central terminals of the fibres. Nerve fibres are simply special conducting agents, having at one extremity a special terminal, or nerve cell, for sending impulses, and at the other end a nerve cell for receiving the impulses. These terminal organs, between which the nerve fibres pass, are the agents which determine the direction in which the impulse is to travel along the nerve. The sending organ may be at the peripheral end of the nerve, and the receiver in the nerve centres, as in the case of an ordinary cutaneous nerve, which carries impulses from the skin to the brain; or the sending organ may be at the centre, and the receiving organ at the periphery, as in the case of the nerves conveying impulses from the brain to the muscles.

The former kind of nerves are called afferent, carrying centripetal impulses, and the latter efferent, carrying centrifugal impulses. Nerves are capable of carrying impulses in either direction, as has been proved by cutting the afferent lingual and the efferent hypoglossal nerves, and causing the proximal end of the former to unite with the distal end of the latter, which is distributed to the muscles of the tongue. When the union has taken place, a stimulus applied to the upper part which was nor

mally afferent, or sensory, carries motor impulses to the muscles, i. e., acts as an efferent nerve.

Protoplasm, though not formed into fibres, can conduct impulses, as is seen in the transmission of an impulse in textures and animals which seem to have no special conducting elements or nerve fibres. Thus, in the hydra all the cells act as nerves, and in the higher animals an impulse, producing a wave of contraction, can pass from one muscle cell to the other directly, as is seen in the ureter, or in the heart of cold-blooded animals.

The only essential part of a nervous conductor is a delicate protoplasmic fibril. Single, thin, thread-like fibrils are found carrying impulses in the nerve centres. In the nerves distributed about the body, one does not meet these single protoplasmic threads (except where the fibrils are interwoven to form terminal networks, as seen in the cornea), but the fibrils are clustered together in large bundles, so as to make one nerve fibre. In the peripheral nerves this bundle of protoplasmic fibrils is covered,

and is called the axis cylinder of the nerve fibre. In some nerve fibres there is but one very thin transparent covering, termed the primitive sheath, while in others there is a thick layer of doublyrefracting fluid inside the primitive sheath, in immediate contact with the fibrils of the axis cylinder. This is called the medullary sheath, or white substance of Schwann, because its peculiar refractive properties make it look white when viewed in a direct light. As the nerves have or have not this medullary sheath, they have been termed "white" or "gray." The former are by far the most plentiful, since they make up the greater part of the ordinary nerves, while the gray fibres only predominate in the sympathetic nerve and its ramifications, and parts of the special sense organs.

An ordinary nerve, then, is made up of a large number of fibres, held together by connective tissue, each fibre containing a vast number of fibrils within its sheath.

FUNCTIONAL CLASSIFICATION.

Nerve fibres may be classified, according to their function, in the following way :

I. AFFERENT NERVES, which bear impulses from the surface to the nervous centres. These may be further divided into :

(a) Sensory nerves, when the impulse they convey gives rise to a "perception." The perceptions may be the special sensations which are transmitted from the organs of special sense, or those of general sensation, giving rise to pleasure or pain.

(b) Excito-reflex nerves communicate impulses to central nerve elements, and give rise to some action, without exciting mental perception. Such nerves regulate the viscera. According to the result of the excitation arising from their impulse, they are termed excito-motor, excito-secretory, and excito-inhibitory, etc.

(c) Mixed nerves act as sensory and reflex nerves; these are the most numerous, the sensory or reflex action depending upon the condition of the nerve centres.

II. EFFERENT NERVES, which carry impulses from the centres to the various organs throughout the body. According to the effect produced by their excitation, they are termed :