FUNCTIONS OF FIBRES AND CELLS. 29 dependent whole. When a draught of cold air strikes the body, apart from the voluntary motion which it may occa sion, its effects may be felt throughout the entire body. The heart and lungs may modify their activity; some of the involuntary muscles may contract; and a shudder may run through the entire physical organism.1 Martin well says that in common life "the very frequency of this uniting activity of the nervous system is such that we are apt to entirely overlook it. We do not wonder how the sight of pleasant food will make the mouth water and the hand reach out for it; it seems, as we say, 'natural,' and to need no explanation. But the eye itself can excite no desire, cause the secretion of no saliva, and the movement of no limb. The whole complex result depends on the fact that the eye is united by the optic nerve with the brain, and that again by other nerves with saliva-forming cells, and with muscular fibres of the arm; and through these a change excited by light falling into the eye is enabled to produce changes in far-removed organs, and excite desire, sensation, and movement." " - Functions of Fibres and Cells. This general survey of the functions of the nervous system enables us to anticipate in an indefinite way the work to be done by the two elements of the nervous system. The fibres, or nerves composed of fibres, will have as their function to transmit stimulations from the surface or outer part of the body to the nerve centres, and to transmit impulses from those centres to the muscles. The cells, or centres composed of cells, will have as their function to receive the stimula1 Cf. Ladd, Physiological Psychology, p. 19. 2 Martin's Physiology, p. 208. tions transmitted by the nerves, and to send impulses along the nerves to the muscles. -The nerves, accord Afferent and Efferent Nerves. ingly, may be divided into two classes: the first class connect some sensitive structure as the skin, the retina, the nervous membrane of the stomach, at their peripheral termination, with the centre; the second connect the centre with the muscles to which they are attached at their peripheral termination. The first class are excited to activity by some structure at their peripheral termination, and transmit nervous action to the centre. They are, therefore, called afferent, in-carrying, or centripetal nerves. The second class are excited to activity by the nerve centres with which they are connected, and transmit nervous excitation to the muscles with which they are connected at their peripheral extremity. They are, therefore, called efferent, out-carrying, centrifugal, or motor nerves. The most important of the afferent nerves for Psychology are those which are called sensory nerves, because they connect the sense organs — eyes, ears and so on — - with the nerve centres. The most important of the motor nerves for Psychology are those which connect the nerve centres with the "voluntary" muscles-those of the hands, arms, legs, eyes, for example. Nature of the Sense Organs. -The greater part of the sense organs consist largely of mechanical contrivances whose function is to modify the external stimulus, and convey the impulse imparted by it to the nerves of sense. NATURE OF THE SENSE ORGANS. 31 For example, the nose consists in large part of a mechanism for bringing the particles of odorous substances in contact with that part of the mucous membrane of the nose in which the olfactory nerve terminates. In order that an object may be smelled, it is not enough that an odorous substance be held near the nose. A current of air containing particles of the odorous substance must be drawn through the nose, and thus brought into contact with the terminal fibres of the olfactory nerve. In like manner the ear consists for the most part of a mechanism whose function is to modify the waves of sound, and transmit them so modified to the internal ear, in which the fibres of the auditory nerve terminate. When the vibrations of air reach the tympanum, they have too large an amplitude, and too little intensity, to occasion these vibrations in the elements of the internal ear, which are essential to the excitation of the auditory nerve. The tympanum modifies these vibrations so as to adapt them to the excitation of the terminal fibres of the auditory nerve, and at the same time transmits them to the internal ear. So likewise, the eye consists in part of an optical instrument, in part of a sensitive nervous membrane called the retina, on which the image resulting from the optical instrument is formed. The eye, as an optical instrument, transmits the stimulations received from light to the nervous elements in the retina in which the optic nerve terminates. The nerve centres with which Psychology is especially interested are those which are found in the encephalon, or contents of the skull, and the spinal cord. - Gray and White Matter. These centres consist of masses of gray and white matter. The white matter consists chiefly of nerve fibres; the gray matter, of nerve cells. These cells, as we have seen, have prolongations or outgrowths called fibres, of which the axis-cylinder is the most essential element. After the axis-cylinder leaves the cell, it divides into two or more parts. Some of these parts enter the white mass, composed chiefly of nerve fibres, and become part of these fibres. Some pass through this white mass and unite with the parts into which the axis-cylinders, extending from other cells, are divided. Gray Matter of the Brain. The gray matter of the brain is not found in a single compact mass. The cerebrum, located in the upper and front part of the brain, has a covering of gray matter, "like a thin rind," called the cerebral cortex, from to of an inch in thickness. Within the cerebrum, and separated from the gray matter of the cortex by a mass of white matter, are found the large ganglia masses of gray matter which are called the optic thalami. Behind these are the corpora quadrigemina, and behind these, and forming a part of the outside surface of the brain, is the cerebellum. These, with the gray masses of the spinal cord, and the medulla oblongata, the body in which the spinal cord terminates, are the gray masses of the nervous system in which Psychology is especially interested. Spinal Cord. The spinal cord and the brain are continuous. There is no point where we can say that the one stops and the other begins. Physiologists have, however, agreed to regard the cord as commencing opposite the outer margin of the foramen magnum of the occipital bone. Its average diameter is about of an inch; its length, 17 inches; and its weight, 1 ounces. It is nearly divided into right and left halves by two fissures, one on the ventral, and the other directly opposite, FIG. 3. The spinal cord and nerve-roots. A, a small portion of the cord seen from the ventral side; B, the same seen laterally; C, a cross-section of the cord; D, the two roots of a spinal nerve; 1, anterior (ventral) fissure; 2, posterior (dorsal) fissure; 3, surface groove along the line of attachment of the anterior nerveroots; 4, line of origin of the posterior roots; 5, anterior root filaments of spinal nerve; 6, posterior root filaments; 6', ganglion of the posterior root; 7, 7', the first two divisions of the nerve-trunk after the union of the two roots. (Martin.) on the dorsal side. If we examine a transverse section of the cord, we shall find that it is composed of white and gray matter, and that its white matter surrounds its gray matter, which is arranged "somewhat in the form of a capital H," the horizontal bar representing the gray matter which connects the gray matter in the right and |