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centres of the yellow spots, both images are seen separately, and we have double vision. In squinting, the axes of the two eyes do not converge equally towards the object viewed. In consequence of this, when the centre of the image formed by one eye falls on the centre of the yellow spot, the corresponding part of that formed by the other eye does not, and double vision is the result.

For simplicity's sake we have supposed the images to fall on the centre of the yellow spot. But though vision is distinct only in the yellow spot, it is not absolutely limited to it; and it is quite possible for an object to be seen as a single object with two eyes, though its images fall on the two retinas outside the yellow spots. All that is necessary is that the two spots of the retinas on which the images fall should be similarly disposed towards the centres of their respective yellow spots. Any two points of the two retinas thus similarly disposed towards their respective yellow spots (or more exactly to the points in which the optic axes end), are spoken of as corresponding points; and any two images covering two corresponding areas are conceived of as coming from a single object. It is obvious that the inner (or nasal) side of one retina corresponds to the outer (or cheek) side of the other.

19. In single vision with two eyes, the axes of the two eyes, of the movements of which the muscular sense gives an indication, cut one another at a greater angle when the object approaches, at a less angle when it goes further off.

Conversely, if without changing the position of an object, the axes of the two eyes which view it can be made to converge or diverge, the object will seem to approach or go further off.

In the instrument called the pseudoscope, mirrors or prisms are disposed in such a manner that the angle at which rays of light from an object enter the two eyes, can be altered without any change in the object itself; and consequently the axes of these eyes are made to converge or diverge. In the former case the object seems to approach; in the latter, to recede.

20. When a body of moderate size, ascertained by touch to be solid, is viewed with both eyes, the images of it, formed by the two eyes, are necessarily different (one showing more of its right side, the other of its left side).

Nevertheless, they coalesce into a common image, which gives the impression of solidity.

Conversely, if the two images of the right and left aspects of a solid body be made to fall upon the retinas of the two eyes in such a way as to coalesce into a common image, they are judged by the mind to proceed from the single solid body which alone, under ordinary circumstances, is competent to produce them.

The stereoscope is constructed upon this principle. Whatever its form, it is so contrived as to throw the images of two pictures of a solid body, such as would be obtained by the right and left eye of a spectator, on to such parts of the retinas of the person who uses the stereoscope as would receive these images, if they really proceeded from one solid body. The mind immediately judges them to arise from a single external solid body, and sees such a solid body in place of the two pictures.

The operation of the mind upon the sensations presented to it by the two eyes is exactly comparable to that which takes place when, on holding a marble between the finger and thumb, we at once declare it to be a single sphere (§ 4). That which is absolutely presented to the mind by the sense of touch in this case is by no means the sensation of one spheroidal body, but two distinct sensations of two convex surfaces. That these two distinct convexities belong to one sphere, is an act of judgment, or process of unconscious reasoning, based upon many particulars of past and present experience, of which we have, at the moment, no distinct consciousness



1. THE sensory organs are, as we have seen, the channels through which particular physical agents are enabled to excite the sensory nerves with which these organs are connected; and the activity of these nerves is evidenced by that of the central organ of the nervous system, which activity becomes manifest as a state of consciousnessthe sensation.

We have also seen that the muscles are instruments by which a motor nerve, excited by the central organ with which it is connected, is able to produce motion.

The sensory nerves, the motor nerves, and the central organ, constitute the greater part of the nervous system, which, with its function of innervation, we must now study somewhat more closely, and as a whole.

2. The nervous apparatus consists of two sets of nerves and nerve-centres, which are intimately connected together and yet may be conveniently studied apart. These are the cerebro-spinal system and the sympathetic system. The former consists of the cerebro-spinal axis (composed of the brain and spinal cord) and the cerebral and spinal nerves, which are connected with this axis. The latter comprises the chain of sympathetic ganglia, the nerves which they give off, and the nervous cords by which they are connected with one another and with the cerebrospinal nerves.

Nerves are made up entirely of nerve-fibres, the structure of which is somewhat different in the cerebro-spinal and in the sympathetic systems. (See Lesson XII., § 16.)

Nerve-centres, on the other hand, are composed of nervecells or ganglionic corpuscles, mingled with nerve-fibres (Lesson XII., 16). Such cells, or corpuscles, are found in various parts of the brain and spinal cord, in the sympathetic ganglia, and also in the ganglia belonging to spinal nerves as well as in certain sensory organs, such as the retina and the internal ear.

3. The cerebro-spinal axis lies in the cavity of the skull and spinal column, the bony walls of which cavity are lined by a very tough fibrous membrane, serving as the periosteum of the component bones of this region, and called the dura mater. The brain and spinal cord themselves are closely invested by a very vascular fibrous tissue, called pia mater. The numerous blood-vessels supplying these organs run for some distance in the pia mater, and where they pass into the substance of the brain or cord, the fibrous tissue of the pia mater accompanies them to a greater or less depth.

The outer surface of the pia mater, and the inner surface of the dura mater, pass into a delicate fibrous tissue, lined by an epithelium, which is called the arachnoid membrane. Thus one layer of arachnoid coats the brain and spinal cord, and another lines the dura mater. As these layers become continuous with one another at various points, the arachnoid forms a sort of shut sac, like the pericardium; and, in common with other serous membranes, it secretes a fluid, the arachnoid fluid, into its interior. The interspace between the internal and external layers of the arachnoid of the brain is, for the most part, very small; that between the corresponding layers of the arachnoid of the spinal cord is larger.

4. The spinal cord (Fig. 81) is a column of greyishwhite soft substance, extending from the top of the spinal canal, where it is continuous with the brain, to about the second lumbar vertebra, where it tapers off into a filament. A deep fissure, the anterior fissure (Fig. 82, 1), divides it in the middle line in front, nearly down to its centre: and a similar cleft, the posterior fissure (Fig. 82, 2), also extends nearly to its centre in the middle line behind. The pia mater extends into each of these fissures, and supports the vessels which supply the cord with blood. In consequence of the presence of these fissures, only a narrow

bridge of the substance of the cord connects its two halves, and this bridge is traversed throughout its entire length by a minute canal, the central canal of the cord (Fig. 82, 3).

Each half of the cord is divided longitudinally into three equal parts, the anterior, lateral, and posterior columns (Fig. 82, 6, 7, 8), by the lines of attachment of two parallel series of delicate bundles of nervous filaments, the roots of the spinal nerves. The roots of the nerves which arise along that line which is nearer the posterior surface of the




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A. A front view of a portion of the cord. On the right side, the anterior roots, A. R., are entire; on the left side they are cut, to show the posterior roots, P.R.

B. A transverse section of the cord. A, the anterior fissure; P, the posterior fissure; G, the central canal; C, the grey matter; W, the white matter; A.R., the anterior root, P.R., the posterior root, Gn. the ganglion, and 7, the trunk, of a spinal nerve.

cord are called posterior roots; those which arise along the other line are the anterior roots. A certain number of anterior and posterior roots, on the same level on each side of the cord, converge and form anterior and posterior bundles, and then the two bundles, anterior and posterior, coalesce into the trunk of a spinal nerve; but before doing so, the posterior bundle presents an enlargement-the ganglion of the posterior root.

The trunks of the spinal nerves pass out of the spinal canal by apertures between the vertebræ, called the intervertebral foramina, and then divide and subdivide, their ultimate ramifications going for the most part to the muscles and to the skin.

There are thirty-one pairs of these spinal nerves, and, consequently, twice as many sets of roots of spinal nerves given off, in two lateral series, from each half of the cord.

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