Text Book of Biology, Part 1: Vertebrata

by H. G. Wells

Section 107. Voluntary actions may, by constant repetition, become quasi-reflex in character. The intellectual phase is abbreviated away. Habits are once voluntary and deliberated actions becoming mechanical in this way, and slipping out of the sphere of mind. For instance, many of the detailed movements of writing and walking are performed without any attention to the details. An excessive concentration of the attention upon one thing leads to absent-mindedness, and to its consequent absurdities of inappropriate, because imperfectly acquired, reflexes.

Section 108. This fluctuating scope of mind should be remembered, more especially when we are considering the probable mental states of the lower animals. An habitual or reflex action may have all the outward appearance of deliberate adjustment. We cannot tell in any particular case how far the mental comes in, or whether it comes in at all. Seeing that in our own case consciousness does not enter into our commonest and most necessary actions, into breathing and digestion, for instance, and scarcely at all in the details of such acts as walking and talking we might infer that nature was economical in its use, and that in the case of such an animal as the Rabbit, which follows a very limited routine, and in which scarcely any versatility in emergencies is evident, it must be relatively inconsiderable. Perhaps after all, pain is not scattered so needlessly and lavishly throughout the world as the enemies of the vivisectionist would have us believe.

7. _The Nervous System_

Section 109. A little more attention must now be given to the detailed anatomy of the peripheral and central nerve ends. A nerve, as we have pointed out, terminates centrally in some ganglion cell, either in a ganglion or in the spinal cord or brain; peripherally there is a much greater variety of ending. We may have tactile (touch) ends of various kinds, and the similar olfactory and gustatory end organs; or the nerve may conduct efferent impressions, and terminate in a gland which it excites to secretion, in a muscle end-plate, or in fact, anywhere, where kataboly can be set going and energy disengaged. We may now briefly advert to the receptive nerve ends.

Section 110. Many sensory nerves, doubtless, terminate in fine ends among the tissues. There are also special touch corpuscles, ovoid bodies, around which a nerve twines, or within which it terminates.

Section 111. The eye ([Figure 8]) has a tough, dense, outer coat, the sclerotic (sc.), within which is a highly vascular and internally pigmented layer, the choroid, upon which the percipient nervous layer, the retina (r.) rests. The chief chamber of the eye is filled with a transparent jelly, the vitreous humour (v.h.). In front of the eye, the white sclerotic passes into the transparent cornea (c.). The epidermis is continued over the outer face of this as a thin, transparent epithelium. The choroid coat is continued in front by a ring-shaped muscle, the iris (ir.) the coloured portion of the eyes. This iris enlarges or contracts its central aperture (the black pupil) by reflex action, as the amount of light diminishes or increases. Immediately behind this curtain is the crystalline lens (l.), the curvature of the anterior face or which is controlled by the ciliary muscle (c.m.). In front of the lens is the aqueous humour (a.h.). The description of the action of this apparatus involves the explanation of several of the elementary principles of optics, and will be found by the student in any text-book of that subject. Here it would have no very instructive bearing, either on general physiological considerations or upon anatomical fact.

Section 112. The structure of the retina demands fuller notice. [Figure 9] shows an enlarged, diagram of a small portion of this, the percipient part of the eye. The optic nerve (o.n. in [Figure 8]) enters the eye at a spot called the blind spot (B.S.), and the nerve fibres spread thence over the inner retinal surface. From this layer of nerve fibres (o.n. in [Figure 9]) threads run outward, through certain clear and granular layers, to an outermost stratum of little rods (r.) and fusiform bodies called cones (c.), lying side by side. The whole of the retina consists of quite transparent matter, and it is this outermost layer of rods and cones (r. and c.) that receives and records the visual impression. This turning of the recipient ends away from the light is hardly what one would at first expect-- it seems such a roundabout arrangement-- but it obtains in all vertebrata, and it is a striking point of comparison with the ordinary invertebrate eye.

Section 113. We may pause to call the student's attention to a little point in the physiology of nerves, very happily illustrated here. The function of a nerve fibre is the conduction of impressions pure and simple; the light radiates through the fibrous layer of the retina without producing the slightest impression, and at the blind spot, where the rods and cones are absent, and the nerve fibres are gathered together, no visual impressions are recorded. If there is any doubt as to the existence of a blind spot in the retinal picture, the proof is easy. Let the reader shut his left eye, and regard these two asterisks, fixing his gaze intently upon the left-hand one of them.