§ 2. Function of the Sub-Cortical Nervous Mechanism.— The portion of the nervous system which lies below the cortex is partly contained within the cranium. This portion consists of nuclei of grey matter imbedded in white matter. The nuclei of grey matter constitute what are called the subcortical centres. The white matter consists of nervefibres, serving to conduct impulses between the cortex and the subcortical centres, and between the subcortical centres themselves. Running through the trunk of the body, behind the viscera, there is another important portion of the nervous system—the spinal cord. At its upper end it enters the cranium, and this portion of it receives a separate name, and is called the medulla oblongata, or simply, the bulb. Nervefibres connect the muscles and the surface of the body with the spinal cord, and strands of nervefibre pass upwards along the cord itself to the subcortical centres.

* Dr. Waller, Human Physiology, p. 518.

+ The student should make a point of reading the chapters on the nervous system in some good Physiology. Lessons I. and XL in Huxley's Elementary Physiology (Macmillan & Co., price 4s. 6d.), and Ch. IX. in Davis's Elementary Physiology (Blackie's Science TextBooks, price 2s.),

These subcortical portions of the nervous system serve to convey and modify impulses passing between peripheral organs and the cortex; but they also discharge functions which are independent of their connexion with the cortex. They are organs of what is called reflex action. Reflex actions are such as take place in a fixed and uniform manner in response to an appropriate external stimulus. Without the actual presence of the external stimulus they do not occur, and whenever the external stimulus operates they occur inevitably and invariably, unless they are interfered with by the simultaneous operations of another external stimulus, or by processes going on in the cortex. Their typical characteristics are best seen when interference on the part of the cerebral cortex is excluded, which may be effected by simply removing the cerebral hemispheres from the brain of an animal. "We may perhaps broadly describe the behaviour of a frog, from which the cerebral hemispheres only have been removed, by saying that such an animal, though exhibiting no spontaneous movements, can, by the application of appropriate stimuli, be induced to perform all or nearly all the movements which an entire frog is capable of executing. It can be made to swim, to leap, and to crawl. Left to itself, it assumes what may be called the natural posture of a frog, with the fore limbs erect, and the hind limbs flexed, so that the line of the body makes an angle with the surface on which it is resting. When placed on its back, it immediately regains this natural posture. When placed on a board, it does not fall from the board when the latter is tilted up so as to displace the animal's centre of gravity ; it crawls up the board until it gains a new position in which its centre of gravity is restored to its proper place. Its movements are exactly those of an entire frog, except that they need an external stimulus to call them forth. They differ, moreover, fundamentally from those of an entire frog in the following important feature: they inevitably follow when the stimulus is applied; they come to an end when the stimulus ceases to act. By continually varying the inclination of a board on which it is placed, the frog may be made to continue crawling almost indefinitely; but directly the board is made to assume such a position that the body of the frog is in equilibrium, the crawling ceases; and if the position be not disturbed the animal will remain impassive and quiet for an almost indefinite time. When thrown into water, the creature begins at once to swim about in the most regular manner, and will continue to swim until it is exhausted, if there be nothing present on which it can come to rest. If a small piece of wood be placed on the water the frog will, when it comes in contact with the wood, crawl upon it, and so come to rest. If disturbed from its natural posture, as by being placed on its back, it immediately struggles to regain that posture; only by the application of continued force can it be kept lying on its back. Such a frog, if its flanks be gently stroked, will croak; and the croaks follow so regularly and surely upon the strokes that the animal may almost be played upon like a musical, or at least an acoustic instrument. Moreover, provided that the optic nerves and their arrangements have not been injured by the operation, the movements of the animal appear to be influenced by light; if it be urged to move in any particular direction, it seems in its progress to avoid obstacles, at least such as cast a strong shadow; it turns its course to the right or left or sometimes leaps over the obstacle. In fact, even to a careful observer, the differences between such a frog and an entire frog, which was simply very stupid or very inert, would appear slight and unimportant except in this, that the animal without its cerebral hemispheres is obedient to every stimulus, and that each stimulus evokes an appropriate movement, whereas with the entire animal, it is impossible to predict whether any result at all, and if so what result, will follow the application of this or that stimulus."* The characteristic of reflex action which Professor Foster here emphasises is its lack of spontaneity—its thoroughgoing dependence on the actual present operation of a stimulus external to the nervous system. Experiments of the kind he describes have been performed on birds and rabbits as well as on frogs. The results are, broadly, similar, except that in the case of birds there is some appearance of spontaneity when the animal has had sufficient time to recover from the shock of the operation. But this spontaneity is too small in degree and too ambiguous in its nature, to invalidate the general conclusion that the function of the subcortical centres when working by themselves is almost wholly reflex. Closely connected with lack of spontaneity there is another and equally important characteristic of reflex action. It takes place invariably in the same way without being modified in accordance with the results of past actions. Whether it is accompanied by any sort of experience or not, we may at least affirm that it is characterised by the absence of the process of learning by experience.

It will put him in possession of the most essential facts. If he wishes to go more deeply into the matter, Dr. Waller's Human Physiology (Longmans, Green, & Co.), may be safely recommended. For the advanced student, Parts III. and IV. of Dr. M. Foster's TextBook of Physiology (Macmillan & Co.) are necessary. What I have said in the text is the roughest possible sketch, and can only serve, at most, as a reminder.

* TextBook of Physiology. By Dr. M. Foster. Sixth Edition. Part III., "The Central Nervous System," pp. 1000, 1001.