Physiological psychology investigates the two kinds of action-centripetal and centrifugal—and traces their paths and termini. It finds that the sensor current may come to the cerebrum before a corresponding motor-current originates, or it may only proceed so far as some nervous ganglion. In the former case there is conscious volition; in the latter case there is only reflex action of some kind. Great efforts have been made to discover the several turning points, or centres, in which centripetal and centrifugal currents are connected. It is certainly one of the most worthy objects in natural science to trace out these relations of the mechanical, vital, and spiritual. No field of Nature has demanded more patience and skill on the part of scientific men than the nervous system. We may be sure that no field of Nature will yield more valuable results. As the science of physiological psychology is in its infancy, it is too early to expect much from it yet. I shall endeavour to sum up the more significant of its discoveries in the next two chapters.

CHAPTER XIII.

Brain Centres of Sensation and Motion.

855. THE distinction between sensor and motor nerves is made by Rufus Ephesius in his work on the names of different parts of the body. He lived in the time of Trajan (97-117 a. D.), but he refers this discovery to the famous physician and anatomist Erasistratus, who lived 300 в. C., and discovered the cause of the illness of the king's son. Antiochus, the son

of Seleucus Nicator, King of Syria, was pining away, and no physician could detect the cause until Erasistratus noticed the quickening of his pulse on the approach of the beautiful Stratonice (etc., see the classical dictionary). Although this important discrimination between efferent and afferent nerves is two thousand years old, yet the connection between these two orders of nerves was only vaguely known until recently. Erasistratus, according to Galen, had discovered by dissection that the motor nerves arise in the substance of the brain, while the sensor nerves connect only with the cerebral membranes. Modern researches show that both sets of nerves arise in the great ganglia at the base of the brain.

§ 56. The spinal cord, after passing from the spine into the skull, thickens and forms the medulla oblongata. Above this it expands laterally, sending out bundles of nerve-fibres to connect and unite the spinal cord with the two hemispheres of the brain, thus forming a sort of bridge, called the pons Varolii. Above this, one on each side of the middle line, are the optic thalami, "continuous with the gray matter of the spinal cord, which thus ascends into the interior of the brain." * Above and before the

* J. Luys, in Appletons' International Scientific Series, The Brain and its Functions.

optic thalami, and also farther outward from the middle line on each side, are two ganglia of gray matter, called the corpora striata, or "streaked bodies." The important concern for us is the function of these great ganglia. It is supposed that the optic thalami furnish the co-ordinating centre for all the nerves of sensation, while the corpora striata furnish a like centre for all nerves of motion; co-ordinating in the sense that they adjust, harmonize, and reduce to unity contrary and conflicting nerve impulses. The sense-impressions from the surface of the body are collected, by aid of the spinal cord, in the optic thalami, and thence transmitted to the gray matter (cortex) of the large brain (cerebrum). Here some elaborative process goes on. When the mind is "made up 99 to act, there proceeds a motor-impulse to the corpora striata, and thence to the muscles of the body that are to be moved.

A French specialist, who has made discoveries in this field, describes it: " Through the tissues of the optic thalami pass vibrations of all kinds, those which radiate from the external world, as well as those which emanate from vegetative life (i. e., from the digestive organs). There in the midst of their cells, in the secret chambers of their peculiar activity, these vibrations are diffused, and make a preparatory halt; and thence they are darted out in all directions, in a new and already more animalized and more assimilable form to afford food for the activity of the tissues of the cortical substance which only live and work

under the impulse of their stimulating excitement." The same author, with his somewhat lively imagination, describes the functions of both ganglia (sensor and motor) thus: "The elements of the optic thalami purify and transform by their peculiar metabolic action impressions radiating from without, which they launch in an intellectualized form toward the different regions of the cortical substance. The elements of the corpus striatum, on the contrary, have an inverse influence upon the stimula starting from these same regions of the cortical substance. They absorb, condense, and materialize them by their intervention; and, having amplified and incorporated them more and more with the organism, they project them in a new form in the direction of the different motor ganglions of the spinal axis, where they thus become one of the multiple stimulations destined to bring the muscular fibres into play."*

857. These two ganglia are, moreover, interconnected by nerve-fibres, and there is possible a direct communication between the optic thalami and the corpora striata, as well as the indirect communication through the gray matter of the cerebrum. Here is a physiological basis for the distinction between reflex movements and deliberative movements. The spinal cord and medulla oblongata are especially the seat of a large number of reflex actions, such, for example, as the closing of the eye to keep out a cinder, or the involuntary gesture of the hand to protect the head from a blow aimed at it. The sensory impulse is con

*Luys, pp. 45 and 58.

verted into a motor impulse through a central organ, a ganglion of the spinal cord. It is a column or pile of centres in which such conversion takes place. Automatic actions do not need external stimulus, and are not reflex, although they seem to be impelled from the same centres as reflex actions; breathing, digestion, movement of the heart are automatic. More complicated reflex action and automatic centres in the medulla oblongata, and still more complex reflexes, involve both the optic thalami or corpora quadrigemina (for the sensory side) and the corpora striata (for the motor side). The medulla oblongata is claimed to be the centre of a large number of automatic centres, such as breathing, swallowing, sneezing, coughing, vomiting, laughing, weeping, etc. If there is a direct communication between the optic thalami and the corpora striata without the mediation of the cerebrum, we have a reflex action of a higher order than those which go out from centres in the spinal cord. If, finally, there are two higher centres in the gray matter of the cerebrum—a sensory centre and a volitional centre-the action becomes deliberative, conscious, and responsible.

It must be understood that the optic thalami form a sort of crown to the back portion of the spinal cord, while the corpora striata crown in like manner the front portion. In the cord the sensory regions occupy the back