The trophic condition of the neurons depends partly upon the stimulus to which they respond with their "specific energy" (sensation, movement, secretion, vascular contraction or dilatation), and in this respect there exists a sufficient stimulus, one too small and one too great. Further, the various parts of the neurons are trophically dependent one upon the other. If the nerve fibre is separated from its ganglion cell it degenerates (Joh. Muller); this is a useful fact, since with the help of the microscope we can use it for studying the anatomy of the nerve paths. If the peripheral area of activity of a ganglion cell is taken from it, its nerve fibre, with its end apparatus, degenerates (v. Gudden and others).

With regard to the speed of impulses in the nerve fibres, it is apparently both in sensory and motor nerves about 34 metres a second, and thus normally in no part of the human organism do they require more than a very small fraction of a second to travel their whole path.

The spinal cord forms, as we see from the above, and as every medical student knows, an organ not only for conduction, but also an intermediary station for centres between the periphery and the brain. These centres receive stimuli both from the brain and from the periphery, and, as they possess a certain autonomy, the spinal cord forms specially an organ for reflex action.

I would remind my reader that reflexes may also be sent out from ganglion cells higher than those of the spinal cord, and that particularly in the floor of the fourth ventricle there are reflex centres to be found.

By reflex action we mean the outcome or effect of every stimulus on a centripetal, i.e., a sensory, nerve, which is thrown back from one or more nervous centres to the periphery. To stimulate a centrifugal motor nerve does not produce a reflex. As the reflexes are carried to the periphery by motor, secretory, and vaso-motor nerve fibres, they may also have motor, secretory, or vaso-motor effects. One jumps on hearing a shot, feels the mouth water on seeing a basket of strawberries, goes pale when one sees a tram run over a fellow creature, blushes with mortification over the sad phenomena of human baseness - all reflexes ! (Personally I may say that when as a young man I came to know a little more physiology I often wondered if there were phenomena from the nervous system other than reflex action.) The physiologists, however, do not acknowledge the presence of sensory reflexes in the ordinary sense of the word, although it is true that a sensory nerve may conduct centri-fugally. That perceptions may arise in a peripheral area on stimulating another peripheral area is explained by the expression "irradiation," etc but is still partially unexplained.

I would remind the reader also of the presence of "inhibitory reflexes." By stimulating the vagus (the central stump) the secretion from the pancreas may be made to cease. Other things being equal, the reflex from the depressor nerve of the vagus (Ludwig and Cyon), by means of which the mesenteric vessels are strongly dilated and the pressure, particularly in the aorta, consequently markedly decreased, is also a vaso-constrictor inhibitory reflex.

We have classified and named reflexes chiefly after the easily observed reflex movements, and know that these arise according to definite laws, even when they are "irregular" (see below). With a weak stimulus a simple local reflex produces a jerk at the level of the place stimulated and in the same limb; with a stronger stimulation one sees that the reflex becomes double-sided and the jerk arises also at the analogous spot in the other half of the body. One sees that on extreme stimulation or increased irritability (produced by strychnine) the reflex becomes widespread.

Reflexes are involuntary, but some of them may to some extent - not always, and not always completely - be controlled (inhibited) by the will. If I am prepared for a shot to fall in my vicinity, by determination I am able to avoid starting.

It is the motor reflexes with contractions of the striated muscles under the power of the will which can be controlled by the will; our other reflexes can only be controlled by preventing the stimulus. Reflex movements of non-striated muscle fibres (especially vaso-motor reflexes) and secretory reflexes cannot be controlled.

By a careful study of our movements we arrive without difficulty at the conclusion, which is well worthy of attention, that between an entirely involuntary, simple muscle reflex and a fully conscious, voluntary and complicated co-ordinate, purposeful movement there are any number of intermediate forms. One particular argument for this opinion is the existence in the lower vertebrates of the extremely interesting so-called "spinal mind of Pfluger." By this we mean the power that these animals possess, without a brain, of carrying on purposive, apparently conscious and deliberate, complicated co-ordinate movements. As the brain has been removed we must suppose that these movements are reflexes from the spinal cord, and call them by a special name, "ordered reflexes." If after having cut out the frog's brain one waits until the animal has recovered from the worst shock of this severe operation and then pinches its hind leg, we find that it performs parrying movements with this leg or with the other hind leg. It can also sit, take ordinary swimming strokes, etc., all phenomena from its spinal mind. It is true that we human beings are able neither to parry, sit, nor swim after our heads have been taken off, and that our spinal mind is far inferior to that of the frog. But its presence in the frog, along with other facts, obtained from observations on lower mammals, allows us to conclude, not without satisfaction, that something similar exists in ourselves.

A fact to be inferred from the above-mentioned facts, of which every medical gymnast ought to be fully aware, is, as already mentioned concerning the various forces which decide the strength of our movements, that our motor apparatus consists not only of centrifugal motor neurons, but also of centripetal sensory ones.