the periphery, from which in many cases filaments emerge; while in other instances these loops are merely recurrent communicating branches. Hyrt has given numerous instances of these. Some of the more familiar are the loops of the hypoglossal, the return upwards of the recurrent laryngeal, and the palmar or plantar arcades. The chiasm of nerves on opposite sides of the body is more rare. That of the optic nerves in man seems to be in him the sole instance of a true physiological chiasm, since Vulpian* thinks he has proved that in the case of the intercommunication of the right and left hypoglossals pointed out by Hyrtl, there is no transmission of power from side to side; and, indeed, if I correctly understand the passage, he would seem to have some doubt as to whether there is really an interchange of fibres.

In man the cerebro-spinal nerves, at least, seem to be physiologically limited to their own side of the body, a proposition which becomes of some importance in the recognition and limitation of paralytic affections. I have once or twice felt doubtful as to the strict truth of this law as applied to certain traumatic palsies of the face, where in rare cases the motor fibres of the upper lip have appeared to me to cross over, so that irritation of the nerve on the sound side seemed to cause slight muscular contractions on the diseased side.

In the lower animals, the existence of any true physiological chiasm, other than that of the optic nerves, was first demonstrated by the author, and his friend Dr. Morehouse, in the case of the chelonians, all of which have a chiasm between the two superior laryngeal nerves, so that irritation of the left nerve acts on both the left and right lips of the larynx, and vice versa. Guided by this discovery, my friend Prof. Wyman found similar chiasms in

* Vulpian, Leçons sur la Phys. Comp., etc., p. 160.

reptiles and birds. He has also discovered that in frogs. the nerves of the skin cross the central line of the body. I have recently examined frogs to learn whether this anatomical crossing involves physiological consequences. To test this, the skin of the frog's back or belly was divided longitudinally at varying distances from the middle line, and then irritants, as biliary and stronger acids, were applied to the skin between the central raphé and the line of section; no response was obtained to such irritations, even in frogs which had been previously rendered excitable by strychnia. The purpose, therefore, of such interchange of nerves must still be considered. doubtful.

During their distribution, the nerves from more or less remote parts of the nervous system in some localities run together, and then separate to reunite and part anew, thus forming, by the free communication between neighboring branches, a plexus or network, from which emerge finally the nerve trunks which are to be distributed to a particular member. This intricate interlacing seems to be merely an arrangement for the interchange of fibres, since those which enter the plexus acquire in it no physiological properties which they did not previously possess.

A knowledge of these networks is essential to the study of nerve wounds, because injuries of the plexuses, especially that of the neck, are very common in war, and their arrangements enable us to explain in many cases the transmission of disease from one nerve to another, which is closely related to it in the plexus, or as its parent stem. After leaving the plexus, the limb nerves are usually distributed with great regularity, anomalies being rather uncommon.

While in most men we may take for granted that nerves obey the normal law, it is still important to remember that in the arm especially, peculiar and exceptional ar

rangements may occur in the way of unusual interchanges of nerve fibres.

Chassaignac, and, more recently, Mr. Hilton, in his thoughtful book on "Pain and Rest," have pointed out, as one of the elements of protection to nerves, the fact that in most instances the motor nerves enter their respective muscles on the under side, so that the whole thickness of the muscle is interposed between the nerve and the exterior sources of injury. We may add that on their first entrance to a limb, the larger nerves lie in the leg at the back, or in the arm, upon the inside, where they are singularly secure from harm.

CHAPTER III.

NEURO-PHYSIOLOGY.

Varieties of nerves.-The nerves which connect a limb with the centres fulfill the following functions: They convey outwardly motor impressions, resulting in voluntary or involuntary motion, as the case may be. They carry centripetally the myriads of impressions which constitute sensations, or which, unfelt as sensations, are excitomotory in purpose. Lastly, a system of nerves, known as sympathetic, exercises control over the flow of blood to the tissues. Section of these nerves paralytically dilates the vessels; galvanization contracts them. This system originates in the spinal cord, but there is also evidence that certain spinal nerves, unrelated to the sympathetic, possess the power to cause directly enlargement of arterioles and increase of vasal supply.

Through this compound group of nerves occur such nutritive changes as depend on increased or lessened flow of blood, but these are insufficient to account for all the trophic changes which we witness from disease or injury; and hence has arisen the belief that there are also trophic nerves proper, which, apart from variable blood supplies, or in connection with these, bring about in the tissues alterations, examples of which I shall have to point out from time to time.

In connection with nerve wounds and their consequences, we have to deal with alterations due to the destruction or irritation of the various fibres of nerves

which minister to pain, touch, the thermal sense, motion, calorification, and nutrition.

I do not think it desirable to enter as largely into these subjects as they would be dealt with in a physiological treatise, but I cannot avoid some reference to such portions of their physiology as either aid us to comprehend and relieve nerve wounds, or as are set in new or clearer light by the phenomena of these accidents.

Trophic nerves.-The question which most often presents itself for answer to the neuro-pathologist is as to the existence or non-existence of nerves directly affecting the nutrition of the tissues, apart from the changes induced in them by the section or irritation of vaso-motor nerve fibres. Without attempting to treat fully of this matter, for which, indeed, the physiological groundwork is still deficient, I shall content myself with stating the impressions in this direction which a large experience of nerve wounds has left upon my mind.

When the physiology of the vaso-motor nervous system was first elucidated by the labors of Bernard, Brown. Séquard, Schiff, and others, it was supposed that it would enable us readily to explain the many obscure phenomena which arise out of nerve wounds; but despite the able arguments of Handfield Jones* and some other observers, this hope has gradually faded away, and it is now generally admitted that we must seek elsewhere for a satisfactory explanation of the facts in question.

The effects of injuries or sections of these nerves can be only to cause either contraction or dilatation of vessels, or to put a stop to the pulsatile movements which have been seen in minute vessels in many parts of the animal economy. These alterations, with the consequent changes in blood supply which they bring about, may be direct

St. George's Hospital Reports, 1868, p. 89 et seq.