Numerous experiments have been made to determine the degree of dependence of the secretion of the gastric fluid upon the nervi vagi ; to these experiments copious references have elsewhere been given*, and we shall therefore only here allude to their results. The temporary suspension of the digestive process appears to be an invariable result of the complete division of the par vagum on both sides ; and many of those who have witnessed this result have somewhat hastily concluded, that the secretion of gastric fluid is dependent upon nervous agency conveyed through that nerve. But it has been observed, in several instances, that the digestive powers have returned after a time, animals which were becoming much emaciated having re covered their flesh ; and it is obvious, there fore, that the secretion of the gastric fluid can not be dependent upon the supply of nervous agency through the par vagum, as some have supposed it to be. It is true, that in a large proportion of the experiments made to deter mine this question, there has been no appear ance of any return of the digestive power, after complete section of the par vagum on both sides ; hut there are various modes of account ing for this fact. The animals on which this experiment has been made, usually live for only a short time afterwards, on account of the disorder of the respiratory processes, which is one of the results of the operation ; so that all which is proved by the great bulk of the experiments is, that the digestive process is generally arrested during the short time that the animal lives after the vagi have been di vided or tied. And such negative results, as Dr. J. Reid has very justly observed, " can never overthrow the results derived from positive experiments, provided that these have been accurately performed, and are free from all sources of fallacy." With these facts before us, it is much to be desired that the experiments just cited, as to the influence of section of the renal nerves upon the secretion of the kidney, had been sufficiently prolonged to ascertain whether the effects described are transient, and whether the real secretion would be restored if time were permitted. And it is obvious that, as they at present stand, no such experiments can serve as an adequate foundation for the hypothesis entertained by some, that the act of secretion is dependent upon nervous in fluence, or, in other words, that nervous agency supplies a condition without which it cannot take place.
There is another group of phenomena bear ing upon this question, though less closely related to it, — namely the changes in the state of nutrition in parts whose nerves have peen injured, and which are thereby rendered insensible. The close affinity, however, al ready shown to exist between the functions of Nutrition and Secretion, is sufficient to make it apparent that they must stand upon the same footing in this respect, and that whatever is true as to the relation of either of them to the nervous system, must be true also of the other. Now it is an observation very frequently made, that parts whose nerves have been paralysed are peculiarly disposed to suffer from destructive inflammation, or to undergo a gradual wasting. The latter of these changes is easily accounted for on the general principle dwelt on under the head of NUTRITION, that the degree of nourishment which any organ or tissue receives, depends upon its functional activity; and thus not merely the muscles, but all the textures of a paralysed limb gradu ally waste away, the disuse of its muscles occa sioning a stagnation in the circulation through the entire part. Of the former result it is ne cessary to make a careful examination, that we may be prepared to estimate it at its true value. One of the cases most frequently quoted in this connection, is the effect of section of the trigeminus in producing destruc tive inflammation of the eye-ball, as first shown by Magendie, and confirmed by many sub sequent experimenters. A full account of
these effects has been already given in another part of this work (see FIFTH PAIR), and it is therefore unnecessary to repeat them here. A corresponding result may be produced by disease. A case is related by Mr. Stanley*, in which there was impairment of the w hole nutrition of one side of the face, with frequent attacks of erysipelatous inflammation, bleeding from the nose, central penetrating ulceration of the cornea, and, at last, destructive inflam mation of the tunics of the eye, in consequence (as it would appear) of destruction of the trunk of the trigeminal nerve of that side by the pressure of a tumour near the pons. No such destructive effects ensue on section of any of the other cranial nerves ; the only injurious influence exercised on the eye by any such operation, being the tendency to inflammation from irritants which the para lysed orbicularis palpebrarum does not shut out or help to remove. But, on the other hand, cases are occasionally to be met with (of which the author has himself witnessed more than one) of the complete paralysis of the ophthalmic division of the fifth pair, which has existed for some time without any other result than a degree of dryness of the surface of the eye from deficient secretion, and a disposition to superficial inflammation from irritating par ticles of whose presence no warning was given by sensation, and for whose removal there was consequently no provision. Such exceptional cases must be admitted as proving that, however unfavourable may be division or injury of the trigeminus to the continued healthy nutrition of the eye, still this nzay be maintained; and that it is consequently no more essentially dependent upon " nervous influence," supplied through that channel, than is the secretion of gastric fluid upon the power supposed to be transmitted by the par vagum.
That the nutritive operations of other parts, however, are usually less vigorously and cor rectly performed when the nerves have been paralysed, than when they retain their entire integrity, would appear from numerous other facts, of which the following are examples. A case is related by Mr. Swan* in which a man's wrist having been injured by a cord having been very tightly drawn round it, there was partial paralysis of the hand, with constantly repeated ulcerations of its dorsal surface ; and on amputation seven years afterwards, there was found to be induration of the inedian nerve, with adhesion of the tissues beneath the annular ligament. The following case, stated by Mr. Paget -I- on the authority of Mr. Hilton, is still more remarkable. " A man was at Guy's Hospital, who, in consequence of a fracture at the lower end of the radius, re paired by an excessive quantity of new bone, suffered compression of the median nerve. He had ulceration of the thumb, and fore and middle fingers, which had resisted various treatment, and was cured only by so binding the wrist, that the parts on the pain-tar aspect being relaxed, the pressure on the nerve was removed. So long as this was done, the ulcers became and remained well ; but as soon as the man was allowed to use his hand, the pressure on the nerves was renewed, and the ulceration of the parts supplied by it returned." That the reparative processes are affected, as well as those of ordinary nutrition, by the loss of nervous power, is a matter of familiar ob servation. A striking example to this effect is mentioned by Mr. Travers:I A man was rendered paraplegic by fracture of the lumbar vertebrm, the sarne accident having also frac tured his humerus and his tibia. The former, in due time, united ; the latter did not.