This peculiar affection of the nutritive pro cesses appears rather dependent upon lesion of the sensory than of the motor nerves. Thus we have seen that the disorganisation of the eye after section of the fifth pair, takes place when only the sensory nerve of the part is affected, and that no such result occurs when only the motor nerves of the orbit are divided. In cases of disease or injuries of the spine, it has been noticed that sloughing of the blad der or other parts has occurred earlier and more extensively when sensation, than when motion alone, has been lost. And Mr. Cur ling has noticed § that two men having been taken at nearly the same time to the London Hospital with injury of the spine, one of whom had lost only the power of motion in the lower extremities, whilst the other had lost both motion and sensation, at the end of four months the atrophy of the lower extre mities had advanced much further in the latter case than in the former. These phenomena would seem to harmonise with the view, that it is especially through the sytnpathetic system of fibres that the peculiar influence is exerted, whose contindal agency we only recognise by the results of its withdrawal. For if, as already remarked, the fifth pair may be con sidered as the sympathetic of the head, the Gasserian ganglion may probably be regarded as belonging to the sympathetic system ; and it has been observed by Magendie, and con firmed by Longet, that the destructive inflam mation of the eye ensues more quickly after division of the trigeminal nerve in front of the Gasserian ganglion, than when the division is made between that ganglion and the brain. If this be true of the Gasserian ganglion, it is probably true, also, of the ganglia on the posterior roots of the spinal nerves ; and thus the disordered nutrition which results from in jury to the trunks of these nerves, and which is not to be accounted for by the mere disuse of parts, inay be attributed, with some show of probability, to the interruption of the connec tion with the sympathetic system, which is specially established hy these ganglia and their communicating cords. But it is to be remembered, on the other hand, that defec tive or disordered nutrition is a marked result of injuries of the spinal cord, whilst the sym pathetic centres remain uninjured ; and that general' atrophy is a frequent consequence of chronic diseases of the brain. Fresh evi dence is much required, therefore, to deter mine the relative shares of the cerebro-spinal and sympathetic centres, in regard to the in fluence exerted by them over the organic functions.
By the survey we have now taken, we are in some degree prepared to estimate the degree and nature of the influence exerted by the nervous system on the nutritive and se cretory functions, and to inquire into the validity of the several doctrines which have been propounded on the subject :— 1. The first of these theories may be stated in the words of Dr. Wilson Philip, one of its most distinguished advocates :—" It appears," he says, " that the nervous influence is neces sary to the function of secretion. It either bestows on the vessels the power of decom posing and recombining the elementary parts of the blood, or effects those changes by its direct operation on this fluid. From many facts stateci or referred to in my inquiry, it appears that the vessels possess no powers but the muscular and elastic ; and that the former, as well as the latter, is independent of the nervous system. Nor is it possible to con ceive any modification of these powers by which they could become chemical agents, and thus be enabled to separate and recom bine the elementary parts of the blood. The
first of the above positions may, therefore, be regarded as set aside, and the necessary infer ence seems to be, that in the functions of secretion the vessels only convey the fluids to be operated on by the nervous influence." It will, perhaps, be sufficient to say of this hypothesis, that having been pui forth at a time when the real nature of the secreting structure was altogether unknown, and when the choice seemed to lie only between the in fluence of the nerves and that of the vessels, it is totally fallacious now that a third agent has been discovered, to which all analogy would lead us to refer, at any rate, the chief instrumentality in the operation. The prin cipal experiment adduced in support of this hypothesis, and of the identification attempted by Dr. Philip between nervous agency and galvanism, was the effect of section of the par vagum in checking the secretion of gastric fluid, and the renewal of the process under the influence of gilvanism. We have already shown the utter invalidity of this result as a ground for any such inference ; and it only remains to show the inconsistency and insuffi ciency of the hypothesis itself, which is easily done. For, as Dr. Prichard has justly re marked *, " if we begin by supposing the ex istence of the cause assigned, we shall find that there is one agent, namely the galvanic fluid, operating on one rnaterial, which is the blood, and effecting its decomposition. How, then, we may ask, does it happen that so many different substances are, in different examples of the same process, the results of this single operation ? In other chemical de compositions, as when water is decompounded by the galvanic fluid, the result is the same and uniform. But in the instance supposed, the operation of the same chemical agents upon each other is followed by the formation of products of the most different descriptions: in one part of the vascular system the blood is converted into bile ; in another, by the operation of the same chemical agent, into milk ; in another, into tears." This variety of effects can only be explained by attribut ing them to the special endowments of the several secreting organs through which the nerv:ms power is supposed to act ; arid if it be thus necessary to admit that such special endowments do exist, by which the particular nature of the secretion is determined, the question naturally arises, Of what use is the nervous power at all ? 2. The second hypothesis, framed to rneet this objection, supposes, to use the language of Prof. Miiller, that " the influence of the nerves on the glands merely enables the se creting substance, in each gland, endowed with peculiar properties, to exert its chemical ac tion." In order to sustain this hypothesis, it is necessary to show that the processes of secretion and nutrition are not only modified by the division of the nerves by which their organs are supplied, but that they are alto gether suspended by that operation ; the secreting or growing structures having no functional power of their own, save when connected with certain nervous centres, which are supposed to transmit to them the requisite vital force : much as in a factory there may be seen a great variety of rnachines, each of them constructed to perform a certain special action, but all of them dependent for their power of carrying it into effect upon a general motive power transmitted to each. We shall, perhaps, more conveniently and satisfactorily examine into the merits of this hypothesis, by bringing it into comparison with the next.