4. Louence of the nervous system upon the circulution.—It is a very general opi nion among physiologists that a considerable influence is exerted by various parts of the nervous system upon the function of circu lation as a whole, and through it upon the different processes of the economy concerned with nutrition, as digestion, secretion, growth, animal heat, &c. There is some difficulty, however, in ascertaining the exact relation which subsists between particular parts of the nervous and circulatory systems. It is mani fest that in many instances the circulation in the bloodvessels is modified by a nervous in fluence which operates on the heart alone, while in others it is affected by an alteration of the vital powers of the bloodvessels themselves. We refer the reader to the articles CONTRAC TILITY and HEART for an account of the modifications to which the circulation is liable from the operation of nervous influence on the heart alone. We shall only remark in this place that 'although the heart may be excited to contraction by the direct stimu lation of its muscular substance, and although the effect upon the heart's action of bodily exertion, of emotions of the mind, and of severe injuries of the brain and spinal mar row, all of which can be supposed to act upon the heart through the nerves only, are un doubted ; yet it is well ascertained that the heart cannot in general be excited to con traction by the direct stimulation of its nerves, and that its action may be regarded as auto matic to a certain degree, and little dependent upon the immediate transmission to it of any nervous influence from the cerebro-spinal or ganglionic nervous systems, since the rythmic contraction of the heart continues to go on for a time in some animals after the division of its nerves, and in others even after its complete separation from the body. It has also been frequently found that after the complete de struction of the brain and spinal mtarow of an animal the circulation of the blood can be maintained for some time by means of artificial respiration,—an experiment which proves that the motion of the blood in the vessels is not immediately dependent upon nervous influ ence.* Many circumstances, however, seem 'to shew that the state of the vessels, and in consequence of this the velocity and force of the blood, are susceptible of very considerable modification from local affections of the nerves belonging to the part in which they may have been observed to occur, or from general alterations of the nervous powers of the system. It is probable that nervous influence operates much more powerfully in modifying the circulation through the small than through the large vessels, indeed we. know of no direct satisfactory experiments which demonstrate the effect of nervous in fluence upon the larger arteries exclusively.
The experiments which seem to prove most satisfactorily the influence of the nervous system on the circulation in the small vessels are those performed on cold-blooded animals by Legal lois,t W. Flourens, and particularly those of Marshall Hall,§ the general result of which may be stated as the following: that after the destruction, whether sudden or gradual, of the brain or spinal marrow, the flow of blood in the remote parts becomes more languid and is gradually more and more circumscribed, while the action of the heart continues, and its power seems not to be diminished in a propor tional degree. But in such experiments as those just mentioned, performed in general in cold blooded animals it must be at all times ex ceedingly difficu'lt to find an accurate mode of measuring the force of the heart, and conse quently they cannot be regarded as affording sufficient evidence that there did not occur along with the languid state of the circulation a certain diminution in the heart's power.
They do not at least entitle us to conclude that the decreased velocity and stagnation of the blood in the remote parts is caused mainly by the loss of the vital powers of the capillary vessels, for these changes of the circulation may in a great measure be the effect of other causes, as the loss of power of the heart, and that more permanent alteration of the textures which very probably accompany the severe injury done to the body. On the other hand it may be remarked that the coldness and im paired nourishment common in palsied limbs, the known increase or diminution of the various secretions from mental emotions, and direct or sympathetic affections of the nerves belong ing to the glands or other secreting organs, the phenomena of blushing, erection, inflam mation, and the like are all very direct and satisfactory proofs that the small vessels and the capillary circulation may be influenced by affections of the nerves. As a further confirma tion of this may be mentioned, 1, the inflamma tion and other consequences of the division of the fifth pair of nerves which occur in the eye; 2, the statement of some, as Treviranus, that the division of the nerves of the leg of a frog impedes the circulation: 3, the assertion by others, as Baumg'artner, that after the division of the nerves or the destruction of the spinal marrow, the peculiar oscillations which he, along with Doellinger and Kaltenbrunner, has observed to precede the formation of new blood vessels do not occur ; and 4, the observations of Nasse, which are stated to shew that the reunion of wounds is retarded or put a stop to by the division of the nerves belonging to the wounded part. Krimer,* whose experiments on this subject are numerous and remark able, states that the circulation was always much impaired by the abstraction of nervous influ ence from the division or ligature of the nerves; that the jet from the femoral artery of a qua druped was much less strong after the division of the crural nerve ; that the capillary circula tion of the frog's web ceased soon after the nerves were cut or tied ; that the arterial blood passed through the systemic capillaries without undergoing its proper change into venous ; and that salt did not produce the accustomed effect of dilating the capillaries when the nerves of the part were injured, but that these effects were induced when galvanic irritation was applied to the divided nerve.
In reference to these experiments it may be remarked that most of them are at variance with experiments of a similar nature performed by others, more especially those of Haller, Spallanzani, Whytt, Fontana, Legallois, W. Philip, Flourens, and M. Hall, none of whom remarked so immediate and complete a stoppage of the circulation from removal of the nervous influence. Again, in palsied limbs the circu lation is frequently little or not at all disturbed, and sometimes the secretions, natural growth of parts, and reunion of wounds have been found to be little impaired by injuries of the nerves. We may therefore form the conclu sion, that although the circulation in the small vessels is obviously liable to be modified by the state of the nerves in their neighbourhood, or perhaps by affections of the nervous system in general, there is no reason to consider the capillary circulation as more immediately de pendent on nervous influence than the action of the heart.