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Letter 7800

Foster, Michael to Darwin, C. R.

4 June [1871]

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    Sends answers to CD's queries on expression.

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[Enclosure: 1]

The reddening of the face before the fire is probably a process of mixed nature, being partly the result of a direct local action of the heat and partly of an indirect reflex action on the vaso-motor centres. The proportion of these two factors seems to depend on the amount of heat affecting the skin—a powerful heat acting directly & locally in a large measure.

Hence when the sympathetic of the neck is divided and the blood-vessels of the face cut off from their vaso-motor centres, the consequent dilitation of the arteries and suffusion of the face would not be removed over the face generally by the application of such a small amount of cold as would, while the connection with the vaso-motor centre was intact, produce contraction of the blood vessels generally by reflex action. More powerful cold would however produce contraction of the arteries and pallor over an area spreading for some distance around the point at which the cold was actually applied. The greater the cold the wider probably would be the area—

I do not feel quite clear about the amount of consensus there is among the arteries of any given area, over and beyond the union which is afforded by their common dependence on a vaso-motor centre. Lister believes largely in ganglia (not detected) scattered among the blood vessels, acting subordinately to either the ganglia in the sympathetic trunks or to the vaso-motor centres in the spinal cord.

The suffusion due to section of the sympathetic seems to pass away before the divided trunk is itself restored to action, the existence some subordinate action, rising in absence of the ordinary governing power, to greater importance seems indicated by this and several other facts.

When the attention is strongly directed to any part, to the tip of the finger for instance there is always a tendency for obscure volitional impulses to be transmitted to the the muscles moving the finger; there is very frequently for example an effort (of which the mind is dimly or not at all conscious) to keep the finger quite still, by the combined actions of antagonistic muscles. And the sensations of this effort are frequently referred to the tip itself of the finger. Putting this on one side, I am not aware of any evidence that any impulse is sent directly from the brain backwards along the sentient nerves, or along any other nerves not exclusively motor nerves.

It cannot but be that sensory impulses are continually originating at sentient points and continually travelling upwards towards the brain without however our being conscious of them. It according seems by far the simplest view to suppose that in ordinary circumstances certain ``stops'' exist in the central nervous system, which cause these sensations to be shunted off in some direction other than towards the seat of consciousness,—and to be used up in some purpose of the economy. By the act of attention these stops are removed; the sensations in consequence go straight up to the seat of consciousness and are accordingly recognized.

Nor does this view seem to me incompatible with the fact that the mind's attention may cause changes in glands &c; for the mechanism by which these changes are brought about is probably a very complex one.

Supposing for simplicity's sake, that the changes in question are brought about by the changes in the blood-supply only— The normal condition of the circulation in any area or organ is kept up by the action of the vaso-motor centres, situate for the most part in various regions of the spinal cord. (More particularly in the medulla oblongata)

It is by working upon these vaso-motor centres that the circulation is generally modified. By certain influences these vaso-motor centres are paralysed, (in a manner analogous to the paralysis or inhibition of the heart by irritation of the pneumogastric nerve) the blood vessels consequently dilate, and more blood is carried thro' the part; or the centres are excited to increased energy (analogy of action of sympathetic on heart), the vessels are contracted and less blood is carried. Further while we know that these centres are peculiarly subject to influences of emotion**, we have no evidence that they can be directly influenced by the will, on the contrary wherever we find evidence of the mind acting on these centres, there is always room for the supposition that emotions (of some kind or other) are at work.

We may therefore explain what goes on when the mind's attention is drawn to an organ as follow.

The direction of the attention gives rise to certain ideas, emotions or similar processes connected to a certain extent with the function of the organ. These ideas &c &c probably form part of a long series of processes which ultimately come to bear on the vaso-motor centres with which they have a distinct physiological (possibly anatomical) relation

This centrifugal action may or may not be accompanied by a marked centripetal process; in way of sensation the two have no necessary connection. There may be little or no sensation from the organ, yet if the fact of attention give rise to the series of emotions &c, a great effect may be produced. On the other hand, much sensation may be felt but no centrifugal action result in consequence o[f] the absence of the chain of emotions &c.

It is perhaps hardly safe to suppose that the changes caused by the nervous system in the nutrition of any organ are to be explained by reference to vaso-motor nerves only. It is more than probable that in the case of glands, nerves, neither sentient nor strictly motor, may act directly on the cells of the glands. Such nerves are however apparently very analogous to vaso-motor nerves, and subject to the same laws.

In the case of glands the connection between vaso-motor &c nerves and certain sets of ideas &c is very marked, whereas ``attention'' does not bring to light any marked sensations. In the case of finger we have no evidence of any connection between that organ and any set of emotions or ideas, while the sensations brought to light by attention are often very strong.

**The word `emotion' as above must be taken in a very wide & indefinite sense for want of a better word

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