V
A still more characteristic phenomenon in the picture of fear is goose-skin. Let us see how and why the skin corrugates in this way. We know that besides the sudoriparous glands there are other glands at the surface of the body which secrete a peculiar fat called sebum, which oils the surface of the skin and gives it that gloss which we notice on the face of some people.
If we take a vertical section of the skin, we can see with the microscope a close network of muscular fibres which traverse the skin in an oblique direction, and surround every hair in the manner of the ribs of an umbrella. It is wonderful to see this mechanism under the microscope, how every hair has its own gland, its own muscle and its own nerve, its own arteries and veins. When these muscles contract, the meshes of the skin contract likewise and express the contents of the glands. We do not notice these movements of the skin, because the muscles contract very slowly.
Sometimes special muscles appear in the skin called cutaneous muscles, which play an important part in the life of animals.
We all know how the hedgehog rolls himself into a ball on the approach of danger. This movement, as we have already stated, is executed by means of a muscle covering the whole of the body, like a hood or purse which may be drawn together on one side. In the mole, too, these muscles are very strong, and we have already mentioned that dogs and horses twitch the skin to rid themselves of flies, and that this movement is due to a rapid contraction of one of these muscles. When animals curl themselves up, with the muzzle close to the tail, as the sleeping dog does, head and limbs are more easily held in this position by means of these muscles. I have found them more or less in all superior animals, and shall now consider the possible uses of these muscles which exist also in man.
It does not seem to me correct to say that they serve to drive the flies away, because they are well developed in reptiles and fish, and in many animals of which the skin is insensible to the stings of insects; also, if the fly-hypothesis were the correct one, the cutaneous muscles should be best developed on those parts of the animal which cannot easily be reached with the head, the leg, or the tail, but the contrary is the case.
Certainly the muscles are made use of for this purpose, but this is an accidental fact, as is also, I believe, the circumstance that these muscles serve to erect the hair when the animal is excited or afraid. When one dog approaches another in a hostile mood, there is such an agitation of his nervous system that he begins to tremble, not through fear, but excessive excitement. All the muscles contract, those of the blood-vessels, of the bladder, of the intestines, therefore it is comprehensible that the cutaneous muscles should also contract, raising the hair on the dog’s back.
If we look at the skin of the arms or legs when we step into a cold bath, or when we uncover ourselves on rising in the morning while the temperature of the room is low, we notice the appearance of goose-skin.
Whenever there is, for some reason, a contraction of the blood-vessels, these muscles contract also and the hair rises. The simultaneous appearance of these two phenomena is, I believe, useful to the animal, because, in raising the hair or feathers, the stratum of air enclosed by these appendages is increased, the loss of heat in this way diminished, and the cooling of the skin prevented. It is, perhaps, for this reason that horses, dogs, cats, and birds ruffle their hair or feathers when cold.[27]