Many psychology textbooks refer to Charles Darwin's "The Expression of Emotion in Man and Animals". (The two I recently saw were "The Principles of Psychology" and "Handbook On Stress and Anxiety".) You may find a few references on the Web as well. Since most quotes use just a few sentences of Darwin's work I thought that I should read more. I did.

Herein is his complete description of fear and terror. (I left out only the descriptions of people so striction.)

People suffering from PTSD have many of these symptoms without ever knowing why, i.e. without encountering anything "scary" — they just suddenly feel this way during daily routines or even when alone or with friends, family or loved ones.

Darwin's book can be found online: Google Books and Project Gutenburg.

THE EXPRESSION OF EMOTION IN MAN AND ANIMALS

BY CHARLES DARWIN

1899

Fear, Terror

The word 'fear' seems to be derived from what is sudden and dangerous; and that of terror from the trembling of the vocal organs and body. I use the word 'terror' for extreme fear; but some writers think it ought to be confined to cases in which the imagination is more particularly concerned. Fear is often preceded by astonishment, and is so far akin to it, that both lead to the senses of sight and hearing being instantly aroused. In both cases the eyes and mouth are widely opened, and the eyebrows raised. The frightened man at first stands like a statue motionless and breathless, or crouches down as if instinctively to escape observation.

The heart beats quickly and violently, so that it palpitates or knocks against the ribs; but it is very doubtful whether it then works more efficiently than usual, so as to send a greater supply of blood to all parts of the body; for the skin instantly becomes pale, as during incipient faintness. This paleness of the surface, however, is probably in large part, or exclusively, due to the vasomotor centre being affected in such a manner as to cause the contraction of the small arteries of the skin. That the skin is much affected under the sense of great fear, we see in the marvellous and inexplicable manner in which perspiration immediately exudes from it. This exudation is all the more remarkable, as the surface is then cold, and hence the term a cold sweat; whereas, the sudorific glands are properly excited into action when the surface is heated. The hairs also on the skin stand erect; and the superficial muscles shiver. In connection with the disturbed action of the heart, the breathing is hurried. The salivary glands act imperfectly; the mouth becomes dry, and is often opened and shut. I have also noticed that under slight fear there is a strong tendency to yawn. One of the best-marked symptoms is the trembling of all the muscles of the body; and this is often first seen in the lips. From this cause, and from the dryness of the mouth, the voice becomes husky or indistinct, or may altogether fail. “Obstupui, steteruntque comae, et vox faucibus haesit.”

As fear increases into an agony of terror, we behold, as under all violent emotions, diversified results. The heart beats wildly, or may fail to act and faintness ensue; there is a death-like pallor; the breathing is laboured; the wings of the nostrils are wildly dilated; “there is a gasping and convulsive motion of the lips, a tremor on the hollow cheek, a gulping and catching of the throat;” the uncovered and protruding eyeballs are fixed on the object of terror; or they may roll restlessly from side to side, huc illuc volvens oculos totumque pererrat. The pupils are said to be enormously dilated. All the muscles of the body may become rigid, or may be thrown into convulsive movements. The hands are alternately clenched and opened, often with a twitching movement. The arms may be protruded, as if to avert some dreadful danger, or may be thrown wildly over the head. The Rev. Mr. Hagenauer has seen this latter action in a terrified Australian. In other cases there is a sudden and uncontrollable tendency to headlong flight; and so strong is this, that the boldest soldiers may be seized with a sudden panic.

As fear rises to an extreme pitch, the dreadful scream of terror is heard. Great beads of sweat stand on the skin. All the muscles of the body are relaxed. Utter prostration soon follows, and the mental powers fail. The intestines are affected.