Jill was presented to us after having been hit by a car. Although his left foreleg was not broken, it was limp and he could not move it. On close examination of his eyes, the left pupil was smaller than the right.
We suspected the car had hit his left shoulder violently pushing it backwards and in doing so, had damaged the radial nerve that supplies the left foreleg. The radial and other nerves leave the spinal cord in the lower neck region. With this type of injury, the roots of the nerves can be "pulled out" from the spinal cord.
One of these other nerves travels back up the neck to supply the eyeball with sympathetic (adrenalin-like) activity. This nerve, gives "fright/alert" action to the pupil and tones up the muscles that move the eye. In other words, it makes the pupil dilate e.g. if frightened or in shock.
If this nerve is damaged, there is loss of this sympathetic tone and the eyeball constricts. The eyeball muscles lose their tone and it recedes into the bony socket making the eyelids come slightly closer together and the 3rd eyelid to cover part of the eye.
Unfortunately, this type of injury is usually permanent as nerves have very poor healing properties, even if surgery is attempted to join them back. If there is no improvement over the next 2-3 weeks, amputation of the "dead" foreleg is the best option to prevent it becoming traumatized.