This is rare, however. At other times its form is very irregular, being most prominent at the part where the accumulation of the blood is least resisted. Pulsation in the tumour is distinct from the first, and is painful to the patient; and in the external aneurisms it is so strong as to be perceived by a bystander at a considerable distance. The tumour is at first compressible, and completely disappears on firm pressure being applied, either directly to the sac, or to the artery above, the sac being thereby emptied of its contents, or prevented from being filled. It may sometimes be difficult to form an accurate diagnosis, from the circumstance that tumours, not aneurismal, receive a pulsatory movement from an artery or from arteries immediately beneath them; such difficulty is obviated by attention to this simple test—that in an aneurism the pulsation is felt equally in all directions. Besides, if the tumour is moveable, it can be partially displaced, so as not to lie immediately over a large artery, and, if it be not aneurismal, it will then be found to possess no pulsation; if it be an aneurism, its pulsation will not be diminished by any change of position. The blood contained within the aneurismal sac, being comparatively motionless, coagulates, and the coagulum is attached to the inner surface; at first it contains red globules, but it afterwards loses them, and becomes of a pale hue, consisting solely of fibrin. This coating gradually increases, and attains no small thickness, fresh portions of fibrin being superadded in concentric laminæ. These layers are chiefly deposited from the blood within the cavity, but they also appear to receive addition from lymph being effused by the vessels proper to the original parietes of the tumour. By such thickening, it can be easily conceived that the pulsation will be somewhat lessened. In large aneurisms the accumulation and deposit of fibrin may be much greater at some points than at others, and hence pulsation may be rendered “not equal in all directions.” It is not, however, diminished to any great extent; for absorption of one or more points occurs, and the coating is again attenuated.