In the great majority of cases the blow was applied to the temporal region, the patient was rendered unconscious, bleeding freely from the ear, nose, and mouth, and remained under treatment for some considerable time. From these facts it may be assumed that the patient suffered from a fracture of the base of the skull--anterior or middle fossa, or both fossæ. Evidence has already been brought forward to show (1) that the majority of fractures involving both anterior and middle fossæ traverse the body of the sphenoid, and (2) that the cavernous sinus is very susceptible to injury. It was also demonstrated that the internal carotid artery is only liable to injury in that part of its intracranial course where it is embedded in the outer wall of the cavernous sinus. Two instances of such a lesion have been described in the section dealing with injuries to intracranial vessels (see p. 148, and Figs. 53 and 54). From these facts and from the post-mortem evidence supplied by the few cases that have come to autopsy, it may, I think, be concluded (1) that traumatic orbital aneurysm implies a condition of arterio-venous aneurysm with direct communication between the internal carotid artery and the cavernous venous sinus, and (2) that the aneurysmal formation is dependent on a basic fracture which traverses the region of the body of the sphenoid bone. The exact nature of the communication between the two vascular channels may be estimated from the fact that proptosis--usually the first symptom to make its appearance--seldom becomes apparent till one or two months have elapsed after the accident. It would appear, therefore, probable, that the artery is not ruptured but merely weakened at the time of the injury, the weakened arterial wall gradually giving way before the force of the blood-stream till a definite communication is formed between the two channels. The male sex, by reason of its greater liability to severe injuries, is generally the one involved, especially between the ages of 20 and 30. =Symptomatology.= 1. _Proptosis_--usually the first symptom to develop--seldom makes its appearance till after the lapse of one to two months. The eyeball is protruded in the downward and outward direction, the main pulsating tumour presenting at the upper and inner quadrant of the orbit. Proptosis is generally steadily progressive, and, in the most marked cases, the globe may be almost pushed out of its socket. In any case the ‘whites’ of the eye are unduly prominent.