The parts of the brain directly pressed upon become anæmic, while the other parts become congested, and the nutrition of the whole brain is thus seriously interfered with. Different parts of the brain and cord show varying powers of resistance to this circulatory disturbance. The cortex is the least resistant part, and next in order follow the corona radiata, the grey matter of the spinal cord, the pons, and, last, the medulla oblongata. Hence it is that the respiratory and cardiac centres hold out longest. _Depressed Bone as a Cause of Compression._--It is more than doubtful whether a depressed portion of bone is of itself capable of inducing symptoms of compression of the brain. When such symptoms accompany depressed fracture, they are to be attributed either to associated hæmorrhage, or to interference with the circulation and consequent œdema which the displaced bone produces. Fragments of bone may, however, aggravate the symptoms by irritating the cerebral tissue on which they impinge. _Foreign Bodies._--The rôle of foreign bodies, such as bullets, in the production of compression symptoms is similar to that of depressed bone. That foreign bodies of themselves are not a cause of compression seems evident from the fact that it is not uncommon for them to become permanently embedded in the brain substance without inducing any symptoms. Not only have bullets, the points of sharp instruments, and other substances remained embedded in the brain for years without doing harm, but in many cases the patients have continued to occupy important and responsible positions in life. _Differential Diagnosis._--It not infrequently happens that a patient is found in an insensible condition under circumstances which give no clue to the cause of his unconsciousness. He is usually removed to the nearest hospital, and the house-surgeon under whose charge he comes must exercise the greatest care and discretion in dealing with him. In attempting to arrive at the cause of the condition, numerous possibilities have to be borne in mind, but it is often impossible to make a definite diagnosis. The chief of these causes are trauma, apoplexy or cerebral embolism, epileptic coma, alcohol and opium poisoning, uræmic and diabetic coma, sunstroke, and exposure to cold. The commonest error is to mistake a case of cerebral compression for one of drunkenness. It is scarcely necessary to say that a man who smells of alcohol is not necessarily intoxicated; the drink may have been given with the object of reviving him. It may be that one or other of the above-named conditions has caused the patient to fall, and in his fall he has incidentally sustained an injury to the head, which, however, is in no way responsible for his unconsciousness. Whenever there is the least doubt, therefore, the patient should be admitted to hospital.