n fails; during the
rise the medulla is again supplied with blood, and breathing is
resumed (Eyster).

The parts of the brain directly pressed upon become anaemic, 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
haemorrhage, or to interference with the circulation and consequent
oedema 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 role 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, uraemic and diabetic coma, sunstroke, and exposure to cold.
The commonest error is to mistake a case of cerebral compression fo