_Treatment._--Little can be done after the air has actually entered the vein beyond endeavouring to maintain the heart's action by hypodermic injections of ether or strychnin and the application of mustard or hot cloths over the chest. The head at the same time should be lowered to prevent syncope. Attempts to withdraw the air by suction, and the employment of artificial respiration, have proved futile, and are, by some, considered dangerous. In a desperate case massage of the heart might be tried. THE NATURAL ARREST OF HÆMORRHAGE AND THE REPAIR OF BLOOD VESSELS #Primary Hæmorrhage.#--The term primary hæmorrhage is applied to the bleeding which follows immediately on the wounding of a blood vessel. The natural process by which such hæmorrhage is arrested varies with the character of the wound in the vessel and may be modified by accidental circumstances. (a) _Repair of completely divided Artery._--When an artery is _completely_ divided, the circular fibres of the muscular coat contract, so that the lumen of the cut ends is diminished, and at the same time each segment retracts within its sheath in virtue of the recoil of the elastic elements in its walls, the tunica intima curls up in the interior of the vessel, and the tunica externa collapses over the cut ends. The blood that escapes from the injured vessel fills the interstices of the tissues, and, coagulating, forms a clot which temporarily arrests the bleeding. That part of the clot which lies between the divided ends of the vessel and in the cellular tissue outside, is known as the _external clot_, while the portion which projects into the lumen of the vessel is known as the _internal clot_, and it usually extends as far as the nearest collateral branch. These processes constitute what is known as the _temporary arrest of hæmorrhage_, which, it will be observed, is effected by the contraction and retraction of the divided artery and by clotting. The _permanent arrest_ takes place by the transformation of the clot into scar tissue. The internal clot plays the most important part in the process; it becomes invaded by leucocytes and proliferating endothelial and connective-tissue cells, and new blood vessels permeate the mass, which is thus converted into granulation tissue. This is ultimately replaced by fibrous tissue, which permanently occludes the end of the vessel. Concurrently and by the same process the external clot is converted into scar tissue. If a divided artery is _ligated at its cut end_, the tension of the ligature is usually sufficient to rupture the inner and middle coats, which curl up within the lumen, the outer coat alone being held in the grasp of the ligature. An internal clot forms and, becoming organised, permanently occludes the vessel as above described. The ligature and the small portion of vessel beyond it are subsequently absorbed.