4. Inhalation of air deficient in oxygen. _Symptoms of Asphyxia._--Divided into three stages. _First stage_: deep, frequent, and laboured respiration; the extraordinary muscles of respiration are called into play. _Second stage_: the inspiratory muscles are less active than expiratory, convulsions of nearly all the muscles of the body occur. _Third stage_: paralysis of respiratory centres, dilated pupils, loss of consciousness, absence of reflexes. Gasping inspirations with prolonged intervals precede dissolution. _Post-mortem Signs._--Engorgement of the pulmonary artery, the right cavities of the heart, and vent cavæ; but on the left side of the heart the cavities, together with the aorta and pulmonary veins, are either empty or contain but little blood. It must be remembered, however, that cases of asphyxia do sometimes occur where the cavities on each side of the heart are _empty, or nearly so_. This is the case in the syncopal asphyxia of some writers. If also the obstruction to respiration be imperfect, the circulation may be continued for some time, congestion of one or more of the internal organs being the result. The blood is dark-coloured, contains much CO₂, and the hæmoglobin is almost completely reduced. The blood coagulates slowly. =Coma.=--Insensibility ending in death. _Causes._--Concussion of the brain, cerebral hæmorrhage, embolism, thrombosis, tumour, depressed fracture of skull, meningitis, and serous effusions; effects of poisons such as opium, alcohol, ptomaines, arsenic, barium, oxalic and carbolic acids; in certain diseases of kidneys and liver, uræmia, cholæmia, acetonæmia, profound anæmia, _e.g._ pernicious, and as a terminal stage to acute or chronic diseases. _Symptoms of Coma._--Coma is generally preceded by stupor, from which the patient may be roused to a certain extent, but only temporarily. The reflexes in this stage may be exaggerated, and the power of swallowing fluids may be retained. When coma is present there is complete abolition of consciousness, sweating, the patient is powerless, the breathing stertorous. The temperature may vary according to the cause; normal or subnormal generally, it may rise in lesions of the pons Varolii. The pulse is generally full and bounding, the pupils dilated or contracted and insensitive to light, the conjunctival reflex absent. Mucus collects in the air-passages and causes “the death rattle,” and the breathing becomes more and more embarrassed and irregular. The reflexes are lost, and the sphincters relaxed. _Post-mortem Appearances._--Causal lesions are found in the brain or other organs; there is usually hyperæmia of the brain and spinal cord and their membranes, unless there be profound anæmia preceding death. The condition of the heart and lungs is not constant; the general appearances resemble those in death from asphyxia. Table giving the Diagnosis of Several Forms of Insensibility