The influence of poisons on the brain seems to be sometimes induced, not immediately, but indirectly through the intervention of a more direct influence on the pulmonary circulation. Thus Mr. Blake appears to have succeeded in proving that the insensibility and tetanic convulsions which immediately precede death, when certain substances, such as the salts of soda, are injected into the veins, depend simply on the obstruction directly produced in the pulmonary circulation causing increased pressure in the systemic veins, and consequently upon the brain and nervous centre generally. For when the jugular vein was opened after the development of tetanic convulsions, and blood was allowed to flow out, the nervous symptoms ceased, and the animal continued for two hours sensible and without any return of convulsions, dying eventually of hemorrhage.[45] But more generally the effect produced on the brain is direct and specific. Thus opium and its active principle morphia suspend the functions of external relation, which are peculiarly dependent on the brain; while for a long time the respiration and circulation are little affected. Even when the poison is admitted directly into the veins, the pulmonary capillaries are not obstructed, and the heart is only somewhat enfeebled in its contractions;[46] and in ordinary cases of poisoning with these substances the heart continues to pulsate, and the lungs also discharge their office, long after sensibility is extinguished and voluntary motion arrested,—until at length the circulation and respiration become affected consecutively by the depressed state of the nervous system.