To this is due the selective action of many poisons.

Morphine, for example, acts chiefly on the cells of the brain; strychnine acts on the cells of the spinal cord which excite motion and thus causes the characteristic muscular spasm.

The poisonous substances produced by bacteria, as in the case of diphtheria, act on certain of the organs only.

Different animal species owe their immunity to certain poisons to their cells being so constituted that a poison cannot gain entrance into them; pigeons, for example, cannot be poisoned by morphia.

Individual variations play an important part also; thus, shellfish are poisonous for certain individuals and not so for others.