=Arrest in diastole.=--The arrest may be preceded by the contractions becoming weaker and weaker, or after the so-called heart peristalsis; or it may be preceded by a condition in which the auricle shows a different frequency to the ventricle. The final diastole may be the diastole of paralysis or the diastole of irritation. The diastole of irritation is produced by a stimulus of the inhibitory ganglia, and only occurs after poisoning by the muscarine group of poisons. This condition may be recognised by the fact that contraction may be excited by mechanical and electrical stimuli or by the application of atropine solution; the latter paralyses the inhibitory nervous centres, and therefore sets the mechanism going again. The diastole of paralysis is the most frequent form of death. It may readily be distinguished from the muscarine diastole; for, in muscarine diastole, the heart is full of blood and larger than normal; but in the paralytic form the heart is not fully extended, besides which, although, if normal blood replace that which is poisoned, the beats may be restored for a short time, the response is incomplete, and the end is the same; besides which, atropine does not restore the beats. The diastole of paralysis may depend on paralysis of the so-called excito-motor ganglia (as with iodal), or from paralysis of the muscular structure (as with copper). § 30. =The effect of poisons on the iris.=--Several poisons affect the pupil, causing either contraction or dilatation. The most suitable animal is the cat; the pupil of the cat readily showing either state. =Toxic myosis, or toxic contraction of the pupil.=--There are two forms of toxic myosis, one of which is central in its origin. In this form, should the poison be applied to the eye itself, no marked contraction follows; the poison must be swallowed or injected subcutaneously to produce an effect. The contraction remains until death. The contraction in such a case is considered to be due to a paralysis of the dilatation centre; it is a “_myosis paralytica centralis_;” the best example of this is the contraction of the pupil caused by morphine.