4.  An example of the fourth circumstance above mentioned, where both the primary and secondary parts of a train of motions proceed with energy less than natural, may be observed in the dyspnoea, which occurs in going into a very cold bath, and which has been described and explained in Sect.  XXXII. 3. 2.

And by the increased debility of the pulsations of the heart and arteries during the operation of an emetic.  Secondly, from the slowness and intermission of the pulsations of the heart from the incessant efforts to vomit occasioned by an overdose of digitalis.  And thirdly, from the total stoppage of the motions of the heart, or death, in consequence of the torpor of the stomach, when affected with the commencement or cold paroxysm of the gout.  See Sect.  XXV. 17.

II. 1.  The primary and secondary parts of the trains of sensitive association reciprocally affect each other in different manners. 1.  The increased sensation of the primary part may cease, when that of the secondary part commences. 2.  The increased action of the primary part may cease, when that of the secondary part commences. 3.  The primary part may have increased sensation, and the secondary part increased action. 4.  The primary part may have increased action, and the secondary part increased sensation.

Examples of the first mode, where the increased sensation of the primary part of a train of sensitive association ceases, when that of the secondary part commences, are not unfrequent; as this is the general origin of those pains, which continue some time without being attended with inflammation, such as the pain at the pit of the stomach from a stone at the neck of the gall-bladder, and the pain of strangury in the glans penis from a stone at the neck of the urinary bladder.  In both these cases the part, which is affected secondarily, is believed to be much more sensible than the part primarily affected, as described in the catalogue of diseases, Class II. 1. 1. 11. and IV. 2. 2. 2. and IV. 2. 2. 4.

The hemicrania, or nervous headach, as it is called, when it originates from a decaying tooth, is another disease of this kind; as the pain of the carious tooth always ceases, when the pain over one eye and temple commences.  And it is probable, that the violent pains, which induce convulsions in painful epilepsies, are produced in the same manner, from a more sensible part sympathizing with a diseased one of less sensibility.  See Catalogue of Diseases, Class IV. 2. 2. 8. and III. 1. 1. 6.

The last tooth, or dens sapientiae, of the upper jaw most frequently decays first, and is liable to produce pain over the eye and temple of that side.  The last tooth of the under-jaw is also liable to produce a similar hemicrania, when it begins to decay.  When a tooth in the upper-jaw is the cause of the headach, a slighter pain is sometimes perceived on the cheek-bone.  And when a tooth in the lower-jaw is the cause of headach, a pain sometimes affects the tendons of the muscles of the neck, which are attached near the jaws.  But the clavus hystericus, or pain about the middle of the parietal bone on one side of the head, I have seen produced by the second of the molares, or grinders, of the under-jaw; of which I shall relate the following case.  See Class IV. 2. 2. 8.