When the active process runs smoothly and uninterruptedly, these bodily activities [circulation and respiration] progress with rhythmic regularity.  Relatively tense, strained attention is generally characterized by more vigorous bodily accompaniments than is low-level, gentle, and relatively relaxed attention (drowsiness, for instance); but both agree, so long as their progress is free and unimpeded, in relative regularity of bodily functions.  Breaks, shocks, and mal-co-ordinations of attention are accompanied by sudden, spasmodic changes and irregularities in bodily processes, the amount and violence of such changes being roughly proportioned to the intensity of the experiences.

Now, emotions represent psychological conditions of great instability.  Especially is this true when the emotion is profound.  The necessity is suddenly thrown upon the organism of reacting to a situation with which it is at the moment able to cope only imperfectly, if at all.  The condition is one in which normal, uninterrupted, coordinated movements are for a time checked and thrown out of gear.[241]

And again, in concluding their admirable study: 

All the processes with which we have been dealing are cases of readjustment of an organism to its environment.  Attention is always occupied with the point in consciousness at which the readjustment is taking place.  If the process of readjustment goes smoothly and evenly, we have a steady strain of attention—­an equilibrated motion in one direction.  The performance of mental calculation is a typical case of this sort of attention.  But often the readjustment is more difficult.  Factors are introduced which at first refuse to be reconciled with the rest of the conscious content.  The attentive equilibrium is upset, and there are violent shifts back and forth as it seeks to recover itself.  These are the cases of violent emotion.  Between these two extremes comes every shade of difficulty in the readjustment, and of consequent intensity in emotional tone.  We have attempted to show in the preceding paper that the readjustment of organism to environment involves a maintenance of the equilibrium of the bodily processes, which runs parallel with the maintenance of the attentive equilibrium, and is an essential part of the readjustment of the psycho-physical organism.

The more motile organisms are constantly, by very reason of their motility, encountering situations which put a strain upon the attention.  The quest for food leads to encounters with members of their own and of different species; the resulting fight, pursuit, and flight are accompanied by the powerful emotions of anger and fear.  The emotion is, as Darwin has pointed out, a part of the effort to reaccommodate, since it is a physiological preparation for action appropriate to the type of situation in question.[242] The strain upon the attention, the affective bodily condition, and the motor activity appear usually in the same connection, and, from the standpoint of biological design, the action concluding the series of bodily activities is of advantage to the organism.