=276.= The nature of sensation is more particularly described in the case of sight and hearing. In the first case there proceed from the eyes rays, which cause tension in the air, reaching towards the object seen[68]; this tension is cone-shaped, and as the distance from the pupil of the eye increases, the base of the cone is increased in size, whilst the vigour of the sight diminishes. This human activity effects vision of itself in one case; for we say ‘darkness is visible,’ when the eye shoots forth light at it, and correctly recognises that it is darkness[69]. But in complete vision there is an opposing wave-motion coming from the object, and the two waves become mutually absorbed: hence Posidonius called sight ‘absorption’ (σύμφυσις)[70]. Similarly, in the case of hearing, the pulsation (which, as we have seen, comes in the first instance from the principate) spreads from the ear to the speaker, and (as is now more distinctly specified) from the speaker to the hearer; this reverse pulsation being circular in shape, like the waves excited on the surface of a lake by throwing a stone into the water[71]. Of the sensations of smell, taste and touch we only hear that they are respectively (i) a spirit extending from the principate to the nostrils, (ii) a spirit extending from the principate to the tongue, and (iii) a spirit extending to the surface of the body and resulting in the easily-appreciated touch of an object[72]. [Sidenote: Other activities.] =277.= The Stoic account of the functions of the soul displayed in the ordinary activities of life is either defective or mutilated; for even a slight outline of the subject should surely include at least breathing, eating (with drinking), speech, walking, and lifting. We need not however doubt that these, equally with the five senses, are all ‘spirits stretching from the principate’ to the bodily organs. This is expressly stated of walking[73]. Of all such activities we must consider voice to be typical, when it is described as the sixth function of the soul. Voice is described as ‘pulsating air[74],’ set in motion by the tongue[75]; but we can trace it back through the throat to some source below, which we can without difficulty identify with the heart, the seat of the principate[76]. The voice is indeed in a special relationship to the principate, since the spoken word is but another aspect of the thought which is expressed by it[77]. [Sidenote: Procreation.]