But it is to be observed that, even in the cover, there are some parts more fixed than others. Of all the senses none of these animals are divested. We have already explained what may be their sensation of feeling. What may be the nature of their smelling and taste we know not, but we are assured they all enjoy the sense of seeing, and perhaps that of hearing also. The senses may be considered, then, as another essential part of the animal economy, as well as the brain, from which sensation derives its origin. Even insects, which differ so much in the centre of the animal economy, have a part analogous to the brain, and its functions resemble those of other animals; and such as the oyster, which seems to be deprived of a brain, ought to be considered as only half-animated, and as filling up an intermediate space between the animal and the vegetable kingdoms. As the heart is the centre of the interior part of the animal, so is the brain the centre of the cover. In like manner as the heart, and all the interior parts, communicate with the brain and exterior cover, by means of the blood-vessels, the brain communicates with the heart, and with all the interior parts, by means of the nerves. This union appears to be intimate and reciprocal, and though of these two organs the functions are absolutely different, yet they can never be separated without the instant death of the animal. The heart and the whole interior part acts continually without interruption, and independent of any exterior cause; but the senses and exterior part act only by alternate intervals, when affected by external causes. Objects act upon the senses, the senses modify this action, and carry the impression modified into the brain, where it becomes what we term sensation. In consequence of this impression the brain acts on the nerves, and communicates the vibration it has received; and this vibration it is which produces progression, and all the other exterior actions of the body. Whenever a cause acts upon a body, we know that the body also acts upon the cause. Thus objects act upon animals by means of the senses, and animals act upon the object by its exterior movements. In general action is the cause, and re-action the effect.