We have seen that though during sleep the operations of the senses are entirely suspended as regards the effects of ordinary impressions, the purely animal functions of the body continue in action. The heart beats, the lungs respire, the stomach, the intestines, and their accessory organs digest, the skin exhales vapor, and the kidneys secrete urine. With the central nervous system, however, the case is very different; for while some parts retain the property of receiving impressions or developing ideas, others have their actions diminished, exalted, perverted, or altogether arrested. In the first place, there is, undoubtedly, during sleep, a general torpor of the sensorium, which prevents the appreciation of the ordinary excitations made upon the organs of the special senses. So far as the nerves themselves are concerned, there is no loss of their irritability or conducting power, and the impressions made upon them are, accordingly, perfectly well conveyed to the brain. The suspension of the operations of the senses is not therefore due to any loss of function in the optic nerve, the auditory nerve, the olfactory nerve, the gustatory nerve, or the cranial or spinal nerves concerned in the sense of touch, but solely to the inability of the brain to take cognizance of the impressions conveyed to it. In regard to the cause of this torpor, I have given my views in a previous chapter. Now it must not be supposed that because mild excitations transmitted by the nerves of the special senses are incapable of making themselves felt, that therefore the brain is in a state of complete repose throughout all its parts. So far from such a condition existing, there are very decided proofs that several faculties are exercised to a degree almost equaling that reached during wakefulness, and we know that if the irritations made upon the senses be sufficiently strong, the brain _does_ appreciate them, and the sleep is broken. This ability to be readily roused through the senses constitutes one of the main differences between sleep and stupor, upon which stress has been already laid.