CRANIOLOGY.
The consideration of the various men tal powers belongs to the science of me taphysics, and will be pursued under the proper articles.
Sleeping and Watching. Sleep is the repose of the organs of sense, and of the voluntary motions, by means of which the communication of the senses with exter nal objects is interrupted. It is the re sult of that law, which subjects the ac tions of the exterior or animal life to pe riods of intermittence. The most perfect sleep is that in which all the functions of this class are suspended, as the sensa tions, perception, imagination, memory, judgment, locomotion, and the voice ; the least perfect affects only a single organ. Between these extremes, every gradation may occur ; and, from the partial suspen sion of some functions, while others are going on, arise dreams, and the various phenomena of somnambulism. It is, however, the same principle, whether observed in the relaxation that follows the contraction of a voluntary muscle, or in the entire suspension of the animal life.
Watching may be considered as a state of considerable effort and expenditure of the sensitive and moving principle, by the organs of our sensations and mo tions. This principle would soon have been exhausted, if its reparation were not facilitated by long intervals of rest. Sleep and watching, therefore, call for each other, and are of reciprocal neces sity.
Sleep, however, only suspends that por tion of life, the design of which is to main tain a commerce with external objects necessary to our existence. The interior, or assimilating, functions are still carried on. Digestion, absorption, circulation, respiration, secretion, and nutrition, are continued: the two former, indeed, seem to be performed with greater energy, while the rest are rather diminished. The pulse is slower, respiration less frequent, perspiration and urinary secretion less abundant.
Numerous causes of excitation con stantly acting on our senses during the day, keep them in a state of activity ; and the absence of these at night is favour able to the repose of our organs. By
multiplying and increasing stimuli, the period of repose may be put off; but these gradually lose their powers, and after a certain time, nothing can hinder its ap proach. Exhausted by fatigue and watch ing, the soldier sleeps at the side of the cannon ; the slave reposes under the blows of his master ; and the criminal sinks to rest amidst the agonies of tor ture.
Sympathy. All parts of the living body are united by certain relative connections, namely, sympathies, which establish a concord and harmony between the actions of the animal machine. The nature of this phenomenon is still obscure : we know not why, when one part is irritated, an other very distant organ should perceive this irritation, and even contract ; nor are we agreed on the peculiar instruments of sympathy, that is, on the organs which connect two parts, one of which perceives or acts while the other is affected. That the nerves cannot be considered as the exclusive means of it is obvious ; since muscles, supplied from the same source do not always sympathise, while a close intercourse sometimes subsists between parts whose nerves have no immediate connection. Often, also, the sympathy is not reciprocal. Examples of this princi ple may be seen in the swelling of the breasts from distension of the uterus ; itching of the nose from worms in the in testines, and of the glans penis from stone in the bladder ; contraction of the dia phragm from irritation of the pituitary membrane ; pain in the shoulder from in flamed liver, &c.
The chief, and perhaps most extensive source of sympathy, must be referred to the nerves, and particularly to a reaction of the sensorium. When a part is stimu lated, and the sensorium affected by its stimulation, the latter reacts through the nerves on another organ, and incites it to action, although there may be no imme diate nervous connection between them. The motion of the iris, arising from the impulse of light on the retina ; that of the diaphragm in sneezing, from irritation of the pituitary membrane, are examples.