5. The room should be dark; light, like sound, is inimical to sleep. The head should be moderately raised. The temperature of the room for robust persons need not be artificially raised. =Sleeplessness=, as a rule, occurs only when some physiological law has been broken. To relieve it, it is essential to equilibrate muscular and mental functions. Increase of muscular exercise is an important element in its treatment. In addition it is advisable not to have any severe mental work during the evening, nor to indulge in late suppers. Sleeplessness is the bane of many men of a nervous temperament, and chiefly attacks those of sedentary habits. It is apt to recur, and for this reason, if for no other, narcotics ought to be scrupulously avoided. The habit of taking such soporifics is unfortunately becoming much more common, and is productive of many evils. Death from accidental overdose is a frequent calamity; and, apart from this possibility, the invalid’s nervous system is completely ruined by persistence in the habit, his power of will is annihilated, and he becomes the miserable slave of an evil habit, whose end is death (see also page 54). CHAPTER XXXIX. PERSONAL HYGIENE (_continued_)—CLEANLINESS. =Physiological Considerations.=—The skin consists of a superficial part or epidermis, and a deeper part called the dermis or cutis. Tubes of two kinds open on the surface of the skin, penetrating at their deeper ends into the cutis, viz. sweat or _sudoriparous glands_ and _sebaceous glands_. The sudoriparous glands are simple tubes, the lower ends of which lie coiled up in the dermis. Each tube when straightened out is about a quarter of an inch long. It has been estimated that in the palm of the hand there are 3,528 orifices of sudoriparous and sebaceous glands on a square inch of surface; reckoning each gland at 1∕4 inch long, this means 73½ feet of tubes in this small space. Assuming that there are 2,800 tubes to every square inch, and that the amount of surface in a man of ordinary height and bulk is 2,500 square inches, it follows that there are seven million pores in a man—that is, 1,750,000 inches, or nearly twenty-eight miles. The perspiration secreted by the sudoriparous glands is constantly evaporating from the surface of the body. It is very important that the orifices of these glands should be kept open in order that the secretion may not be interfered with. Animals have been killed by covering their skin with gelatine, and so preventing the escape of perspiration. The sebaceous glands are shorter than the sudoriparous, and commonly end alongside the hairs before the latter issue from the skin. They secrete an oily material which serves the purpose of a natural pomade. The sebaceous secretion also keeps the general surface of the skin unctuous and supple. The smell of the sebaceous secretion may be unpleasant, especially in the arm-pits and some other parts. Frequent washing is therefore desirable.