52 THE INDIVIDUAL ORGANISM 



soles of the feet, and on the forehead. These glands have separate open- 

 ings on the skin surface, the pores, there being as many as 2^ million 

 of these on the entire body. Sweat is, in a sense, a waste product, and 

 sweating plays a part in the elimination of body wastes; but a much 

 more important function of sweating is the control of body heat through 

 the evaporation of water. 



We have seen that body heat is largely produced by the muscles. This 

 heat is taken up by the blood and distributed throughout the body, which 

 has a normal temperature of approximately 98 to 99°F. The skin, with 

 its abundant blood supply, is so situated that it acts as an effective heat- 

 eliminating surface. Contraction of the muscle fibers of the dermis and 

 of the walls of its blood vessels reduces the amount of warm blood 

 entering the capillaries and there exposed to loss of heat; relaxa- 

 tion of the muscle fibers and dilation of the blood vessels allow a more 

 rapid loss. In addition to the considerable loss of heat by radiation, the 

 skin is cooled by the evaporation 1 of a watery fluid, sweat, from the body 

 surface; the amount of cooling is varied as required, by changes in the 

 activity of the sweat glands. 



Teeth. A conspicuous feature of the skull is the double row of teeth 

 imbedded in the upper and lower jawbones, similar to 'bones in their 

 hardness and appearance. Structurally, however, teeth are a part of the 

 skin and are more comparable to such skin appendages as nails and hair 

 than to the bones of the skeleton. Their relation to the skull is entirely 

 secondary, their implantation in the jawbones being merely an arrange- 

 ment for strength and support. Teeth, being used for the breaking up 

 of food materials in the process of chewing, are functionally most closely 

 associated with the digestive system. 



Like other mammals, man has two sets of teeth. The temporary first 

 set, the deciduous or "baby" teeth, are 20 in number. They are "cut" 

 (or erupted) between the ages of six months and two and one-half years 

 and are gradually replaced by the teeth of the second or permanent set. 

 The latter are normally 32 in number and comprise, on each side of each 

 jaw, 2 incisors, 1 canine (cuspid or "eyetooth"), 2 premolars (bicuspids), 

 and 3 molars. The incisors are the "front" teeth — flat, edged, used for 

 cutting off morsels of food ; the canines are heavier and more pointed and 

 correspond to the tearing and stabbing "tusks" of carnivorous mam- 

 mals; the premolars are broad-surfaced grinding teeth with two elevated 

 cones on the grinding surface; and the molars are larger grinding teeth 

 with four (sometimes five) elevated cones on the grinding surface. The 

 last pairs of molars are the so-called "wisdom teeth," which in man are 



1 It is this large loss of heat by evaporation that permits the human body to main- 

 tain its normal temperature, even though the air temperature may be well above 

 100°F. 



