Chap. 9 PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 153 



off from the other toes and face about with its sole side toward them. It cannot 

 act like a thumb. Compared with the importance of toes in other mammals, 

 that of the human toes is lessening. 



Skull. The human skull is a group of bones (22) that forms the house of 

 the brain. It holds most of the sense organs, the gateways to the brain, and 

 the entrance way for food. The skull is divided into the cranium, holding the 

 brain and the face with the eyes, nose, and ears arranged around the mouth. 

 In man the cranium is large in proportion to the face; in a frog the cranium is 

 relatively small and the face large. The uniquely human features of the skull 

 are the rounded dome of the cranium and the chin (Fig. 9.15) ; the latter was 

 not well developed in primitive man nor is it now in infants. 



The 22 bones of the adult skull include a number that are fused together. 

 In the newborn infant even the main immovable joints of the cranium have 

 not closed and there are six spaces or fontanelles where the hard matter of the 

 bone has not been formed. At birth the edges of these bones overlap as the 

 baby's head is squeezed through the pelvic girdle. The skeleton of the human 

 face is comparatively light in weight because it is so full of cavities. The promi- 

 nent openings of eye sockets, nostrils, and mouth occupy a goodly area and 

 there are also extensive cavities (sinuses) within certain bones (frontal, 

 ethmoid, sphenoid, maxillary), all of which open by small passages into the 

 nasal chambers. Painful inflammation of the lining of the sinuses commonly 

 originates with colds and congestion in the nasal chambers and spreads 

 through the passageways that open into them. 



Teeth. Teeth are actually outgrowths of the integument or skin tissues and 

 their ancestry goes back to the scalelike structures which develop about the 

 mouths of sharks and other fishes. They are discussed with the intake of food 

 and mechanical digestion, their main functions (Chap. 11). 



Broken Bones and Dislocated Joints 



These are common disorders of the skeleton (Fig. 9.17). Breaks or frac- 

 tures are either simple, in which the skin is unbroken, or compound, if jagged, 

 broken ends of bone protrude outside the flesh. With any fracture nerves and 

 blood vessels are broken and there is pain and bleeding, the latter often within 

 the flesh. In treating a break the bones are first put back into normal position. 

 This is known as reduction. As a broken bone heals bone-forming cells, mostly 

 from the newly formed fibrocartilage in which bone regenerates, gradually 

 grow into the area surrounding the break. Limy salts characteristic of bone are 

 deposited in an enlargement, a callus, that is later resorbed. 



Sprains are due to the wrenching or twisting of ligaments that bind bones 

 together at a joint. Severe ones may tear the ligaments and even the periosteum 

 of the bone, but even moderate ones disturb nerves and blood vessels. 



