382 



ORGAN SYSTEMS OF MAN 



lent freedom of movement. For this reason 

 the hip joint is not nearly so apt to dislocate 

 as the shoulder joint. The leg can circum- 

 scribe a narrow cone but not a wide one 

 like the arm. The leg is primarily concerned 

 with the business of carrying the body for- 

 ward in progression and consequently is 

 constructed to function essentially in a for- 

 ward and backward motion. 



At the distal end (end farther from the 

 body) the femur flattens out, forming a 

 hinge with one of the two lower leg bones, 

 the tibia or shin bone. The other lower leg 

 bone is the fibula, which is smaller and lies 

 on the outside of the leg. Together with the 

 tibia it affords a point of contact, in tvirn, 

 with one of the two large ankle bones. The 

 other forms the heel. These two, together 

 with the metatarsals and phalanges, form 

 the foot. This part of the skeleton is man's 

 contact with the ground and is a very im- 

 portant part of his anatomy. When some- 

 thing goes wrong here he is practically 

 helpless. 



There are two arches in the foot, longi- 

 tudinal and transverse, which are primarily 

 supported by stretched tendons that come 

 from muscles in the lower leg. Being always 

 under tension, they possess a resilience that 

 puts a "spring in one's step" and they also 

 take away the shock from sudden contact 

 with the substratum. Flat feet may be 

 caused by undue stress such as comes from 

 overweight or they may be inherited. Such 

 dislocation of the bones of the feet may 

 cause considerable pain and make normal 

 walking difficult. 



We have considered in some detail the 

 arrangement of the structural units of the 

 vertebrate skeleton. Let us now examine 

 the composition of these units. 



The composition of bone 



If a long bone like a femur is cut in cross- 

 section, it will be found to be hollow with 

 a soft spongy material, the marrow, occupy- 

 ing the cavity (Fig. 4-4). The outer por- 



tion is very hard and resists breaking. The 

 tubular nature of the bone makes it even 

 stronger than a solid piece of equal weight; 

 to understand this, one has only to compare 

 solid and tubular rods of steel with respect 

 to strength where bending and twisting is 

 concerned. The hard part of bone is com- 

 posed of calcium carbonate, or lime, and 

 potassium phosphate, as well as an organic 

 matrix which resembles cartilage. This can 

 easily be demonstrated by placing the bone 

 in an acid solution which dissolves out the 

 minerals, leaving the matrix. Although the 

 bone still retains its original shape it is very 

 soft and pliable and as such could certainly 

 be of no use to an animal. On the other 

 hand, the organic matrix can be removed 

 by heating the bone for some time so that 

 only the minerals are left. Such a bone also 

 retains its original shape but if disturbed 

 crumbles into ashes. Again a bone of this 

 composition would be of no use to an ani- 

 mal. Minerals and matrix taken together, 

 then, are necessary to produce satisfac- 

 tory material of which to construct skeletal 

 units. 



Bone growth 



It is obvious that the bones of a child, 

 while fully formed and quite solid, must in- 

 crease both in length and diameter as 

 growth occurs. This is accomplished by a 

 rather elaborate bone-destroying and bone- 

 building process going on within the bone 

 itself. The bone is covered on the outside 

 by a thin cellular membrane, the perios- 

 teum, which has to do with the increase in 

 the diameter of the bone. At the ends, 

 called the epiphyses (singular — epiphysis), 

 there is also active cellular growth which 

 causes the increase in length. As bone is 

 produced by both periosteum and epiphys- 

 eal cells, a simultaneous bone destruction 

 is going on within the marrow cavity. In 

 other words, as the bone cells produce bone 

 on the outside and at the ends of the bone, 

 similar cells are destroying bone on the 



