206 



THE SKELETON 



provision is according!}^ made, by the addition of a strong calcaneo-navicular liga- 

 ment, for the support of the head of the talus. This ligament is in turn supported 

 by its union with the deltoid ligament of the ankle, and by the tendon of the 

 tibialis posterior which passes beneath it to its insertion. 



Besides being arched longitudinally, the foot presents a transverse arch formed 

 by the metatarsal bones in front and the distal row of the tarsus behind. It is 

 produced by the marked elevation of the central portion of the medial longitudinal 

 arch above the ground, whereas the lateral longitudinal arch is much less raised, 

 and at its anterior end becomes almost horizontal. Both the longitudinal and 

 transverse arches serve the double purpose of increasing the strength and elasticity 

 of the foot and of providing a hollow in which the muscles, nerves, and vessels of 

 the sole may lie protected from pressure. 



Homology of the Bones of the Limbs 



That there is a general correspondence in the plan of construction of the two extremities is 

 apparent to a superficial observer, and this becomes more marked when a detailed examination 

 of the individual bones, their forms and relations, their embryonic and adult peculiarities, is 

 systematically carried out. In each limb there are four segments, the shoulder girdle corre- 

 sponding to the pelvic girdle, the arm to the thigh, the forearm to the leg, and the hand to the 

 foot. These parts have been variously modified, in adaptation to the different functions of the 

 two limbs, particularly as regards the deviations or changes from what is regarded as their primi- 



FiG. 253. — Diagrammatic Representation of the Bones of the Two Limbs, 

 SHOW Homologous Parts. (Modified from Flower.) 



Infiaspinous fossa 



Subscapular fossa 



Greater tuberosity 

 Humerus 



Lateral 

 ondyle Radius 



Iliac surface 



P' 



Lesser Lesser Medial Ulna 



trochanter tuberosity epicondyle 

 Femur 





"^< 



'C3 -57 



Medial condyle 



Tibia 



ISCHIUM 



Great trochanter Lateral condyle 



Fibula 



tive position, and as a knowledge of these changes is essential to a clear understanding of the 

 homologous bones, it will be advantageous to refer briefly to the relations of the limbs in the 

 earliest stages of development. 



The limbs first appear as flattened, bud-like outgrowths from the sides of the trunk. Each 

 [)re.scnts a dorsal or extensor surface, and a venlral or flexor surface, as well as two borders, 

 an anterior, or cephalic, directed toward the head end of the embryo, and a posterior or caudal, 

 directed toward the tail end. In reference to the axis of the limb itself, the borders have 

 been culled pre-axial and post-axial, respectively. When, somewhat later, the various divisions 

 of the limb rnakc their af)pcarance, it is seen that the greater tuberosity, tlie lateral epicondyle, 

 the radius, and the thumb lie on the pre-axial border of the anterior extremity, and the small 

 trochanter, the medial condyle, the tibia, and the great toe on the ijre-axial border of the 

 posterior extremity. Further on the post-axial border of the anterior extremity are seen the 

 lesser tuberosity, the medial epicondyle, the ulna, and little finger, whilst on the corresponding 

 border of the i)o.stcrior limb are the great tro(;lianter, the lateral condyle, the fibula, and the 

 little toe. The i)arts now enumerated on the corresponding borders of the two limbs must 

 therefore be regarded as serially homologous (fig. 2.'33). 



