726 MAMMALIAN ANATOMY 



up to the knee-joint. It is very slender, except at the ends, which are 

 flattened and expanded. Its upper end articulates with the outer 

 tuberosity of the tibia ; its lower end articulates with the lower end 

 of the tibia and clasps the side of the astragalus. It presents a shaft 

 and two extremities. 



The proximal extremity, or Head, is compressed transversely. 

 When seen from the side it has the form of a pentagon. It is not 

 clearly denned by a neck from the shaft ; its surfaces and some of its 

 borders pass into the surfaces and borders of the shaft. It has three 

 surfaces, one on the upper, or proximal, end, and two lateral, which, 

 from their position when the bone is in place in the leg, are called 

 posterior-external and anterior-internal. 



The posterior-external surface (Fig. 554), as has been said, has 

 the outline of an irregular pentagon, limited by the four borders and 

 an imaginary line drawn transversely to the shaft. 



The anterior-inferior border is a continuation of the anterior- 

 internal border of the shaft. It is emarginate and directed upward, 

 outward, and forward, forming a right angle with the anterior-superior 

 border above. Its lower part is sharp, but above it is wider and more 

 rounded. 



The anterior-superior border is directed upward, backward, and 

 inward from the right angle which it makes with the anterior-inferior 

 border ; it forms with the superior border an angle somewhat greater 

 than a right angle. This angle is the most proximal point of the 

 entire bone. The border is short, straight, or slightly emarginate ; it 

 is rough and rounded. 



The superior border is the external edge of the proximal surface. 

 It is almost transverse to the long axis of the bone, passing downward 

 a little at its posterior end. It is longer than the other borders, and 

 emarginate, or incised, behind the middle. 



The posterior-superior border runs downward, backward, and out- 

 ward from the obtuse angle which it forms above with the superior 

 border, and ends below by joining the posterior-inferior border in an- 

 other obtuse angle. It is longer than the anterior-superior border but 

 shorter than the proximal border, and is straight but somewhat swollen. 



The posterior-inferior border is directed downward, inward, and 

 forward into the posterior-external border of the shaft. It is the 

 shortest of the borders, and is emarginate and roughened. 



