112 THE CAT. [CHAP. iv. 



The shaft of the tibia is generally triangular in section, being 

 produced into a sharp crest (c) in front, on the internal side of which 

 crest the bone is more or less flattened, while it is strongly coacave 

 on its outer side. At the upper end of the front of the shaft is a 

 prominence caUed the median tiibet'osity (t), or tuber ck, which 

 exhibits a flattened surface for the attachment of the patella. 



The lower end of the bone is smaller than the upper. Its lower 

 border has a single wide, vertical groove behind for the tendon of 

 the flexor longus hallucis muscle. Its outer surface is flattened (/) 

 for the reception of the fibula. Its inner margin is produced down- 

 wards into a strongly marked triangular process, called the internal 

 malleolus (im). This forms the bony projection on the inside of the 

 ankle, and articulates with the inner side of the tarsus. The 

 posterior part of the non- articular, or free, surface of the internal 

 malleolus, shows two small strongly marked vertical grooves, the 

 anterior of which transmits the tendon of the tibialis posticus muscle, 

 while the more posterior and outer is traversed by that of the flexor 

 longus digitorum muscle. 



The lower end of the tibia presents an irregular and undulating 

 articular surface, corresponding with the surface of the tarsus, which 

 it joins. This surface presents a median ridge running obliquely 

 backwards and inwards from its front margin, and fitting into the 

 groove on the upper surface of the astragalus. On the outer side of 

 the ridge is a rather wide articular surface (os), which slopes upwards 

 and outwards ; on its inner side is a more concave, but less inclined 

 surface (es), which becomes continuous with the articular surface borne 

 by the internal malleolus. The hinder margin of the articular sur- 

 face of the tibia (when the bone is vertical) descends sKghtly below 

 its front border, which nevertheless exhibits a descending process (p) 

 which corresponds with the front end of the median ridge just 

 described. 



§ 16. The FIBULA is the slenderest bone, in proportion to its 

 length, in the body, and extends on the outer side of the leg from 

 close to the knee down to the ankle. 



Its upper extremity is slightly enlarged into what is called the 

 head, which articulates with the outer side of the head of the tibia, 

 and gives insertion to the external lateral ligament. It docs not 

 mount upwards so high as does the tibia, and its outer surface is 

 concave. 



The shaft of the bone is irregularly triangular in section. Its 

 lower end is expanded into what is called the crtcrnal malleolus, 

 which forms the bony projection on the outer side of the ankle, and 

 articulates with the outer side of the tarsus. 



This malleolus docs not project downwards so far as does the 

 internal malleolus. On its inner side it articulates anteriorly with 

 the outer side of the lower end of the tibia (as already mentioned) 

 below and behind this it articulates with the outer side of the 

 astragalus. The external malleolus is deeply grooved behind for 

 the passage of the tendon of the 2Jeroncus brcvis muscle, while that 



