THE CALCANEUM 103 



Blood-supply. — The astragalus is supi:»lied l)y tlio dorsalis pedis artery and 

 its tarsal l)rancli. 



Ossification. — The astragalus is ossified from one, oeeasionally from two, 

 nuclei. The principal centre for this bone appears in the middle of the cartilagi- 

 nous astragalus at the seventh month of embryonic life. The additional centre is 

 de])()sitecl in the posterior portion of the bone, and forms that part of the astragalus 

 ■which, when it remains separate from the rest of the bone, is known as the os 

 trigonum. At birth, the astragalus presents some im})ortant i)eculiarities in the 

 disjjosition of the articular facet on the tibial side of its Ixxly, and in the oblicjuity 

 of its neck. If, in the adult astragalus, a line be drawn through the middle of the 

 trochlear surface parallel with its inner border, and a second line be drawn along 

 the outer side of the neck of the bone so as to intersect the first, the angle formed 

 by these two lines will express the obliquity of the neck of the bone. This in the 

 adult varies greatly, but the average may be taken as 10°. In the foetus at birth 

 the angle averages 35°, whilst in a young orang it measures 45°. In the normal 

 adult astragalus the articular surface on the tibial side is limited to the body of the 

 l)one. In the fa?tal astragalus it extends for some distance on to the neck, and 

 sometimes reaches almost as far forward as the scaphoid facet on the head of the 

 bone. This disposition of the inner malleolar facet is a characteristic feature of 

 the astragalus in the chimpanzee and the orang. It is related to the inverted 

 2)osition of the foot which is found in the human endoryo to near the period 

 of birth, and is of interest to the surgeon in connection with some varieties of 

 clul)-foot. (Shattock and Parker. ) 



THE CALCANEUM 



The calcaneum, or os calcis, is the largest tarsal bone. It is cul)oidal in 

 sliaj)e, and presents, for examination, six surfaces. 



The superior surface has in its nnddle a large, oval, convex, articular facet 

 for the under aspect of the body of the astragalus; behind the facet, the bone is 

 rough and convex laterally. In front of the facet the bone presents a deep depres- 

 sion, the floor of which is rough for the attachment of ligaments, especially the 

 calcaneo-astragaloid, and the origin of the extensor brer is digitorum muscle; when 

 tlie calcaneum and astragalus are articulated, this portion of the bone forms a floor 

 to a cavity sometimes called the sinus pedis. Internally, this u])per surface of the 

 bone presents a well-marked lip, th(^ sustentaculum tali, furnished Avith an elon- 

 gated concave facet, occasionally divided into two, for articulation with the neck of 

 the astragalus. 



The inferior surface is narrow and rough; it ends posteriorly in tAvo tuber- 

 cles : the inner is the larger and broader, the outer is narrower but prominent. 

 The inner tubercle affords origin to the abductor hallucis, the flexor breris digitorinn, 

 and the abductor minimi digiti ; the last muscle also arises from the outer tubercle, 

 and the ridge of bone connecting the tubercles. The outer tubercle aftords attach- 

 ment to the abductor ossis metatarsi quinti. The rough surface in front of the tul)er- 

 cles gives attachment to the long plantar ligament (calcaneo-cuboid) and the outer 

 head of i\ie flexor accessorius. Near its anterior end this surface forms a rounded 

 eminence, the anterior tubercle, from which (and the shallow groove in front) the 

 short plantar (calcaneo-cuboid) ligament arises. 



The external surface is rough and slightly convex. Near the middle of this 

 surface there is a small tubercle for the middle fasciculus of the external lateral 

 ligament of the ankle-joint. Anteriorly, w^e notice the two shallow peroneal 

 grooves, separated by a tubercle, which is sometimes very prominent. The 

 upper groove is for the tendon of the peroneus breris, and the lower lodges the 

 tendon of the peroneus longus. 



The inner surface is deejdy concave, the hollow lieing increased l)y the over- 

 hanging sustentaculum tali in front and above, and the ])rominent inner tubercle 

 posteriorly. The under aspect of tlie sustentaculum is deeply grooved for the 

 tendon of the flexor longus hallucis, whilst the hollow below receives the plantar 

 vessels and nerves. Its lower border serves for the attachment of the inner head 



