23 OSTEOLOGY. | ay 
Tae Os CAncis: 
The os calcis (calcaneus) is the largest of the tarsal bones. It supports the 
astragalus above and articulates with the cuboid in front. Inferiorly and behind, its 
ica extremity or tuberosity forms the heel on which so large a proportion of 
the body weight rests. The long axis of the bone inclines forwards and a little 
outwards. 
The wpper surface of the os calcis is divisible into two parts—a posterior non- 
articular part and an anterior articular portion. The length of the former varies 
according to the projection of the heel; rounded from side to side, it 1s slightly con- 
cave from before backwards. In front of this there is a convex articular area of 
variable shape (facies articularis posterior), sometimes nearly circular, at other times 
oval and occasionally almost triangular. This is directed upwards and forwards, 
and articulates with the posterior calcanean facet on the under surface of the astra- 
galus. Anterior to this facet the bone is deeply excavated, forming a fossa from 
Cupoip CuBOID 
Surface for 
attachment 
of short 
plantar 
“A ligament 
;BINTEROSSEOUS GROOVE 
GROOVE FOR 
FLEXOR 
LONGUS 
HALLUCIS 
FACETS FOR ASTRAGALUS 
Susrenta-// 
CULUM 
TALI 
Surface for 
attachment 
of long 
plantar liga- 
ment 
PERONEAL SPINE) 
INTERNAL 
“TUBERCLE 
TUBEROSILY '‘PUBEROSITY 
: Fie. 177.—THE RicHt Os CALCIS AS SEEN Fic. 178.—TuHE Ricgut Os CALCIS AS SEEN 
FROM ABOVE. FROM BELOW. 
which a groove (sulcus caleanei) leads backwards and inwards around the antero- 
internal border of the articular surface. When the os calcis is placed in contact 
| with the astragalus, this groove coincides with the sulcus on the under surface of 
the latter bone and so forms a canal or tunnel (sinus tarsi), in which the strong 
interosseous ligament which unites the two bones is lodged. To the front and inner 
side of this groove, there is an elongated articular facet directed obliquely from 
; behind forwards and outwards, and concave in the direction of its long axis. This 
is frequently divided into two smaller oval areas by an intermediate non-articular 
surface. Of these facets the hinder (facies articularis media) articulates with the 
middle calcanean facet on the under surface of the astragalus, whilst the anterior 
S (facies articularis anterior) supports the under surface of the head of the astragalus 
: (facies articularis calcanei anterior). The outer side of the upper surface of the 
anterior extremity of the bone is rough, and hereto is attached the origin of the 
short extensor muscle of the toes. 
The inferior surface of the bone is slightly concave from before backwards, and. 
convex from side to side. The under aspect of the tuberosity is provided with two 
tubercles, an inner (processus medialis tuberis caleanei) and an outer (processus 
lateralis tuberis calcanei), of which the former is the larger. From this the short — 
flexor of the toes and the abductor hallucis muscle arise, Svielse from both tubercles : 
