382 



THE ARTICULATIONS 



Synovial Membrane. The synovial membrane in this joint is distinct. It 

 lines the inner surface of the ligaments. 



Actions. The movements permitted between the os calcis and cuboid are 

 limited to a slight gliding upon each other. 



3. THE LIGAMENTS CONNECTING THE Os CALCIS AND NAVICULAR. 



Though these two bones do not directly articulate, they are connected together 

 by two ligaments : 



Superior or External Calcaneo-navicular. 

 Inferior or Internal Calcaneo-navicular. 



The Superior or External Calcaneo-navicular (Fig. 264) arises, as already 

 mentioned, with the internal calcaneo-cuboid in the deep hollow between the 

 astragalus and os calcis ; it passes forward from the inner side of the anterior ex- 

 tremity of the os calcis to the outer side of the navicular bone. These two liga- 

 ments resemble the letter Y, being blended together behind, but separated in front. 

 The Inferior or Internal Calcaneo-navicular (Fig. 266) is by far the larger 



and stronger of the two ligaments between 

 these bones ; it is a broad and thick band of 

 fibres, which passes forward and inward from 

 the anterior margin of the sustentaculum tali 

 of the os calcis to the under surface of the 

 navicular bone. This ligament not only 

 serves to connect the os calcis and navicular, 

 but supports the head of the astragalus, form- 

 ing part of the articular cavity in which it is 

 received. The upper surface presents a fibro- 

 cartilaginous facet, lined by the synovial 

 membrane continued from the anterior cal- 

 caneo-astragaloid articulation, upon which 

 the head of the astragalus rests. Its under 

 surface is in contact with the tendon of the 

 Tibialis posticus muscle ; l its inner border is 

 blended with the fore part of the Deltoid 

 ligament, thus completing the socket for the 

 head of the astragalus. 



Surgical Anatomy. The inferior caleaneo-nav- 

 icular ligament, by supporting the head of the astrag- 

 alus, is principally concerned in maintaining the 

 arch of the foot, and when it yields the head of 

 the astragalus is pressed downward, inward, and for- 

 ward by the weight of the body, and the foot becomes 

 flattened, expanded, and turned outward, constituting 

 the disease known as flat-foot. This ligament con- 

 tains a considerable amount of elastic fibre, so as to 

 give elasticity to the arch and spring to the foot; 

 hence it is sometimes called the "spring" ligament. 

 It is supported, on its under surface, by the tendon 

 of the Tibialis posticus, which spreads out at its 

 insertion into a number of fasciculi which are attached 



Fio. 266.-Ligaments' of the plantar surface to most of the tarsal and metatarsal bones ; this pre- 

 of the foot. vents undue stretching of the ligament and is a pro- 



tection against the occurrence of flat-foot. 



4. ARTICULATION OF THE ASTRAGALUS WITH THE NAVICULAR BONE. 

 The articulation between the astragalus and navicular is an arthrodial joint : 



the rounded head of the astragalus being received into the concavity formed by 

 1 Mr. Hancock describes an extension of this ligament upward on the inner side of the foot, 

 which completes the socket of the joint in that direction (Lancet, llibb, vol. i. p. o 



