SESAMOID BONES. By this name are designated the small bones met with in the neighbourhood of certain joints, generally in the tendons of the adjacent muscles. They owe their name (alicrapri an) to the figure which they usually possess, resembling that of an Indian grain called sesame. But the present application of the term regards the character of their situation in the course of a tendon, rather than their form ; for in stance, the patella is often said to be a sesa moicl bone, not because it resembles sesame, but because it is placed in the tendon of the extensor cruris nmscles. This character of' their situation in the course of tendons con stitutes their chief point of interest ; it is in this that they are peculiar, and different from other bones.
In the human subject these bones are usually met with only on the palmar aspect of the inetacarpo-phalangeal joint of the thumb and the homologically corresponding part of the great toe, in both of which situations there are usually two. These are not constantly present, and, according to Cloquet, are not met with in children, owing, probably, to their becoming ossified late, though in young Ru minants and Solipeds, as well as in other animals, I have found their ossification as far advanced as it was in the other bones. They are more frequently absent in the hand than in the foot, and in females than in males. The long flexor tendon of the thumb or great toe passes between them, and the two are bound together above and below it by dense fibrous tissue, so that they assist in forming its sheath.
The sesamoid bones of the thumb are very small—usually not bigger than the half of a large pea. They have a somewhat oval out line, are convex on their palmar, and slightly concave on their dorsal aspect, which is arti cular, and covered with cartilage. They arti culate with the head of the metacarpal bone. Those of the great toe are each as large as a horse bean, of a long oval outline, convex on the plantar aspect, and presenting a concave cartilage-covered surface to the head of the metatarsal bone, to which they are adapted.
The little pieces of bone, situated and shaped as above, are enclosed in the tendons of the short flexor rnuscle of the thumb of great toe, the fibres of which have the follow ing relation to thern : — Some of the ten dinous fibres pass over them, and some on each side, whilst their articular cartilage, as I have verified by microscopic examination, be comes mixed with tendinous fibres, passing on their arthrodial aspect, as it approaches the bone. The greater part of the tendon, how
ever, is inserted into their proximal, and arises again, so to speak, from their distal end. The arthrodial surface of their articular cartilage forms part of the synovial surface of the sub jacent metacarpo pha,langeal joint, and they are held in their place by the strong fibrous tissue of the common synovial capsule.
The sesamoid bones consist of finely cancellated osseous tissue, enclosed in a shell of denser bone. The main direction of the osseous columns that surround the can celli is longitudinal, but they intercommunicate in all directions. These columns are much stouter towards the external part than to wards the inner or that which is in contact with the long flexor tendon.
A nzicroscopic examination of sections, taken in the three cardinal directions, shows that they possess much the same minute structure as other similarly shaped bones. The lacunm are large and expanded*, the canaliculi dis tributed arborescently, except a few in the immediate neighbourhood of the cancelli and Haversian canals, where they have the straight and parallel arrangement met with in the shafts of the long bones. The Ha versian canals are but few in number, their place being plentifully supplied by the nu merous cancelli. The dense surrounding shell is stratified parallel to the surface, very markedly so on the articular aspect, where it is thickest. At the points where tendinous fibres are attached, it appears to be laminated at right angles to the strata and the surface, as though the fibres or the tendon were re ceived between the plaitings of osseous laminm, or conversely as though the ossification had extended up in laminm between the tendinous fibres. The lacunm that occur in this crust are mostly large and clumsy, elongated and directed vertically or obliquely towards the surface, particularly the articular surface, and all of them destitute of canaliculi; a condi tion met with in the superficial osseous crust of other articular surfaces, and points of attachment of tendons, especially in old sub jects.—It is probably the form of osseous tissue that results from the ossification of permanent cartilage or white fibrous tissue ; but my researches, in order to ascertain this point, have not been sufficiently extensive.