Developenzent (examined in young Rumi nants).— A small mass of temporary cartilage precedes the osseous condition of these little bones. This becomes ossified from a single central point in the manner of an epiphysis, as described at page 857. Vol. III. art. OSSEOUS TISSUE.
Disease and injztry.—I am not aware that the diseases or accidents affecting the sesa moid bones have ever been noticed, unless the patella be considered a sesamoid bone, which, indeed, it is in structure, by situation in a tendon, and in function. This bone comports itself in disease just as other bones do (see KNEE JOINT, ABNORMAL ANATOMY). Wilerl fractured transversely, it presents the pecu liarity of uniting, by white fibrous tissue, in stead of by bone. I cannot regard this non union by osseous tissue as resulting from any deficiency of nutritive or reparative power in the patella, for new fibrous tissue is always, and when the fracture is longitudinal, even new bone is usually, formed ; nor from want of apposition, for in iliany ununited speci mens the apposition is very perfect. Osseous union, as a result of reparative inflammation, never occurs in situations where the new ma terial of repair is not subjected to pressure, as in the skull, acromion, olecranon, heel, — a hole made in the scapula does not be come filled up with bone. I therefore regard the non-union by bone of transverse fracture of the patella as due to the absence of that stimulus (pressure) which I conceive to be necessary in order to determine the repara tive material to assume the osseous form ; whilst I attribute the union by ligament to the presence of the stimulus (tension) which I regard as necessary, in order to direct the metamorphosis of the adhesive lymph, or rather the mass of new corpuscles or cells, which is formed for the purpose of repair, soon after any accidental breach of continuity has been produced, towards the ligamentous form. These remarks would, of course, ap ply to transverse fractures of the sesamoid bones properly so called, in case such acci dents ever occur.
Other sesanzoid bones.— Sesamoid bones are occasionally met with in the huinan subject in other than the above-named situations.
One is sometimes found at the distal joint of the thumb and great toe ; two at the prox imal joint of the forefinger and second toe ; and one at the corresponding joint of the little finger and toe. There is pretty fre quently one, or even two or three, in the heads of the gastrocnemius, just at the posterior part of each condyle of the femur. An ossi fication often takes place in the tendon of the peroneus longus, just where it doubles round the os cuboides ; and a small bone is not un frequently found in the tendon of the tibialis anticus, near its insertion into the scaphoid.
Comparative anatomy.— Sesamoid bones at the metacarpo-tarso-phalangeal joints exist in much greater number in the quadruped maw malia than in man ; and they seem to be largest in animals that are digitigrade in their progression. I have not had an opportunity of scrutinizing their condition in the Qztadrzt mana ; none are preserved in the skeletons. and as the thumbs are somewhat rudimentary they are probably absent. In the Seal, a pair is situated on the metatarsal joints of both the hallux and the fifth toe, which greatly exceed the other toes in size ; but there are none in the fin-like hands. They do not exist in the paddles of the Cetacca nor in the singularly modified extremities of the Sloth. In all, or nearly all, other Manzmalia a pair occurs opposite every metacarpo- and rnetatarso-phalangeal joint. The two bones of the pairs are not unfrequently anchylosed together, as in the outer digit of the hand and foot of the Elephant. They are always situated in the course of the tendons of the interossei muscles. Often, however, as in Ruminants, their large size is enormously dis- ' proportioned to the small tendons on which they are placed, which, in this order with their muscles, are quite rudimentary', and the large sesamoid bones seem to be embedded in the sheath of the long flexor tendons. In Birds their place is occasionally supplied by large masses of fibro-cartilage. In Reptiles they are wanting.