542 



SESAMOID BONES. 



stouter towards the external part than to- 

 wards the inner or that which is in contact 

 with the long flexor tendon. 



A microscopic examination of sections, taken 

 in the three cardinal directions, shows that 

 they possess much the same minute structure 

 as other similarly shaped bones. The lacunas 

 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 of the tendon were re- 

 ceived between the plaitings of osseous laminae, 

 or conversely as though the ossification had 

 extended up in laminae between the tendinous 

 fibres. The lacunae 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. 



Develapement (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. I II. art. OSSEOUS 

 TISSUE. 



Disease and injury. I am not awaCe 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). When 

 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 many 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, 



* See Vol. III. p. 850. fig. 452. art. OSSEOUS 



TISSUE. 



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 sesamoid bones. Sesamoid bones are 

 occasionally met with in the human 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- 

 frequeritly 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 mam- 

 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 Quadru- 

 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 liallux 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 Cetacea nor 

 in the singularly modified extremities of the 

 Sloth. In all, or nearly all, other Mammalia 

 a pair occurs opposite every metacarpo- and 

 metatarso-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. 



The palcl/a exists in all placcntal Mammals, 



