Appendicular Skeleton

The tibia has an epiphysis at either end, but that for the upper comes down in front so as to include a good deal of the tubercle. In almost any other mammal, and often in man himself, it may be seen that this down-growth is an epiphysis developed in the quad riceps tendon below the patella and joining the main upper epiphysis before uniting with the diaphysis or shaft.

The fibula has two epiphyses, the lower of which appears first. The general rule with the long bones of the extremities is the ischium is the hypoischium of reptiles (see subsection on comparative anatomy). The most anterior of the epiphyses in the acetabulum is the os acetabuli of lower mammals, while the occasional one for the spine of the pubis is often looked on as the vestige of the marsupial bone of monotremes and marsupials. It will thus be seen that many of the secondary centres of the os innominatum are atavistic.

The femur has epiphyses for the head, the lower end, the great and the small trochanters.

The cartilaginous patella does not appear until the third month of foetal life, that is well after the quadriceps extensor cruris, in that the epiphysis nearest the elbow or farthest from the knee is the first to appear and the last to join.

In the tarsus the cartilages are at an early stage arranged in three rows like those of the hand, but in the proximal row the middle one (intermedium), corresponding to the semilunar in the hand, fuses with the one on the tibial side to form the astragalus, though sometimes a vestige of it seems to persist as a little bone at the back of the astragalus, known as the os trigonum.

The centre for the calcaneum appears in the sixth month of foetal life, that for the astragalus in the seventh, the cuboid about birth, the external, middle and internal cuneiforms in the first and second years, while the navicular is the last to appear in the third year.

The calcaneum has an epiphysis developed in the insertion of the tendo Achillis behind.

The development of the metatarsal bones and phalanges of the foot is the same as that of the hand.

For further details and literature see J. P. M'Murrich's Development of the Human Body (London, 1923) and D. J. Cunningham's Text Book of Anatomy.

Comparative Anatomy.

It is only when the class of pisces is reached that paired appendages are found, and there are two main theories to account for their first occurrence. The one which is at present most favoured is that in some ancestral fishes two folds ran along the ventro-lateral part of the body, like the bilge keels of a boat, and that these joined one another in the mid ventral line behind the cloacal orifice to form the median caudal fin. Into these folds the segments of the body, including myo tomes and myocommata, extended. Later on parts of these ridges were suppressed, but in the pectoral and pelvic regions they were retained to form the paired fins. This theory is supported by the fact that in some elasmobranch embryos the whole length of the folds can be traced. The second theory is that the limbs are elaborated gills. It is probable that the limb girdles are of later evolution than the skeleton of the fins themselves.

In the elasmobranch fishes (sharks and rays) there is a cres centic bar of cartilage (pectoral girdle), concave upward, which girdles the ventral and lateral parts of the body; it is divided into a dorsal part (scapula) and a ventral part (precoracoid and cora mals, the serratus magnus muscles forming the chains of the bridge (see fig. 28).

The clavicle is often entirely suppressed in mammals; this is the case in most of the Ursidae, all the Pinnipedia, Manis among edentates, the Cetacea, Sirenia, all Ungulata and some of the Rodentia. It is complete in all the Primates, Chiroptera, In sectivora (except Potamogale), many of the Rodentia, most Edentata, and all the Marsupialia except Perameles. In the Mono tremata it is fused with a well-developed interclavicle, but in coid) by a facet for the articulation of the fin. This of course is the glenoid cavity. In some forms, e.g., the shark Heptanchus, there is a perforation in the ventral part of the bar on each side, which possibly indicates the division between the precoracoid and coracoid elements.

In many of the bony fish (Teleostei) the outline is obscured by a series of bones which connect the girdle with the skull and may be the precursors of the clavicle.