Page:EB1911 - Volume 25.djvu/196

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facet for the articulation of the fin. This of course is the glenoid Pectoral cav ity. I n some forms, e.g. the shark Heptanchus, there zirdle ls 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

Appears about later end of 2nd m. of foetal life

Appears about is years; fuses 22-25 years

Appears about 4th m. of foetal life

Appears about 15 years: fuses 22- 25 years

Appears about 18 years Appears about 18 years

Unite about 10 years

At birth. About 12 or 13 years.

From Arthur Thomson, Cunningham's Text-Book of Anatomy.

Fig. 23. — Ossification of the Innominate Bone.

series of bones which connect the girdle with the skull and may be the precursors of the clavicle.

In the Amphibia the dorsally-placed scapula (fig. 27, S) has more dorsally still a cartilaginous plate, the supra-scapula (fig. 27, S.S), which may be calcified. The precoracoid (fig. 27, P.C) and coracoid (C) are quite distinct, the former being in front (cephalad) and over- laid by a dermal bone, the clavicle (CI). The attachment of the coracoids to the sternum has been noticed in section Axial of this article. Uniting the ventral ends of the precoracoid and coracoid is the epicoracoid on each side (fig. 27, E.C).

In the Reptilia the same general plan is evident, but in the lizards

Appears about early part, of first year Appears about 2-3 years

Fuses with shaft about 18-19 years/

Usually appears in

the gth month of

foetal life

Fuses with shaft about 20-22 years At birth. Abouf*T2 years. About 16 years.

From Arthur Thomson, Cunningham's Text-Book of Anatomy.

Fig. 24. — Ossification of Femur.

the ventral ends of the two clavicles are united by a median dagger- like dermal bone, the interclavicle (fig. 27, I.C), which lies on a plane superficial to the sternum and epicoracoids.

In birds the scapula has the shape of a sabre blade, and there is a rudimentary acromion process, though this is also indicated in some

reptiles. The pre- and epi-coracoids are aborted, but the coracoids are very strong. The clavicles and interclavicle unite into a V- shaped bar which forms the furcula or " merrythought."

In the Mammalia the Monotremata(Ornithorhynchusand Echidna) retain the reptilian arrangement of large coracoids and epicora- coids articulating with the sternum, while the clavicles and inter-

Fuses with shaft about 20-24 years

May appear - Appears^ independently

before birth about 1 1 years

Fuses with shaft about 20-24 years

Appears about 3-4 years

Appears about ij years

Fuses about 1 8th i At birth. About 12 years. About 16 years. From Arthur Thomson, Cunningham's Text-Book of Anatomy.

Fig. 25. — Ossification of the Tibia.

clavicle are also largely developed; the scapula too is more bird- like in shape than mammalian. In the higher mammals the scapula develops a spine and usually an acromial process, and has a triangular outline.^ As long as the forelimb is used for support, the vertebral border is the shortest of the three, and the long axis of the bone runs from this border to the glenoid cavity; but when the extremity is used for prehension, as in the Primates, or for flight, as in the Chir- optera, the vertebral border elongates and the distance from it to the glenoid cavity decreases so that the long axis is now parallel with that of the body instead of being transverse.

Above the monotremes too the coracoid becomes a mere knob for muscles, and no longer articu- lates with the sternum. There is thus a sudden transition from the way in which the forepart of the body is propped up on the forelimbs when the coracoid is functional (as in reptiles) to the way in which it is suspended like a suspension bridge between the two scapulae in pronograde mammals, the serratus magnus muscles form- ing the chains of the bridge (see fig. 2Q.

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, Insectivora (except Potamogale), many of the Rodentia, most Edentata, and all the Marsupialia except Perameles. In the Monotre- mata it is fused with a well- developed interclavicle, but in other mammals the inter- clavicle is either suppressed or possibly represented by the sternal epiphysis of the clavicle of the Primates. The pre- coracoid as a distinct structure

entirely disappears, though vestiges of it may remain in cartilaginous parts of the clavicle.

The chief modifications of the humerus are the development of the pectoral ridge, which is large whenever the pectoral muscles are strong, and is represented in man by the outer lip of the bicipital groove and the supracondylar foramina. In the tuatera lizard (Sphenodoh)

Appears about 2nd year

At birth.

Fuses with shaft about 1 9 years

About About

12 years. 16 years.

From Arthur Thomson, Cunningham's Text-Book of Anatomy.

Fig. 26. — Ossification of Fibula.