consists of a scapho-lunar and a cuneiform.

There is no

central. The distal row has the usual four bones. The pisiform is large, and articulates with the ulna as well as the cuneiform, and there is a small radial sesamoid, articulating with the scapho-lunar. There are also two large sesamoids, sometimes united, in the palmar tendons. The digits are five in number, all with the normal number of phalanges, which are short and broad, except those that bear the long, slightly curved, broad nails, with which the animal scratches and burrows in the ground. The pollex is more slender than the other digits; it is of about the same length as the fifth, the second and fourth are nearly equal and longer, and the third slightly the longest.

In the Ornithorhynchus the manus is comparatively more slender and elongated; but the number and arrangement of the bones are the same as in the Echidna.

CHAPTER XVII.

THE PELVIC GIRDLE.

THE posterior limb consists of a pelvic girdle and three segments belonging to the limb proper, viz. the thigh, the leg, and the foot, or pes.

The PELVIC GIRDLE is present in some form in all Mammals, though in the Cetacea and the Sirenia it is in an exceedingly rudimentary condition.

In all Mammals, except those belonging to the two orders just named, each lateral half of the pelvic girdle consists primitively, like the corresponding part of the anterior limb, of a rod of cartilage crossing the long axis of the trunk, having an upper or dorsal, and a lower or ventral, end. The upper end diverges from that of the opposite side, but the lower end approaches, and, in most cases, meets it, forming a symphysis, without the intervention of any bone corresponding to the sternum.

The pelvic girdle differs from the shoulder girdle in being. articulated to the vertebral column, at a point near to, but not at, the upper end of the rod.

Like the shoulder girdle, it bears on its outer side, near the middle, a cup-shaped articular cavity (acetabulum or cotyloid cavity, Fig. 113, a, p. 313), into which the proximal extremity of the first bone of the limb proper is received.

Like the shoulder girdle it is divided, by its mode of ossification, into an upper (dorsal) and a lower (ventral) segment, and the point of union between these is near the middle of the articular cavity.

Unlike the shoulder girdle of most Mammals, the lower segment is always largely developed, and ossifies from two separate centres, which form an anterior and a posterior

FIG. 113.-External surface of right innominate bone of a young Lamb (Ovis aries), 4.77 ilium (gluteal surface); si supra-ilac border; ab acetabular border; ib ischial border; Is ischium; sp spine; ti tuberosity of ischium; P pubis; s symphysis; thf thyroid or obturator foramen: a acetabulum.

bar, in contact above and below, but leaving a space between them in the middle, filled only by membrane, and called the thyroid or obturator foramen (thƒ).

The upper segment is named the ilium (II), the anterior bar of the lower segment the pubis (P), and the posterior bar the ischium (Is). In the process of growth these three

osseous pieces always coalesce into a single bone, called the os innominatum.

This is further completed by the addition of epiphyses ; one for the upper extremity of the ilium (corresponding to the supra-scapular epiphysis of the shoulder), and others for the most prominent parts of the lower or free borders of the pubis and ischium (symphysis pubis and tuber ischii).

In most Mammalia (perhaps with the exception of the Monotremata, Lemuroidea and Chiroptera) a fourth pelvic bone is found, situated between the ilium, pubis, and ischium. This, the so-called os acetabuli,1 either persists as a separate ossification (it is especially large in Talpa, Sorex, Viverra, and Lepus) or it fuses with one of the three other pelvic bones. Its morphological meaning is as yet unknown, but it can scarcely be considered as an epiphysis.

The three principal bones of the pelvis nearly always enter into the formation of the acetabulum, but in a few genera, as Lepus, the pubis is excluded.o

There is never any secondary osseous bar in the pelvic girdle corresponding to the clavicle of the upper extremity. The ilium of Mammals is essentially an elongated, threesided, or prismatic bone, though the relative size and position of the various surfaces and angles may differ greatly in different species. In the most characteristic form, one of the surfaces is internal, or directed towards the vertebral column, articulating by a flat irregular surface with the lateral "pleurapophysial" ossifications of the sacral vertebræ.

66

1 See W. Krause, " Ueber den Pfannenknochen," Centralblatt f. d. medicinisch. Wissenschaft. 1876, No. 46; W. Leche, 'Zur Anatomie der Beckenregion bei Insectivora." K. Svenska Vetensk. ; Akad.'s Handlingar, xx. 1883; and Bronn's "Clas-en und Ordnungen des Thierreichs, Mammalia."

2 Gegenbaur," Ueber den Ausschluss der Schambeines vom Hueftgelenk," Morph. Jahrb. 1876.

This may be called the sacral surface (see Figs. 114, and 115, p. 317; and 116, ss, p. 325.) Another is directed mainly forwards, and may be called anterior or iliac (is), as it gives origin to the iliacus muscle. The third is posterior or gluteal (gs), as it gives origin to the gluteal muscles.

Of the borders one is external or acetabular (ab), as it ends below at the margin of the acetabulum; another is antero-internal or pubic (pb), and the third is postero-internal or ischial (ib), so called because they end below by joining the pubis and the ischium respectively.

The innominate bone is always placed more or less obliquely to the vertebral column, the upper or iliac end inclining forwards, and the lower or ischio-pubic end turning backwards, contrary to the usual direction of the scapular arch. In order to give still greater stability and fixity to the pelvic girdle, and to incorporate it more completely for mechanical purposes with the vertebral column, there is, in addition to the articulation between the ilium and true sacral vertebræ, a very strong double ligamentous union between the ischium and the side of the anterior caudal or pseudo-sacral vertebræ, constituting the greater and lesser sacro-sciatic ligaments, which are replaced in some Mammals (as most of the Edentata) by a complete bony union.1

The two innominate bones, together with the sacrum, constitute the pelvis, a complete circle of bone, or rather a short tube. This has two outlets: an anterior (sometimes called inlet or brim) bounded by the inferior surface of the first sacral vertebra above, by the pubic borders of the ilia laterally, and by the anterior borders of the converging pubic bones, meeting at the symphysis below; and a

1 Practically, though not morphologically, the pelvis is a part of the trunk or axial skeleton. The functions of the hind limb in propelling and raising the body necessitate that it should be so.