Animal Diversity: Chordata

10. The sternum lies in the mid-ventral line. It consists of episternum, omosternum, and
xiphisternum.
11. The episternum is a flat, almost circular plate of cartilage.
12. The omosternum is a bony rod connected to the episternum on the anterior side and
clavicle on the posterior side.
13. The mesosternum is a cartilaginous rod lying opposite the omosternum.
14. The xiphisternum is the terminal broad cartilaginous plate lying at the tip of the
mesosternum.
The forelimbs show the fusion of the radius and ulna into a single radius-ulna. There are only
four complete digits with a vestigial one, the prepollex. Only six carpals are present.
Forelimb bones of frog (Figure 15)
Humerus
1. It is the bone of fore-limb and is the
component of upper-arm.
2. It is a short, stout and cylindrical
bone with a slightly curved shaft.
3. Its proximal end is known as the
head which fits into the glenoid
cavity of pectoral girdle.
4. The head is covered with calcified
cartilage.
5. The ridge below the head is known
as deltoid ridge.
6. The distal end forms a rounded trochlea with a condylar ridge on either side.
7. The trochlea articulates with the groove of radius-ulna.
Radius-ulna
1. It is a compound bone of fore-limb and is the component of fore-arm.
2. It is formed by the fusion of radius and ulna bones.
3. Its proximal end has a concavity to receive the trochlea of humerus.
4. The ulna projects into an olecranon process.
5. The distal portion of radius-ulna is somewhat flat having a groove.
6. Distal portion has an articular surface for the metacarpals.
Carpus-metacarpus and digits
1. The bones of the wrist are called carpals.
2. The carpal bones are six in number and arranged in two rows of three each.
3. The bones of the proximal rows are called ulnare, intermedium and radiale. These
bones articulate with the radius-ulna.
38