28 



LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



dorsai surface faces posteriorly and the preaxial surface faces laterally. Conse- 

 quently, the elbow or joint between upper arm and forearm is directed caudad. 

 As a result of the rotation of the upper arm the toes would point posteriorly, but 

 by an additional torsion they have been brought around to the front again. 

 This torsion involves a crossing of the two bones of the forearm, the distal end 

 of the preaxial bone being brought internal to the distal end of the postaxial 

 bone (see Fig. 2 and read R, p. 26). This position of the forearm is known as 



postaxial preaxial 



postaxial 



preaxial 



B 



HIND -LIMB 



FORE-LIMB 



HIND-LIMB 



FORE-LIMB 



FIG. 2. Diagrams to illustrate the torsion of the limb; preaxial surface white, postaxial surface 

 black. A, primitive position of the limbs, seen from above, the limbs extending at right angles to the 

 body, the preaxial surface facing anteriorly, the postaxial posteriorly. B, position of the limbs in 

 mammals, seen from the side; the limbs extend vertically below the body; the upper arm has rotated 

 outward and backward so that the preaxial surface now faces laterally; the forearm has rotated forward 

 again, resulting in a crossing of its bones; the hind h'mb has rotated outward and forward so that the 

 postaxial surface faces laterally. (From Flower's Osteology of the Mammalia.} 



the prone position, and is imitated in the human arm when the arm hangs by 

 the side with the back of the hand directed forward. In this position the crossing 

 of the two long bones of the forearm can be felt. If now the arm is raised side- 

 wise, shoulder high with the palm facing forward, the two bones return to the 

 primitive parallel position, known as the supine position. Thus man can change 

 his forearm from the-prone to the supine position, but in most mammals the fore- 

 arm is fixed in the prone position. 



The position of the parts of the foot in walking is different in different 

 mammals. The rabbit and the cat walk on the digits, with the remainder of 

 the hand and foot elevated. This type of gait is known as digitigrade. Man 

 walks on the whole sole of the foot, the primitive method, known as plantigrade. 

 Horses and cattle and other ungulates walk on their nails, which are broadened 

 into hoofs, a mode of walking called ungnligrade. 



