LIMBS. 



115 



ventral elements which join their fellows in the middle ventral line, 



and are known as the pubis and 



ischium. The limbs are divided into 



three segments the two proximal 



of which are long and contain long 



hollow bones articulated together, 



the third segment being shorter and 



terminal. These segments are called 



brachium, antebrachium and manus 



in the fore-limb; femur, crus and 



pes in the hind-limb. 



The proximal segments (i.e., the 

 brachium and femur) each contain 

 one bone the humerus (H) and 

 femur (Fe) respectively. The middle 

 segments (i.e., antebrachium and crus) 

 each contain two bones the radius 

 and ulna in the former (JK, U), the 

 tibia and fibula (T, F ) in the latter. 

 The distal or terminal segments (i.e., 

 the manus and pes) each contain a 

 large number of elements placed close 

 together. These elements consist of 

 two proximal rows of bones, known 

 in the hand as the carpus, and in 

 the foot as the tarsus; of a middle 

 row, known respectively as the meta- 

 carpus and metatarsus; and of a 

 number of distal bones known as the 

 phalanges, and constituting the 

 skeleton of the fingers and toes. 



The skull varies considerably in 

 form and structure. When the ver- 

 tebral column is membranous and 

 cartilaginous, the skull likewise con- 

 sists of a continuous membrano-ear- 

 tilaginous capsule, which in essential 

 points agrees with the embryonic 

 rudiment of the cranium (primor- 

 dial cranium) of the higher 

 Vertebrates (fig 571) From this primordial cranium the bony 



