XIII 



PHYLUM CHORDATA 



427 



rad 



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fixed in the position of pronation, i.e., the distal end of the radius 

 is rotated inwards, so that, while the proximal end is external to 

 the ulna, the distal end becomes internal, and the digits of the 

 inarms become directed forwards. 



At the proximal end of the humerus are to be recognised : 

 (1) A rounded head for articulation with the glenoid cavity of 

 the scapula ; (2) externally a greater and (3) internally a lesser 

 tubcrositi/ for the insertion of muscles ; (4) a groove, the Jricipital 

 groove, between the two tuberosities. On the anterior surface 

 of the proximal portion of the shaft is a slight ridge, the deltoid 

 ridge. At the distal end are two articular surfaces, one large and 

 pulley-like trochlea for the ulna : the other smaller capitcllum 

 -for the radius : laterally are two prominences or condoles, an 

 internal and an external. 



The radius and ulna are firmly fixed together so as to be 

 incapable of movement, but not actually ankylosed. The radius 

 articulates proximally with the humerus, distally with the scaphoid 

 and lunar bones of the carpus. The ulna presents on the anterior 

 aspect of its proximal end a deep fossa, the greater sigmoicl cavity, 

 for the trochlea of the humerus : 

 the prominent process on the 

 proximal side of this is the olc- 

 cranon process. Distally it arti- 

 culates with the cuneiform. 



The carpal bones (Fig. 1021), 

 nine in number, are all small 

 bones of irregular shape. Eight 

 of these are arranged in two rows 

 -a proximal and a distal ; the 

 ninth, ccntrale (cent.), lies between 

 the two rows. The bones of the 

 proximal row are taken in order 

 from the inner to the outer side, 

 scaphoid (sc.), lunar (or semi-lunar) 

 (lun.), cuneiform (cun.), and pisi- 

 form. Those of the distal row are, 

 reckoned in the same order, trape- 

 zium (trpm^), trapezoid (trpz.), mag- 

 num (mag.), and unciform (unc.y- 



The five metacarpals are all small, but relatively narrow and 

 elongated, bones, the first being smaller than the rest. Each of 

 the five digits has three phalanges, except the first, which has 

 only two. The distal (ungual) phalanges are grooved dorsally for 

 the attachment of the horny claw. 



1 The homologies of these bones are not quite certain, but are very probably 

 as follows: scaphoid = radiale, lunar = 1st centrale, cuneiform = intermedium, 

 pisiform = ulnare, centrale 2nd centrale, trapezium = 1st distale, trapezoid = 2nd 

 listale, magnum = 3rd distale, unciform = 4th and 5th distalia. 



FIG. 1021. Lepus cuniculus. Distal 

 end of fore-arm and carpus, dorsal view, 

 the bones bent towards the dorsal side 

 so as to be partly separated, cent, cen- 

 trale ; cun. cuneiform; Inn. lunar; 

 mag. magnum ; rad. radius ; sc. scap- 

 hoid ; trpz. trapezoid ; trpm, trapezium ; 

 c//'.ulna; unc. unciform ; / V, base.^ 

 of metacarpals. (After Krause.) 



