40 ARBOREAL MAN 



the same condition is found; an ulnar head constitutes 

 a bulky portion of the muscle in the Chelonia (e.g., Testudo) 

 and in the Lacertilia (e.g., Varanus). Among the general- 

 ized Amphibia, Cryptobranchus shows the same thing in 

 the ill-differentiated form we have noted previously. 



The story of the M. pronator quadratus is equally 

 striking. In Man an interosseous membrane unites the 

 two bones of the forearm, and although in this membrane 

 there is, as is usual in such membranes, a crossed arrange- 

 ment of its fibres, nevertheless the great bulk of the 

 strands run from the radius down to the ulna. On a plane 

 altogether anterior to this membrane the muscle bundles 

 of the M. pronator quadratus run on the whole in an 

 opposed direction, from ulna down to radius. It is 

 important to notice that behind the M. pronator quad- 

 ratus the interosseous membrane is quite uninterrupted; 

 the muscle does not replace the membrane, but lies in 

 front of it. 



This condition is typical of all the Primates. It is 

 found in some of the more mobile-limbed members of 

 other Eutherian orders. It is present in Tupaia and in 

 Crocidura, in typically human guise; and it is present 

 again in generalized Reptiles and in Amphibia. Its con- 

 dition in Cxyptobranchus has previously been mentioned. 



A true M. pronator quadratus is not present in those 

 thoroughly quadrupedal animals in which the rotation of 

 the two bones of the second segment is lost. The true 

 M. pronator quadratus is not to be confused with the 

 M. radio-ulnaris, which is a purely interosseous muscle, 

 homologue of the M. tibio-fibularis of the leg, and is 

 situated on a plane posterior to the fibres of the distinct 

 M. pronator quadratus. 



The facts of the occurrence of these muscles in the 

 Vertebrate series must be admitted to be very curious, 

 since the typically human condition is such a rare mam- 

 malian feature, and yet is one so closely matched among 

 the generalized Amphibia and Reptilia. The facts may 



