594 MAMMALIAN ANATOMY 



in the cat. The posterior border is rather the maximum convexity on 

 the posterior aspect of the bone. The external surface is rounded and 

 marked half-way down by the roughened area for the pronator radii 

 teres muscle. The posterior surface is rounded above and below, but 

 flattened in the middle, where the extensor primi internodii pollicis 

 muscle is attached. 



The grooves on the posterior aspect of the lower extremity are 

 slightly different from those presented by the radius of the cat. The 

 larger inner one, which in the cat is for the extensor indicis, is for the 

 tendons of the extensores indicis, communis digitorum, and minimi 

 digiti ; the next one, which in the cat transmits the extensor communis 

 digitorum, transmits the extensor secundi internodii pollicis ; the area 

 external to the median ridge is subdivided into two grooves for the 

 extensores carpi radialis brevior and longior ; and instead of a single 

 groove on the outer aspect for the extensor ossis inetacarpi pollicis, 

 there are two, the inner for that muscle, the outer for the extensor 



primi internodii pollicis. 



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THE CARPUS. 



General Description. The Carpus, or wrist, is the proximal 

 and smallest part of the hand. It is composed of seven small bones, 

 united closely to form a compact mass about twice as wide as it is long 

 and twice as wide as it is thick from the dorsal to the palmar side. 

 The carpus constitutes a very small portion of the upper extremity, 

 as its length is contained twelve or thirteen times in the length of the 

 ulna alone and forms only an eighth or ninth of the total length of 

 the hand. The carpus presents four well-defined surfaces. 



The larger quadrilateral dorsal surface is bounded by approxi- 

 mately straight radial and ulnar borders, and by proximal and distal 

 borders rendered irregular by the slightly differing sizes of the con- 

 stituent bones. The surface is convex from side to side, the maximum 

 convexity being nearer the radial border than the middle line. 



The smaller palmar surface is concave from side to side, and is 

 very irregular, inasmuch as it affords attachment to strong ligaments. 

 At its uluar proximal end it is prolonged into a stout palmar hook. 



The transversely oblong proximal surface is convex in both direc- 

 tions for articulation with the biconcave articular surface formed by 

 the ends of the radius and ulna. 



