THUMB AND FINGERS. , 199 



Thumb and Fingci's. 



The thumb and fore-fingers are each composed of three 

 bones. 



The THUMB is situated obliquely in respect to the fingers ; 

 neither opposite directly to them, nor in the same plane 

 with them. All its bones are much thicker and stronger in 

 proportion to their length, than the bones of the fingers are ; 

 which is extremely necessary, as the thumb counteracts all the 

 fingers. 



The first bone of the thumb has its base adapted to the pecu- 

 liar articulating surface of the trapezium : for, in viewing it from 

 one side to the other, it appears convex in the middle ; but, 

 when viewed from behind forwards, it is concave there. The 

 edge at the forepart of this base is extended farther than any 

 other part ; and round the back part of the base a rough fossa 

 may be seen, for the connexion of the ligaments of this joint. 

 The body and head of this bone are of the same shape as the 

 ossa metacarpi ; only that the body is shorter, the head flatter, 

 and tubercles at the forepart of its root larger. 



The articulation of the upper end of this bone is remarkable ; 

 for, though it has protuberances and depressions adapted to the 

 double pulley of the trapezium, yet it enjoys a circular motion, 

 as the joints do where a round head of the one plays in the 

 orbicular socket of another ; it is, however, more confined, and 

 less expeditious, but stronger and more secure than such joints 

 generally are. 



The second bone of the thumb has a large base formed into 

 an oblong cavity, whose greatest length is from one side to the 

 other. Round it several tubercles may be remarked, for the 

 insertion of ligaments. Its body is convex, or half round behind ; 

 but flat before, for lodging the tendon of the long flexor of the 

 thumb, which is tied down by ligamentous sheaths, that are 

 fixed on each side to the angle at the edge of this flat surface. 

 The lower end of this second bone has two lateral round protu- 

 berances, and a middle cavity, whose greatest extent of smooth 

 surface is forwards and backwards. 



