28 THE ARTISTIC ANATOMY OF ANIMALS 



the tendon of the elevator muscle of the wing, or small 

 pectoral. The importance of the fourchette being, as we 

 have seen, in proportion to the movements of flying, it is 

 easy to understand that the bone is not found in the ostrich. 



The Arm 



A single bone, the humerus, forms the skeleton of this 

 portion of the thoracic limb. 



The Humerus. — The bone of the arm is, in quadrupeds, 

 inclined from above downwards and from before backwards. 



It is, with relation to other regions, short in proportion 

 as the metacarpus is elongated, and as the number of digits 

 is lessened. In the horse, for example, whose metacarpus 

 is long, and in which but one digit is apparent, the humerus 

 is very short. The slight development in length of the 

 humerus explains its close application to the side of the 

 animal as far as the elbow. 



In animals in which the humerus is longer, the bone is 

 slightly free, as well as the elbow, at its inferior extremity. 

 Later on we will return to the consideration of this 

 peculiarity and of the proportions of the humerus, after 

 we have studied the other parts of the fore-limbs. 



The humerus in quadrupeds is inflected like the letter S ; 

 in man this general form is less accentuated, the humerus 

 being almost straight. On its body, which appears twisted 

 on its own axis, we find the musculo-spiral groove,* which 

 crosses the external surface, and is very deep in some animals. 

 Above this groove, and on the external surface, there exists 

 a rough surface which is the impression of the deltoid. In 



* It would be going outside our province to tliscuss whether the 

 humerus is really twisted on its axis. This question, often discussed, has 

 been solved in some recent works in the following manner : the humerus 

 has undergone torsion at the level of its superior extremity, and not 

 at the level of its body ; this does not authorize us further to accord any 

 definite sense to the denomination ' groove of torsion ' (musculo-spiral 

 groove). That which we must especially remember in connection with 

 this fact, is, as we shall afterwards see, the difference of direction which the 

 articular head presents according as the torsion has been more or less 

 considerable; because this is established, according to the same order, in 

 man and in quadrupeds. 



