ANATOMY OF A MONODACTYLOUS FOETUS 401 



This lateral portion forms a sharp fold projectiiis between 

 the humerus and radius and occupies the deeper portion of the 

 skin web previously described as binding the arm in flexion at 

 the elbow. This muscle is very tight and prevents all exten- 

 sion of the radius on the humerus. It is the muscle so placed 

 as to most thoroughly prevent this movement, and the part 

 responsible for this is the lateral portion, due to its insertions 

 down the shaft of the radius. There is no opposition to this force 

 as the triceps is not attached to the radius. 



Although this muscle occupies only about half the projecting 

 extent of the skin web here, it is probably the cause of the web, 

 forcing the skin out in a sharp fold ahead of it. The fold has 

 developed beyond the extent of the muscle later on. 



The lateral portion of the brachialis is responsible for another 

 displacement of the radius. As its insertion is far down on the 

 shaft of the radius, and its pull is all to the one side, it has swung 

 the radius around laterally until the long axis of this bone lies 

 in a plane parallel instead of perpendicular to the line joining 

 the two epicondyles of the humerus. This latter relation is not 

 at first sight apparent, for the forearm appears to be ventral, 

 not lateral to the upper arm. The reason for this is that the 

 scapula, carrying the humerus with it is rotated through a 

 right angle forward and inward on the flattened chest wall. 

 The scapula has medial and lateral surfaces respectively, instead 

 of ventral and dorsal. The humerus similarly has medial and 

 lateral surfaces instead of ventral and dorsal, and the axis at the 

 lower extremity passing through the epicondyles is not medio- 

 lateral in direction, but dorsoventral. The forearm thus lies 

 in a dorsoventral plane although actually rotated laterally 

 through a right angle. 



Triceps brachii (figs. 9 and 10, T^, T.„ 1\) 



Origin. The long head is very large and arises from part of 

 the axillary border of the scapula as well as the infraglenoid 

 tubercle. 



The lateral head arises from the upper third of the posterior 

 surface of the shaft if the humerus above the groove for the 



