EEPORT ON THE SPHENISCID^. 29 



Tlie Humerus. 



The humerus, like all the bones of the wing in the Penguins, is much flattened from 

 side to side, and differs much in form from that of any other group of birds. The 

 articular surface of the proximal end of the bone is crescentic in form, the horns of 

 the crescent being directed towards the middle line of the body. This surface is 

 separated from the rest of the bone by a horizontal groove which affords attachment to the 

 capsular ligament of the shoulder joint. Immediately below this groove on the outer 

 surface of the bone is a deep depression, into which as well as into the ridge in front of it is 

 inserted the tendon of the pectoralis medius. On the inner side of the bone, below its arti- 

 cular surface and close to the anterior border, is another linear groove which affords insertion 

 to the tendon of the pectoralis major. Below the posterior horn of the crescentic arti- 

 cular surface, but separated from it by a horizontal groove, is a well-marked eminence into 

 which is inserted the tendon common to the subscapularis and supraspinatus. Imme- 

 diately below this eminence is a deep circular excavation, which corresponds in position 

 to that of the foramen which in the majority of birds permits the passage of air into the 

 interior of the bone. In the Penguins, however, the bones of which are destitute of 

 air-containing cells, this fossa is blind and merely affords attachment to the third head of 

 origin of the triceps muscle. The "great tuberosity" of the bone is but ill defined, and 

 is represented only by the slightly projecting upper extremity of the shaft. Into it are 

 inserted the pectoralis major and the tensor patagii longus. 



The posterior border of the humeral shaft affords attachment to the fourth head of 

 origin of the triceps, while to the anterior margin are attached the tendons of the greater 

 pectoral and tensor patagii muscles. The lower end of the anterior margin presents 

 a shallow excavation, from which arises the brachialis internus muscle. The outer 

 surface of the bone is smooth, while the inner surface, close to its lower end, presents 

 two or three oblique grooves which accommodate the blood-vessels of the arterial rete 

 mirabile. 



The lower end of the humerus is very obliquely truncated from the anterior to the 

 posterior border, and is provided with four distinct articular surfaces. Of these the two 

 larger are quite continuous with one another, and are adapted to the upper extremities 

 of the radius and ulna. The posterior border of the bone is prolonged inferiorly into a 

 pointed angle, which is marked by two deep parallel grooves. Of these the external is the 

 deeper, and in it glides the external or larger of the two sesamoid bones found in this 

 region. The internal is the shallower of the two, and is adapted to receive the internal 

 or smaller of the idnar sesamoids. The internal and external surfaces of the lower end of 

 the humerus are rough for the attachment of the powerful lateral ligaments of the elbow 

 joint. 



The humerus presents no distinctive features of any importance in any species 

 of Penguin. Even in the different genera special characteristics are looked for in 

 vain. 



