284 THE VOYAGE OF H.M.S. CHALLENGER. 



the Sphenisciclse ; in the presence of two sesamoid bones, developed in connection with the 

 tendon of the triceps muscle ; in the peculiar form and mode of articulation of the carpal 

 bones ; in the union of the first or radial, which, although independent in the embryo, 

 becomes inseparably anchylosed with the second metacarpal bone in the adult ; and in 

 the absence of a free poUex. 



The skeleton of thedeg; of the Peno-uiu is modified to a less extent than is that of the 

 wing, but here likewise certain peculiarities are met with. The patella is of larger size 

 than, and differs somewhat in form from, that of the majority of birds ; the tarso- 

 metatarsus presents features which serve at once to distinguish that bone from the 

 corresponding skeletal element of any other group of birds, being altogether shorter and 

 broader than in these, with the single exception of the genus Frer/atta. From Frcgatta, 

 however, as from all other birds, the Penguin is distinguished by the clearly-defined 

 separation of the metatarsal elements, the shafts of which are differentiated from one 

 another, while in other birds these bones are indistinguishably fused together. 



In respect of the form and mode of articulation of the various joints, with the excep- 

 tion of those of the wing, the Penguins do not differ from other birds. In the wing of the 

 SpheniscidfB, in accordance with the alteration of function of that member, the various 

 joints, with the exception of that at the shoulder, present an amount of rigidity met with in 

 no other group of birds. The shoulder joint is as perfect in the Penguins as in other birds, 

 but the more distal articulations are arranged so as almost entirely to prevent those 

 movements of flexion and extension which are essential to an organ of flight. This limita- 

 tion of movement is partly dependent on the form of the articulations, and partly on 

 the arrangement of the ligaments, which here as elsewhere not only serve to attach the 

 bones to one another, but to limit the amount of movement permissible at the various 

 joints, in accordance with the anatomical arrangement of the muscles, and the physio- 

 logical requirements of the organism. 



The muscular system of the Penguins is characterised by the great development of 

 the cutaneous muscles, which present an arrangement quite peculiar to the grou^D. It 

 has been suggested to me that the large development of the cutaneous muscles in these 

 birds is probably a means whereby water may be readily ex2:)elled from the interstices 

 of the plumage so soon as the bird quits the water. Were it otherwise, in the low 

 temperature of the Antarctic region which the majority of these birds inhabit, the plumage 

 would soon be frozen into an icy mass, the high temperature of the bird being of itself 

 insuSicient to obviate this, seeing that the ready conduction .of heat from the interior of 

 the organism is prevented by the great development of the subcutaneous fatty layer which 

 obtains in every member of the group. The muscular system of the Penguins is further 

 characterised by the great strength of all, and the peculiar disposition of certain, of the 

 extensor muscles of the vertebral column, more especially of the biventer cervicis, which 

 extends from the iliac bone to the skull, these peculiarities being associated with the 



