REPORT ON THE SPHENISCID^. 75 



and anus, as well as from the cartilaginous extremity of the pubic bone. The muscle 

 passes forwards, and is inserted into the whole length of the inner border of the posterior 

 sternal notch. 



The external slip is continuous at its origin with that of the internal portion of the 

 muscle, and is attached by means of a flattened tendon to the posterior half of the free 

 border of the pubic bone, as well as to its cartilaginous extremity. The muscle passes 

 forwards, and is inserted by means of distinct digitations into the outer surfaces of the 

 third, fourth, fifth, and sixth sternal ribs, close to the articulations of these with the breast 

 bone. 



Action. — The rectus abdominis, in contracting, diminishes the capacity of the abdominal 

 cavity. 



Relations. — Both portions of the muscle are concealed by the external oblique, and 

 rest upon a large air sac which intervenes between the rectus and transversalis 

 abdominis. The inner slip towards its insertion overlaps to some extent the inner 

 border of the external portion of the muscle. 



Nerve supjily. — Branches from the lower intercostal nerves. 



Variations. — In Aptenodytes Icngirostris the insertion of the external portion of the 

 muscle extends from the second to the seventh sternal ribs inclusive. In Pygosccles 

 tceniatus and Eudyptes chrysolophus it is inserted into the fourth, fifth, sixth, and seventh 

 sternal ribs. 



Comparative Eemarks. 



The most striking peculiarity of the muscular system of the trunk of the Penguin lies 

 in the great development of the extensor muscles of the neck. Associated with this is the 

 peculiar disposition of the biventer cervicis, a disposition which is found elsewhere only in 

 the Ostrich among birds. This muscle arises as far back as the crest of the iliac bone, 

 and passing forwards is inserted into the occiput, without presenting any trace of the 

 tendinous intersection from the presence of which in the majority of birds this muscle 

 has received its name. The great strength of the extensor muscles in question is 

 doubtless correlated with the peculiar flexures of the vertebral column, which in every 

 member of the group Spheniscidse are more pronounced than in any other birds. 



These flexures are developed to such an extent, that, as already pointed out in the 

 description of the vertebral column, the lower cervical vertebrae are actually in contact 

 with the furculum, and thus the trachea and oesophagus, instead of passing along the middle 

 line of the neck, are thereby laterally displaced, and pass into the thorax, lying altogether 

 to the right of the vertebral column. Associated with this anterior cervical curvature, 

 there falls to be noticed the compensatory curves of the vertebral column, both above 

 and below, — curves the movements of which are necessarily controlled by stronger 

 muscles than are possessed by other birds in which these cervical flexures are less 



