DR. J. MURIE ON THE ANATOMY OF THE WALRUS. 44] 
though thin. This arises by a fascia with a few muscular fibres interspersed from 
about opposite the fourth to the eighth dorsal vertebra, and, forming five broad but 
tolerably distinct slips, is inserted into the fifth, sixth, seventh, eighth, and ninth ribs, 
between the tendons of the sacro-lumbalis and those of the external oblique. 
The serratus magnus sends digitations to eight ribs. The highest, as in Otaria, is 
more or less separate, and is inserted into the dorsum of the scapula between the angle 
and spine on the vertebral border. 
In Trichechus the scalenus anticus springs from the transverse process of the atlas, and 
is inserted by one slip into the third rib, and by another into the fourth rib; the latter 
partly covers the cartilage. 
The supracostal muscle (woodcut, fig. 2, Sp.c) nearly agrees with the position it has in 
the Eared Seal. It extends rather further outwards, however ; and the fibres spring from 
the intercostal interspace and second rib, and reach backwards to the fourth costal 
cartilage. 
The levatores costarum are only fourteen in number—namely, one less than in the 
Sea-lion. 
Besides having muscular slips from the third to the seventh ribs, as obtains in 0. 
Jubata, two extra portions of the triangularis sterni proceed to the eighth and ninth 
rib-cartilages in the Morse. 
Muscles of the Pectoral Limb. 
Between the Walrus and the Sea-lion there is a marked distinction as regards the 
pectorales. Both animals, as likewise the Earless Seals, have a somewhat similar 
elongated powerful expanse of fleshy fibres attached to the thorax; and all have the 
superficial layer of muscle inserted low on the arm; but the boundaries of the muscular 
areas and separation into layers do not agree. 
In Trichechus a thick fleshy pectoralis major is well defined (see woodcut, fig. 2, 
Pma). The basal origin (for it is of an unequal-sided triangular contour) extends 
3 inches anterior to the manubrium, and reaches backwards to the posterior border of 
the ensiform cartilage or xipho-sternum. ‘The muscular fibres converge towards the 
axillary region and the elbow, and at the latter point terminate in a transversely 
arched manner in a thick aponeurotic sheet, which is carried on and is continuous 
with the superficial fascia of the forearm. 
A second, deeper layer to the above, or pectoralis minor (Pm), meets its fellow of the 
opposide side in the middle ventral line, and there extends from the fourth rib-cartilage 
to about six inches behind the ensiform cartilage. Its border overlies the costal 
digitations of the external oblique muscle; and it is itself hidden by the previously 
described pectoralis major, excepting a narrow strip of its outer edge. There are 
no directly transverse fibres as in the upper pectoral, but all assume a forwardly 
oblique direction as they pass to be inserted into the whole length of the shaft of 
