THE TRUNK 



in otariids, but intervertebral flexibility would render it easy for the seal 

 to develop this ability did it have occasion for doing so. 



The number of ribs in the most generalized mammals is thirteen pairs, 

 while in the Pinnipedia they number fifteen, except in the walrus which 

 has fourteen pairs. This is accompanied in the former case by five, and 

 in the latter, by six lumbar vertebrae. So it is seen that the members 

 of this order have responded to a stimulus for a longer thorax, although 

 it is conceivable that this result was attained before aquatic habits were 

 adopted. Ten (usually) pairs of ribs are attached to the vertebrae by 

 both capitulum and tuberculum, and this, I believe, is the situation in at 

 least the majority of terrestrial carnivores. So in the latter detail there 

 has apparently been no alteration. The ribs are not otherwise note 

 worthy. 



In pinnipeds the sternum is usually composed of six or seven bony 

 elements, while in terrestrial carnivores it averages eight or nine. So the 

 sternum in the former group has suffered reduction, this being more ac- 

 centuated in the Phocidae than the Otariidae because in the latter the in- 

 dividual elements have a greater length. In all pinnipeds there is a pres- 

 ternal extension of the manubrium, partly cartilaginous, which is mark- 

 edly well developed, especially in the Phocidae, and this is of very definite 

 importance to the animal. 



With all of the above bony details of the pinniped thorax in mind we 

 can proceed to scrutinize some of the muscular stimuli involved. In 

 otariids the cervical and anterior thoracic muscles are developed not only 

 for agility of neck muscles for enabling the head and neck to act as a 

 balancer both on land and in the water, but to assist in movements of 

 the anterior limb in both situations. The spinal musculature retains to a 

 large extent its regular function of bending the vertebral column in the 

 sagittal plane. The lateral and ventral thoracic and the abdominal mus- 

 cles are mostly subservient to the functions of pulling the anterior limb 

 posteriorly and medially, and pulling the sacral region down and for- 

 ward during terrestrial progression. The musculature controlling the an- 

 terior limbs is discussed more fully under the latter heading, but it may 

 here be noted that the absence of a clavicle in the Pinnipedia (as is usual 

 in Carnivora), allows much freedom in movement of the shoulder, and 

 the specialized functions involved may or may not have had some influ- 

 ence upon the conformation of the anterior thorax. In both the sea-lions 

 and seals the noteworthy forward extension of the manubrium, partly 

 bony and partly cartilaginous, has undoubtedly been brought about in 

 response to the need for more powerful action of the pectoral muscles 



[161] 



