AQUATIC MAMMALS 



cated by any muscle pull or lifting action of the lungs the tendency would 

 be for the cross section of the anterior trunk eventually to assume a 

 shape that was exactly circular, for this is the ideal both for locomotion 

 and for such purposes as retention of body heat, as well as that external 

 pressure may be distributed evenly to all the internal organs. 



The point of greatest bodily circumference may be, and is, shifted ac- 

 cording to aquatic requirements without especial reference to the form 

 of the thoracic cavity itself, as fleshy or fatty tissue may easily be de- 

 posited in the required region. The actual external shape of the an- 

 terior trunk may be influenced by deposits of fat, by the conformation of 

 the shoulders and their muscles, by the height of the vertebral spines and 

 consequent thickness of the spinal musculature, and by the form of the 

 thoracic cavity. 



In the fetal state the shape of the cross section of the thoracic cavity, 

 in all mammals without exception, I believe, is slightly broader trans- 

 versely than in a sagittal direction. In terrestrial sorts its shape in after 

 life will alter according to posture, the fundamental forces concerned 

 being gravity and muscle pull. In a strictly quadrupedal form, and even 

 such a primate as the baboon, gravity will tend to pull down the ster- 

 num. This is doubtless assisted in some obscure way by muscular stress, 

 and by the requirement in swifter, larger mammals that the anterior 

 limbs be as near together as practicable to reduce any propensity to wad- 

 dle. The result will be a thorax narrow transversely. That muscle pull 

 need not be of great influence in the attainment of this result is indica- 

 ted by the situation in the existing sloths, which spend their lives either 

 hanging by their limbs or curled up in sleep. We might expect to find 

 that the constriction of the chest between the pendent arms had resulted 

 in its becoming narrower but on the contrary it is much broader than 

 thick, indicating that muscle pull has not been an important factor in 

 the expected direction, but that the chief influence was the removal of 

 ventral, gravitational pull, which has permitted the ribs to spring out- 

 ward. In mammals of erect posture, as man, there is also no gravita- 

 tional pull in a ventral direction upon the sternum, but here the throw- 

 ing back of the shoulders (ostensibly) has permitted a broadening, usu- 

 ally, of the thorax chiefly by tension of the muscles extending from the 

 arm to the midline, both dorsally and ventrally. Occasionally, in man, 

 a "pigeon-breasted" individual is encountered, in which the thorax is al- 

 most circular, so other and obscure factors are indicated. Or there may 

 be some such unusual situation as is found in bats, which require a broad 

 chest to accommodate a huge mass of pectoral musculature. 



[154] 



