GIANT GROUND SLOTHS GAZIN 353 



These vertebrae are normally symmetrical in mammals and there is 

 no mention by Richard Owen of such a condition though probably 

 present, in dorsal vertebrae of Megatherium. A single reference to a 

 similar condition was found in the case of the mylodont sloth remains 

 from the tar pits at Rancho La Brea in California, as described by 

 Chester Stock (1925). This was noted for all the second dorsal or 

 thoracic vertebrae and certain of the third; again the more slender 

 pedicle is always on the right. 



Speculation as to the cause of this asymmetry may be offered but 

 there is no certainty that any of the suggestions made are a solution 

 to the mystery. The nerves and blood vessels passing between the 

 arches in these positions should be equally developed on the two sides. 

 One might be tempted to speculate on a rather pronounced unilateral 

 development of a certain group of muscles. The nerves emerging from 

 the spinal cord in this region, however, are for the most part related 

 to the back muscles, skin, and certain of the muscles that function in 

 breathing. The nerves of the brachial plexus which control the fore- 

 limbs generally emerge from the lower neck region and from between 

 the first segments of the dorsal series. This distribution in the case 

 of the ground sloths might have been from a more posterior portion 

 of the anterior dorsals, but such an implied "righthandedness" is not- 

 reflected in the bones of the forearm. 



A second suggestion, which probably does not merit serious con- 

 sideration, is that the branches of the spinal nerve in the anterior 

 dorsal region communicating with the sympathetic trunk or nerve 

 might have been strongly involved in an asymmetric arrangement 

 with, for example, the cardiac plexus. The sympathetic, as well as 

 the vagus, nerve patterns are notoriously lacking in symmetry, so that 

 it might not seem too unreasonable to suppose that a nonsymmetrical 

 tie-in with the spinal nerve could have developed in ground sloths. 



As a remaining possibility, in contrast to the foregoing, one might 

 postulate suppression, through some means, of the proper functioning 

 of the nerves on the left side of the column. This would presumably 

 call for compensation through overdevelopment of those on the right. 

 Seeking a direct mechanical cause for possible suppression of nerves 

 on the left one cannot fail to note the proximity of the aorta artery 

 to the left of the vertebral centra. The segments of the vertebral 

 column involved correspond closely to the probable position of the 

 upper portion of the aorta descendens very near the arch of the aorta. 

 This in itself is not peculiar to sloths, but the rather tremendous body 

 bulk, coupled with the assumption of an upright posture much of the 

 time, lends credence to the suggestion that the pressure of the un- 

 doubtedly large aorta could have been responsible. Further pressure 

 on the thoracic cavity transmitted to the left side of the vertebral 



