Cervical Rilis In Man. 95 



The dermal arinonr of the ArmndilU) Ims ])revented uiovenient 

 between the bones of the middle nf the nrik, and they are fused into 

 a rod of bone : this fact suirfrests that :is the necessity for joints 

 departs, they disappear. 



Bradypux. or three-toed sloth, has been compelled to lenjrthen its 

 neck to afford the head a wider ran<re of browsing circumference. As 

 a result of the impiilses generated by its hanging and browsing 

 habits, the first and second thoracic vertebrae have lost their ribs, 

 and thereby these l)ack l)ones have been converted into neck bon?s. 

 The animal s])ends its life hanging l)ody downwards from the branches 

 of trees, and under the impulses resulting froui this mode of life, 

 the fixed mammalian type has been broken down, and the rigidity 

 of the anterior thorax has given place to adaptations that have 

 extended the movements of the neck to the thoracic region. In this 

 mammal it is observed that under the necessity for elongating the 

 neck the first and second thoracic vertebrae are added to the neck, 

 which is not according to the mammalian rule. Tbe giratYe has elon- 

 gated its neck, but it has done so under the control of a strictly 

 quadrupedal gait and consequent!}" its neck is composed of seven 

 cervical vertebra, according to the law which holds the mammalian 

 neck true to type. The three-toed sloth, therefore, shows destruction 

 of the fixed mammalian type, and therefore conclusively proves that 

 fixed type must, when impulses change and assail its fixed characters, 

 yield up its stability, become plastic and change into aberrant type. 

 This fact shows that the law of evolution, like the law of gravity, 

 is ever operative : the evidence which the three-toed sloth offers in 

 this direction is augmented by every mammal that has changed from 

 the fixed mammalian type. 



Choloepus. — There is considerable diversity of opinion as to the 

 number of neck bones that are in the neck of the two-toed sloth. 

 Owen describes seven bones, Thomson six. and AViedersheim writes 

 thus of the ribs of mammals : — '" The cervical ribs in nearly all cases 

 unite completely with the vertebrae, and a vertebrarterial canal is 

 thus formed. The last cen'ical rib may be well developed and mav 

 articulate with the corresponding verteltra ((liohiepnx Hofnianii)." 



This diversity of opinion may ])e due to the different species of 

 two-toed sloths varying from one another, as do the two genera of the 

 order Sirenia, or to -variations within the species itself, as occurs in 

 man. 



The three-toed sloth has a short, thick body which is suspended by 

 short, thick limbs: the body is too short to sag in the middle of the 

 back, and it consequently hangs straight in the horizontal. The two- 

 toed sloths have long, narrow bodies and long limbs, and when 

 suspended in what would produce a horizontal position in the three- 



