C 276 3 
XXII. On the mechanism of the Spine. By Henry Earle, Esq. 
F. R. S . Surgeon to the Foundling, and Assistant Surgeon to 
St. Bartholomew’ s Hospital. 
Read April 25, 1822. 
Hav 1 n g been lately engaged in examining the structure of 
the vertebras in different animals, I have been particularly 
'struck with the mechanism of the spine and spinal canal in 
birds, by which a most remarkable degree of motion is gained 
in the neck, without any injury or pressure on a part of such 
vital importance to the existence of animal life, as the spinal 
marrow ; an extent of motion, so great indeed as completely 
to compensate for the deficiency of it in the dorsal and lumbar 
regions, as well as for the want of any prehensile power in 
the anterior extremities. In attempting to explain the nature 
of this peculiar mechanism, which tends to throw consider- 
able light on the physiology and pathology of the spine, I 
believe that I have not been preceded by any author. The 
cervical vertebras in birds are very numerous, varying from 
nine to twenty-four.* They differ considerably from one 
another, according to their situation, in the form and direction 
of their articulating surfaces, and in the number and shape of 
* This great diversity in the number of the cervical vertebrae in birds, is the 
more remarkable, when contrasted with the uniformity which pervades the class 
mammalia, where the number (with one single exception, the three-toed sloth) is 
constantly seven. The mole, whose head appears lost between the scapulas, has 
precisely the same number as the giraffe and the horse. 
