Mr. Earle on the mechanism of the spine. 277 
the different processes, which afford extensive means of at- 
tachment to the muscles concerned in the different motions of 
the neck. Unlike the vertebras in man and most of the mam- 
malia, they are articulated together by complicated joints, 
which bear a close resemblance to the articulation of the 
olecranon with the humerus in the human subject, but dif- 
fering in some respects ; the vertebras in birds admitting of 
lateral motion as well as flexion and extension, whilst the 
elbow is strictly a hinge-like joint. 
The varying position of these articular surfaces is greatly 
favoured by the interposition of a cartilage, which is curiously 
adapted to the surface of each bone, and is enclosed between 
reduplications of synovial membrane; and thus each joint is 
double, consisting of two synovial cavities, and is analogous 
to the articulation of the lower jaw in man ; a circumstance, 
I believe, not mentioned either by Cuvier, Blumenbach, or 
Macartney. 
The canal of each vertebra is of very unequal calibre, the 
centre being narrowest. It enlarges above and below, and at 
each joint is nearly three times the capacity that it is in the 
centre ; and thus the canal of each individual vertebra may 
not unaptly be compared to an hour-glass. The canal is 
closed in front by the posterior surfaces of the bodies of the 
vertebrae, but behind it is very imperfect ; and in the skeleton 
there is a large lozenge-shaped opening, formed by the di- 
verging inferior articular processes and the converging plates 
which unite to form the back of the canal. This, in a recent 
state, is filled up by a membrane, and is protected by the 
highly elastic and powerful ligamentuin nuchas. 
This mechanism, besides allowing of the greatest possible 
