mechanism of the spine. 283 
In some birds, at the upper part of the neck, a bony arch ex- 
tends from one to the other, to preserve the blood vessels 
which pass under it from pressure. This is the case in the 
heron, and other very voracious birds. 
D. Lunated excavation for articulation with the superior 
vertebra. 
E. Semi-lunar convexity, corresponding to the excavation 
above mentioned. 
F F. Diverging inferior articular processes, with surfaces 
facing obliquely outward. 
G G. Superior articular surfaces, facing obliquely inwards. 
H. I. Spinal canal, imperfect in the skeleton, in consequence 
of the vacancy left between the diverging inferior articular 
processes and the superior converging plates, which unite to 
form the back of the canal K. 
L. Opening for the passage of the spinal nerves. 
M. Foramen for the passage of the carotid artery and 
branches of the spinal nerves. 
Fig. 6. A perpendicular section of two vertebras, showing 
the interarticular cartilages. 
28 , George Street, Hanover Square, 
April z, 1822 . 
