164, Mr. Home's Observations to show 
The fact was readily established, and I could feel the ribs 
with my fingers as they were brought forward ; when a hand 
was laid flat under the snake, the ends of the ribs were dis- 
tinctly felt upon the palm, as the animal passed over it. 
This becomes a more interesting discovery, as it constitutes 
a new species of progressive motion, and one widely different 
from those already known. 
In the draco volans the ribs form the skeleton of the wings, 
by means of which the animal flies, the five posterior ribs 
being bent backwards and elongated for that purpose, so that 
in that instance the progressive motion is performed by the 
ribs, but those particular ribs are superadded for this purpose, 
and make no part of the organs of respiration ; whereas in 
the snake the ribs are so constructed, as to perform their office 
with respect to the lungs as well as progressive motion. 
That ribs are not essential to the breathing of all animals, 
whose lungs are situated in the same manner as in snakes, is 
proved by the syren having no ribs ; but as this animal has 
also gills, and can breathe in water as well as in air, the lungs 
are not so constantly employed, and probably a less perfect 
supply of air to them may suffice. 
In animals in general, the ribs are articulated to the back 
bone by means of a convex surface, which moves upon a 
slightly concave one formed upon two of the vertebrae, partly 
on the one and partly on the other, so that there is a rib situ- 
ated between every two vertebrae of the back ; but in the 
snake tribe, the head of the rib has two slightly concave sur- 
faces which move upon a convex protuberance belonging to 
each vertebra, so th a there is a rib to each of the vertebrae. 
One advantage of this peculiarity is, that it prevents the ribs 
from interfering with the motion of the vertebrae on one 
