the progressive Motion of Snakes. 1% 
another. The vertebrae are articulated together by ball and 
socket joints (the ball being formed upon the lower end and 
the socket on the upper one), and have therefore much more 
extensive motion than in other animals. 
The muscles, which bring the ribs forward, consist of five 
sets, one from the transverse process of each vertebra to 
the rib immediately behind it, which rib is attached to the 
next vertebra. The next set goes from the rib a little way 
from the spine just beyond where the former terminates, it 
passes over two ribs, sending a slip to each, and is inserted 
into the third : there is a slip also connecting it with the next 
muscle in succession. Under this is the third set, which arises 
from the posterior side of each rib, passes over two ribs, 
sending a lateral slip to the next muscle, and is inserted into 
the third rib behind it. 
The fourth set passes from one rib over the next, and is 
inserted into the second rib. 
The fifth set goes from rib to rib. 
On the inside of the chest there is a strong set of muscles 
attached to the anterior surface of the vertebrae, and passing 
obliquely forwards over four ribs to be inserted into the fifth 
rib, nearly at the middle part between the two extremities. 
From this part of each rib a strong fiat muscle comes for- 
ward on each side over the viscera, forming the abdominal 
muscles, and uniting in a beautiful middle tendon, so that the 
lower half of each rib, which is beyond the origin of this 
muscle, and which is only laterally connected to it by loose 
cellular membrane, is external to the beiiy of the animal for 
the purpose of progressive motion ; and that half of each rib 
next the spine, as far as the lungs extend, is employed in 
respiration. 
