ihe progressive Motion of Snakes. S43 



llic 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. 



At the termination of each rib is a small cartilage ia 

 shape corresponding to the rib, only tapering to the point. 

 Those of the opposite ribs have no connection, and, when 

 the ribs are drawn outwards by the muscles, are separated 

 to some distance, and rest through their whole length on 

 the inner surlace of the abdominal scuta, to which they are 

 connected by a set of short muscles : they have also a con- 

 nection with tliose of the neighbouring ribs by a set of 

 short straight muscles. 



These oDservations apply to snakes in general ; but they 

 have been particularly examined in a boa constrictor, three 

 feet nine inches long, preserved in the Hunterian Museum. 

 In all snakes, the ribs are continued to the anus, while the 

 lungs seldom occupy more than one-half of the extent of 

 the cavity covered by the ribs. These lo'ver ribs can only 

 be employed for the purpose of progressive motion, and 

 therefore correspond in that respect with the ribs in the 

 draco volans superadded to form the wings. 



The parts of which a description has been attempted, 

 will be better understood by an inspection of Plates VI. and 

 VII. than bv any explanation that words can convey. 



In Plate Vll. the joints between the vertebra and ribs are 

 represented of the natural size, from the skeleton of a large 

 boa sent from the East Indies by the late Sir William 

 Jones, and deposited in the Hunterian Museum. On tha 

 under surface of the vertebra is a protuberance for the at- 

 tachment of muscles peculiar to this genus; it varies in siz« 

 in the different species, and explains the power attributed 

 to the boa constrictor. 



When the snake is going to put itself in motion, the ribs 

 of the opposite sides are drawn apart from each other, add 

 the small cartilages at the ends of them are bent upon the 

 upper surfaces of the abdominal scuta, upon which the ends 

 of the ribs rest; and as the ribs move in pairs, the scutum 

 under each pair is carried along with it. This scutum, by 

 its posterior edge, lays hold of the ground, and becomes a 

 fixed point from whence to set out anew. This motion is 

 beautifully seen when a snake is climbing over an angle to 

 get upon a flat surface. 



When the atiimal is movinir, it alters its shape from a 

 circular or oval form to sumething approaching to a tri- 

 anjile, of which the surface on the ground forms the base. 



The coluber and boa having large abdominal scuta, which 

 Q 2 may 



