2 



All the cervical and dorsal vertebr?e,* excepting tlie first 

 two of the former, support ribs. Each pair of ribs rakes 

 backwards, and then bending towards the median line 

 terminates in two cartilaginous filaments attached to the 

 ventral ' scute' or shield. The ' hoemal' arch is incomplete, 

 the pair of ribs being connected anteriorly by integumen- 

 tary tissue only. The ventral shield connected with each 

 pair of ribs is an organ of locomotion, and the numerous 

 shields arranged in clincher work act as legs when alter- 

 nately raised and depressed by the muscles of the ribs. The 

 snake uses these organs in difierent ways ; when moving 

 slowly a vermicular action along the ribs causes the ventral 

 shields to catch the ground in succession and the means of 

 progress is nearly imperceptible ; but rapid movement is 

 effected by the alternate contraction of a part of the 

 muscles of either side throwing the body into an S form ; 

 at each curve one side of the ventral shields holdinnf the 

 ground while the other side advances. These mnvements 

 are not done with any visible alternation, but in a progres- 

 sive and undulating manner of considerable gracefulness. 

 When turning round slowly, the movement becomes a kind 

 of counter-marching, where every pair of ribs has to come 

 up to the pivot point before proceeding in the opposite 

 direction. In case of alarm, these movements become more 

 sudden, the snake throws itself into sharp curves, and a 

 jerking motion of the body changes the ordinary imper- 

 ceptible gliding motion into a rapid series of wriggles. 

 Obstacles to other animals are rather favourable to the pro- 

 gress of snakes; whilst they are powerless on a smooth surface, 

 roughness aids them considerably ; a stone, a root or a twig 

 becomes a fulcrum for the anterior parts of the body, and 

 brings the posterior parts rapidly up, so that the ground 



* There is no real distinction between dorsal and cervical vertebrae, 

 the latter term is not, therefore, to be taken in a strict anatomical 

 sense. 



