POISONOUS SNAKES OF NORTH AMERICA. 



367 



of tlie bout' tlR'ie is above a deep (cavity forming the bony walls of the 

 " pit " already referred to, whicli separates two articular surfaces. Tbe 

 upper one at the top of tbe niaxil- 



a 



Fig. ]4. 



rOISON APPARATUS OF THE RATTLESNAKE. 



Right side, a External pterygoid bone; & inter 

 iial iitervgoid bone; c palatal bone; d maxillarj 

 biine; f lachrymal boue. 



., Arter Mitchell.) 



lary forms with the corresponding 

 concave face of t h e lachrymal bon e. 

 which projects from and articu- 

 lates with the frontal bone, a 

 hinge-like joint, allowing consid- 

 erable freedom of motion. The 

 lower surface receives the flat- 

 tened anterior end of the external 

 })terygoid bone. It will be seen 

 from the accompanyiugcut(tig. 14) 

 that if the latter bone («) be moved 

 forward or backward, the maxil- 

 lary hinges on the lachrymal, and 

 that if the pterygoid be pushed 

 forward, the fang is erected. 



There are several muscles engaged in producing this ere<'tion and 

 the opposite motion, the depression of the fang, but we shall only 

 mention the two principal ones. 



The elevator muscle of the fang is the 

 sphenopterygoid muscle (fig. 15, «), which 

 arises along the median ridge of the base of 

 the skull ((?), and running backward is in- 

 serted upon the enlarged posterior end of the 

 pterygoid bone. The contraction of this 

 muscle pulls (direction \-m fig. 16) the ptery- 

 goids forward, which thus push the lower 

 end of the maxillary forward, the upper end 

 being held in position by the lachrymal hinge. 

 The tip of the fang describing part of a cir- 

 cle, finally i)oints downward instead of back- 

 ward. The chief retractor muscle, which 

 antagonizes the elevator muscle by acting 

 in the opposite direction, is the external 

 pterygoid (ecto-pterygoid) muscle (fig. 15, ^), 

 which arising from the joint between the 

 (piadrate bone and the lower jaw, runs for- 

 ward and is inserted on the outside of the 

 maxillary bone a little below the joint of the 

 latter with the outer ])terygoid bone. It 

 will be seen that contraction of this muscle 

 means a pulling backward of the maxillary bone (in the direction y>-e 

 tig. 16), resulting in the backward and upward movement of the point 

 of the fang. 



Fig. 15. 



MUSCLES OF POISON APPARATUS OV 



RATTLESNAKE, PALATAL VIEW. 



Spheno-pterygoid musele; 6 ex- 

 ternal pterygoid mu.scle; « tVisciiil 

 sheath of this mu.scle attached to 

 the capsule of the gland ; <l median 

 ridge of base of skull. 



(AllerMiIilieil. ) 



