POISONOUS SNAKES OF NORTH AMERICA. 



367 



a 



"^ 



Fig. 14. 



POISON APPARATUS OF THE RATTLESNAKE. 



Eight side, a External pterygoid Ijoiie; 6 inter- 

 nal pterygoid bone; c palatal bone; d maxillary 

 bone; f lachrymal bone. 



(After Mitchell.) 



of the bone there is above a deep cavity foriuiiig the bony walls of the 

 " pit " already referred to, which separates two articular surfaces. Tiie 

 upper oue at the top of the in axil- ^ 



lary forms with the corresponding 

 concavefaceof the lachrynuil bone, 

 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 

 pterygoid bone. It will be seen 

 from the accompanymgcut (fig. 14) 

 that if the latter bone {a) 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 erection and 

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

 mtntion 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 [d), and running backward is in- 

 serted upon the enlarged posterior eud of the 

 pterygoid bone. The contraction of this 

 muscle pulls (direction l-m fig. 10) the ptery- 

 goids forward, which thus push the lower 

 end of the maxillary forward, the upper eud 

 being held in position by thelachrymal hinge. 

 The tip of the fang describing part of a cir- 

 cle, finally points 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-pteiygoid) muscle (fig. 15, i), 

 which arising from the Joint between the 

 quadrate 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 pterygoid bone. It 

 will be seen that contraction of this muscle 

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

 fig. 16), resulting in the backwaid and upward movement of the point 

 of the fang. 



Fig. 15. 



MUSCLES OF POISON APPARATUS OF 

 RATTLESNAKE, PALATAL VIF.W. 



a Sphenopterygoid muscle; J; ex- 

 ternal pterygoid muscle; c fascial 

 sheath of this muscle attached to 

 the capsule of the gland ; d median 

 ridge of base of skull. 



; After B^itchell.) 



