152 ANATOMY OF VERTEBRATES. 



The skull of the Boa Constrictor differs from that of the 

 Python, not only in the greater breadth of the frontals, but in 

 that of the nasals ; in the absence of the superorbital, in the 

 more slender and cylindrical form of the ectopterygoid, and in 

 the larger and higher internal border of the coronoid. But the 

 mechanism of the jaws is the same. By the elastic matter join- 

 ins: too-ether the extremities of the maxillary and mandibular 

 bones, those on the right side can be drawn apart from those on 

 the left, and the mouth can be opened not only vertically, as in 

 other vertebrate animals, but also transversely, as in insects. 

 Viewing the bones of the mouth that support teeth in the great 

 constricting serpents, they offer the appearance of six jaws — four 

 above and two below ; the inner pair of jaws above are formed by 

 the palatine and pterygoid bones, fig. 96, 20-24, the outer pair by 

 the mamillaries, ib. 21, the under pair by the mandibles, or 'rami,' 

 as they are termed, of the lower jaw, fig. 97, 31-32. 



Each of these six jaws, moreover, besides the movements ver- 

 tically and laterally, can be protruded and retracted, independently 

 of the other : by these movements the Boa is enabled to retain 

 and slowly engulf its prey, which may be much larger than its 

 own body. At the first seizure the head of the prey is held 

 firmly by the long and sharp recurved teeth of all the jaws, 

 whilst the body is crushed by the overlapping coils of the serpent ; 

 the death-struggles having ceased, the Constrictor slowly uncoils, 

 and the head of the prey is bedewed with an abundant slimy 

 mucus : one jaw is then unfixed, and its teeth withdrawn by 

 being pushed forward, when they are again infixed, further back 

 upon the prey; the next jaw is then unfixed, protruded, and 

 reattached ; and so with the rest in succession — this movement of 

 protraction being almost the only one of which they are susceptible 

 whilst stretched apart to the utmost by the bulk of the animal 

 encompassed by them : thus, by their successive movements, the 

 prey is slowly and spirally introduced into the wide gullet. 



In comparing the skull of a poisonous with that of a constrict- 

 ing Serpent, the differential characters consist, in the Rattlesnake 

 (Crotalus) e.g., chiefly in the modification of form and attach- 

 ments of the maxillary, which is movably articulated to the 

 palatine, ectopterygoid, and lacrymal bones ; but chiefly supported 

 by the latter, which presents the form of a short, strong, three- 

 sided pedicle, extending from the anterior external angle of the 

 frontal to the anterior and upper part of the maxillary. The 

 articular surface of the maxillary is slightly concave, of an oval 

 shape : the surface articulating with the ectopterygoid on the poste- 



