98 



ANATOMY OF VERTEBRATES. 



Disarticulated prosL-in-ciilinlic ! 



segments, of two arches and a common centre ; but tlie consti- 

 tuent bones have been subject to more extreme modifications. 

 The centrum, called ' presphenoid,' fig. 79, 9, is produced far 

 forward, slightly expanding ; tlie neurapophyses, called ' orbito- 



sphenoids,' ib. lo, are small semi- 

 oval plates, protecting the sides of 

 the cerebrum; the neural spine, or 

 key-bone of the arch, called ' frontal,' 

 ib. 11, is enormously expanded, but 

 in the Cod is single ; the diapophyses, 

 called ' post-frontals,' ib. 12, jn-oject 

 outward from the hinder angles of 

 the frontal, and give attachment to 

 the piers of the inverted htcmal arch. 

 The first bone of this arch is com- 

 mon in Fishes to it and to that of the 

 last described vertebra, being the 

 bone called ' epitympanic,' fig. 81, 25 ; 

 this modification is called for by the 

 necessity of consentaneous move- 

 ments of the two inverted arches, in 

 connection with the deglutition and course of the streams of 

 water req^uired for the branchial respiration. The hremal arch 

 of the present segment — enormously developed — is plainly 

 divided primarily on each side into a pleurapophysis and hrema- 

 pophysis ; for these elements are joined together by a movable 

 articulation, whilst the bones into which they are subdivided 

 arc suturally interlocked together. The pleurapophysis is so 

 subdivided into four pieces ; the upper one, articulating Avith 

 the postfrontal and mastoid — the diapophyses of the two middle 

 segments of the skull — is called 'epitympanic,' ib. 25; the hind- 

 most of the two middle pieces is the 'mesotympanic,' \h. 26: the 

 foremost of the two middle pieces is the ' pretympanic," ib. 27 ; 

 the lower piece is the hypotympanic, \h. 2S ; this presents a joint- 

 surface, convex in one way, concave in the other, called a ' glngly- 

 moid condyle,' for the h;\»mapophysis, or lower division of the 

 arch. In most air-breathing vertebrates — the Serpent, fig. 97, 

 e.g. — the pleurapophysis resumes its normal simplicity, and is a 

 single bone, 28, which is called the '' tympanic ; ' in the ecl-tribo, as 

 in the Batrachia, figs. 4.3, 72, 7, h, it is in two pieces. The greater 

 sidjdivision, in more actively In-eathing Fishes, of the tvmpanic 

 pedicle, gives it additiimal elasticity, and by their overlappino-, 

 interlocking junction, greater resistance against fracture : aiul 



