98 



ANATOMY OF VERTEBRATES. 



Disarticulated prosencephalic arch, Cod 

 (Morrhua vulgaris) 



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

 tuent bones have been subject to more extreme modifications. 

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

 forward, slightly expanding ; the neurapophyses, called ( orbito- 



sphenoids,' ib. 10, are small semi- 

 oval plates, protecting the sides of 

 the cerebrum ; the neural spine, or 

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

 ib. 11, is enormously expanded, but 

 in the Cod is single ; the diapophyses, 

 called ( post-frontals,' ib. 12, project 

 outward from the hinder angles of 

 the frontal, and give attachment to 

 the piers of the inverted haemal 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 



O 



water required for the branchial respiration. The haemal arch 

 of the present segment --enormously developed is plainly 

 divided primarily on each side into a pleurapophysis and hgema- 

 pophysis ; for these elements are joined together by a movable 

 articulation, whilst the bones into which they are subdivided 

 are suturally interlocked together. The pleurapophysis is so 

 subdivided into four pieces ; the upper one, articulating with 

 the postfrontal and mastoid the diapophyses of the tAVO middle 

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

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

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

 the lower piece is the hypotympanic, ib. 28 ; this presents a joint- 

 surface, convex in one way, concave in the other, called a ( gingly- 

 moid condyle,' for the hagmapophysis, 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 eel-tribe, as 

 in the Batrachia, figs. 43, 72,^, h, it is in two pieces. The greater 

 subdivision, in more actively breathing Fishes, of the tympanic 

 pedicle, gives it additional elasticity, and by their overlapping, 

 interlocking junction, greater resistance against fracture ; and 



