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 ' prcsphenoid,' fig. 79, o, 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 ' 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 

 water required for the branchial respiration. The hamial arch 

 of the present segment — enormously developed — is plainly 

 divided primarily on each side into a pleurapophysis and ha?ma- 

 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 two 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 hasmapophysis, 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, g, 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 



