. . Hunterian course. 
MR. ST. GEORGE MIVART ON THE VERTEBRATE SKELETON. 987 
parts which are so), in which two longitudinal series of hard parts (having a numerical 
relation to the epaxial hard parts) may be developed, and may coalesce more or less 
completely together after or before solidification— Paraxial parts. 
They are subdivisible into (A) diapophyses and upper ribs, and (B) parapophyses, lower 
ribs and sternum. 
4. An inferior cylinder internal to the pleuro-peritoneal cavity (or continuous with - 
parts which are so), in which hard parts may be developed (having generally a numerical 
relation to the other, axial hard parts) —Hypaxial parts. 
These may be of two very different kinds. They may be (A) in close relation with the 
dorsal vessels— Hypapophyses, including “chevron bones; ” or (B) they may bound ex- 
teriorly the anterior part of the alimentary canal, or be in series with parts so bounding 
it—Visceral arches. 
By “ visceral arches" I mean the branchial arches, the hyoidean arch, the mandible, 
and the trabeculæ cranii. On the authority of Mr. Parker, I do not count the palatine 
structure as a distinct arch, as he informs me it is but a secondary growth. 
Hypaxial parts, then, are skeletal structures either, on the one hand, intervening be- 
tween the axis and the pleuro-peritoneal cavity together with its prolongations, or else 
parts serially homologous with others which do so intervene; or, on the other hand, 
parts placed within the inner wall of the pleuro-peritoneal cavity, together with its pro- 
longations, or else parts serially homologous with others which are so placed. Finally, a 
greater or less degree of coalescence, before or after solidification, may take place between 
epaxial parts and paraxial parts, or between paraxial parts and hypaxial parts. 
In vertebrates generally (Plate LIII. fig. 1) we have, at the anterior end of the axial 
skeleton, hypertrophied epaxial and hypaxial parts, i. e. the brain-case and visceral 
arches. Further back we have hypertrophied paraxial parts with much diminished 
hypaxial ones. Finally, towards the hinder end of the body (except in tailless forms) we 
have, in vertebrates above fishes, a reappearance of hypaxial elements, generally accom- 
panied by coexisting but distinct paraxial parts. In Fishes, in the same region, we have 
generally paraxial parts in union with more or less of the hypaxial element, or we have 
paraxial parts only, or, much more rarely, only hypaxial parts. 
In this way the much discussed theory of the essentially vertebrate nature of the 
skull reappears, but in diminished proportions. As suggested by Professor Huxley, we 
have no vertebral body beyond the sella turcica—the region of the trabeculæ belonging 
to the system of inferior arches, not of neural ones or of bodies. The anterior end of 
the neural canal (in an ossified condition) seems to present us with three segments, 
suggesting three vertebral elements, viz. :—1, the occipital segment ; 2, the alisphenoids 
and parietals; 3, the orbito-sphenoids and frontals. A sense-capsule is at the anterior 
part of each of these three segments. 
All the ossifications of the acoustic capsule belong to a different category. Only two 
distinct vertebral centres (the basi-oceipital and part of the basi-sphenoid), however, cor- 
respond now to these three pairs of neural arches ; but many others may have at one time 
existed and since disappeared. Professor Huxley suggested this as possible in his last 
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