form posterior half. In adults the maxilla developes a thin plate below the origin
of the zygomatic portion of the malar, and the orbital process of the maxilla so expands
over it that the bone is firmly locked to the skull ; while in younger skulls
the malar is easily removed and indeed falls out. The round anterior extremity
projects considerably downwards, defining a deep infra-orbital notch. The head
of the bone anteriorly is also traversed by a longitudinal furrow from the lower
margin of the orbit. The zygomatic portion in the adult is occasionally supported
by a thin sheath derived from the frontal.
Maxilla (PI. XXXTX, figs. 1 and 2, and PI. XL, figs. 17 and 18).—The
dental portion is a long laterally compressed rod, tapering towards its free extremity,
and from the external and superior surfaces of its extremity springs an
upwardly and backwardly projecting portion which overlaps the frontal between its
naso-orbital processes, and which bears the crests which are so distinctive of this
dolphin.
The external surface of the maxilla, behind the open end of the dental furrow,
presents, in young skulls, two short parallel upwardly curved ridges; but these
are confluent in adult skulls and continuous at their upper extremities with the
margins or walls of an upwardly curved fossa which terminates immediately below
the orbit. In young skulls, the fossa communicates by an anterior round slit,
and posterior to this by a long narrow fissuré, with the backward continuation of the
dental cavity which forms the outer wall of the backward continuation of the true
maxillary fossa or antrum ; but it is marked by imperfections of ossification and by
a permanent fissure which leads from the antrum to the fossa. The antrum has
its margins usually connected by long spiculse or bands, opening posteriorly by
two orifices which are formed by the backwardly projecting process of the inner
wall of the fossa. This latter is applied to the palatine and forms the lower
border of the infra-orbital canal, into which the superior orifice leads directly,
while the inferior opens posteriorly by an arched margin along the lower border
of the inner plate of the mam'll a, (defining the outer wall of the fossa) and leads
into the pterygoid sinus. The inner wall of this orifice is formed by the outer
surface of the external plate of the maxilla in the fossa and by a portion of the
palatal. Above the external imperfection of ossification, three to four small oval
foramina open from the infra-orbital canal, and doubtless transmit branches of the
infra-orbital vessels and nerves. The inner plate of the maxilla, from what has been
said, thus apparently forms the outer wall of the fossa, but if the upward twisting
of the bone is kept in view, and also its relations to the pterygoid and palatal bones,
this portion may be regarded as the true palatine surface of the maxilla, as it supports
the lateral, muscular extensors at this part of the palate. Such being the case,
the palatal bone is not so abnormally situated as at first sight one would be led to
suppose. When the palatine is removed from the posterior extremity of the maxilla,
the commencement of the infra-orbital canal is enlarged, as that bone forms its posterior
boundary defining it as a circular orifice, the floor of which is formed by the
palatine ridge-like process of the maxilla, and to the under margin of which the
pterygoid is applied. The palatine process of the maxilla terminates posteriorly
in a hook and groove, the former embracing the palatine, and the latter receiving a
curved sharp ridge of the same bone. Behind this, looking backwards, is the' rough
surface for the articulation of the palatine, above which is the small surface forming
the boundary of the anterior wall of the external nares, posterior to the premaxilla.
Above this is the expanded surface of the maxilla, which is applied to the frontal on
either side of its nasal ridge or crest. This surface, which is the highest portion of
the maxilla, looks backwards and downwards, and its extent varies on the two sides
of the skull owing to its sinistral twisting, the left being by far the smaller. The
right surface is concave in its lower two-thirds and convex above at its internal
extremity, where it rests in a deep concavity on the anterior surface of the' frontal,
external to the nasal ridge. The frontal surface of the left maxilla is narrow and
wholly concave, the corresponding maxillary surface of the frontal being convex.
Anterior to the palatine surface of the maxilla and above the infra-orbital canal there
is a broad, but thin flat surface marked by numerous imperfections of ossification
in the young skull. This is defined above by a broadly arched free margin of the
external plate of the maxillary crest, the arch leading into the cavity that exists
between the two. plates of the crests. This flattened surface is not applied to the
frontal, so that a large space exists between the two bones; it is precluded from
communicating with the nasal cavities by the palatine and is closed below by the
pterygoids. Anteriorly it ife continuous with the orbital cavity, so that the latter
communicates in the dry skull with the interspace between the plates of the maxillary
crests. Immediately above this arch, and below the whole length of the exposed
portion of the maxillary crests, there is an elongated, slightly convex surface, to
which the upper portion of the internal surface of the orbital process or wing of the
frontal is applied. There is a ridge running downwards and forwards from the posterior
angle of the crest to the posterior end of the free arched margin, leading into
the cavity of the crest, and the ridge defines the posterior limit of the application
of the orbital wing of the frontal to the maxilla, the surface of the maxilla posterior
to it looking backwards and its orbito-frontal surface outwards. I t also defines
the posterior limits of the crest, and a line drawn from its lower extremity to the
orbital process, which projects forwards over the upper end of the interspace
(alveolar) between the plates of the maxilla, marks the lower limits of the maxillary
crest. An irregular ridge defines the base of the external portion of the crest
and corresponds to the upper free sharp wavy margin of the orbital wing of
the frontal. The external surface of the crest in young skulls is more or less
quadrangular, a form preserved in the adults, but becoming more constricted
posteriorly. At all ages, the superior posterior angle is rounded. In youth, an
external depression or constriction curves backwards, inwards and then forwards,
terminating immediately behind the superior anterior angle in a well-marked
depression; but with age the external surface of the crest becomes nearly flat,
and the constriction is reduced to a depression at the base of the crest about its
middle. In young skulls a number of fine grooves radiate inwards in a curved