William Kitchen Parker.

The morphology of the skull online

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first branchial, arches;, external branchiie.

47. Before the parachordals can be plainly made out,
a pair of broad flat bars is manifest in the hinder part of
the down-turned cranial floor, behind and below the first
two brain- vesicles, and partly below the third. Their
plane is directed forwards and downwards from the middle
of the base of the third vesicle to the middle of the first.
These trahecuUe (Fig. 1, tr) embrace the pituitary body
and infundibulum by an arcuate inner margin, so as to
leave between the two bars an oval space equal in width
to themselves. In front they very nearly meet behind
the nasal sacs, where they project bluntly towards one
another. From this inner point their boundary sweeps

B. BL 2

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crescentically outwards, and then again beads inwards
in front of the auditory capsules. The trabeculae are ex-
panded where they abut against these capsules, wedging
in for a little distance between them and the para-
chordals. Posteriorly the trabeculae lie on the lore end
of the parachordals, forming a slight elevation directed
backwards, behind the pituitary region : they closely em-
brace the fore part of the beaded portion of the notochord.

48. In front of the trabeculae and between the nasal
sacs there is a median styloid tract of condensed tissue,
broad behind, filling up the angle where the trabeculae
almost meet, and then growing forwards as a pointed rod
between the nasal sacs and beneath the fore-brain. Be-
fore the end of this stage the ear-sacs begin to acquire a
cartilaginous covering : but excepting in the regions which
have been described, the boundaries of the cranial cavity
are entirely membranous and very diaphanous.

49. Each visceral arch has a cartilaginous axis, form-
ing a morfe or lesis arcuate rod (Fig. 1, mn, hy, hr) ; and
these cartilages will in future be referred to as the mandi-
bular, hyoid, and branchial bars or arches. The upper
ends of the branchial bars point directly inwards : but
each of the two anterior arches sends forwards from its
upper extremity a considerable flat process. The process
from the mandibular arch is so large as to be really a
continuation of it ; and it occupies the thick ridge which,
forms the lateral boundary of the mouth on either side :
it is the palato-quadrate plate.

50. There is very little upward expansion of the
mandibular arch where it bends forwards; a little later,
ligamentous fibres connect the region of the bend with
the side wall of the cranium immediately in front of the
auditory capsule and below the exit of the trigeminal
nerve. The palato-quadrate plate of either side grows to-,
wards its fellow, and ultimately meets it below the level
of the eyes and behind the nasal sacs: the two become
connected by ligament, and a deep groove is left between
the bars and the nasal sacs. The anterior process of the

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hyoid ST6h applies itself along the lower side of the
auditory capsule posteriorly. The branchiid arches remaia
aiiaple throughout this period.

51. During the growth of the processes of the first
two arches, the upper part of the cleft between them
becomes much wider, and is finally almost triangular,
with the base above. The succeeding clefts also b^me
wider above than below. The posterior edge of each arch
developes a valance-like ridge or fold tending to cover the
cleft behind it.

The subsequent growth of these folds closes in the clefts
very much, leaving only the adult branchial slits. Such folds
entirely close most oi the visceral clefts in abranchiate verte-
brates. The fold on the hyoid arch is homologous with the
opercular fold in osseous fishes.

52. In the youngest embryos at this stafi^e a series of
rounded papillse, the rudiments of external gills, are found
on all the visceral arches except the last. Later on, these
papillse develope into long filaments, each containing a
single capillary loop. About ten of these spring from the
hyoid and each of the branchial arches ; the mandibular
has four, much shorter than the others, arising from its
upper posterior edge in front of the first cleft. More in-
ternally on the posterior edge of each arch bearing ex-
ternal gills, is a series of cog-like projections, the rudiments
of the internal gills : they form double series on the first
four branchial arches.

53. In embryos of a Skate {Raia maculata), about an
inch and a third in length, (Fig. 3), taken from the purse
seven weeks after oviposition, development has not pro-
ceeded farther than m the Dogfish already described.
The extra length is due to the tail: in other respects
it does not differ markedly from the larval Dogfish. In
the whole cranial region there is no difference which re-
quires special notice.

54. The mandibulo-hvoid cleft (sp) early fills up
below, and is pear-shaped above. The first branchial


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cleft (between the hyoid and the first branchial arch) is
similar to the preceding in its upper region, but retains
the lower slit-like part. All the arches except the last
branchial bear long spatulate external branchial fila-
ments; there are six or seven to each arch except the
mandibular, which has only four. In an earlier stage the
rudiments of the branchiae are seen as very numerous
minute buds arranged in series. Certain of these shoot
out to become the external transitory gill-filaments ; the
others afterwai'ds develope into the permanent gills.

Fig. a

Head of embryo Skate, 1^ in. long. Tr. trabecnla ; pi. pt,nm. palato-
mandibular arch ; m. pt. metapterygoid cartilajge ; h. m. hyomandibolar ;
hy. remainder of hyoid arch ; br. 1. first branchial arch ; pn. pineal gland ;
sp, mandibolo-hyoid cleft or spiracle.

55. The mandibular and hyoid arches are more
markedly bifurcate above than in the Dogfish, the anterior
forks, however, being much the larger. The palato-
quadrate region (pL pt) has the same relations as in
the Dogfish: a separate cartilage {m, pt) arises in the
hinder fork of the arch, in front of the first cleft. It
is placed in front of and below the auditory capsule,
and is the rudiment of the spiracular cartilage.

56. Behind the first cleft, the anterior process of
the hyoid arch is developed as a separate cartilage, lying
below the auditory mass : it is the hyomandibuUir element
(h, m.). The hinder fork of the arch is loosely connected
with the ear-sac behind, and it grows inwards towards

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the axial parts in a pointed manner ; it diverged upwards
and backwards from the ventral part of the arch (Ay),
almost as much as the palato-quadrate diverges forwards
from the mandible proper.

57. The branchial arches (5r) in the greater portion
of their extent, laterally and below, are simple unseg-
mented arcs; but a separate little curved pharyngo-
branchial element early developes in each, above and
mesiad of the main bar, in the pharyngeal roof.

58. The primary basal elements of the skull are
already clearly manifest in these young embryos, the
parachordals forming the base of the hinder cranial
region, and the trabeculae of its anterior portion. The
latter relation is somewhat masked by the mesocephalic
flexure; the trabeculse appearing to be behind rather
than below some parts of the brain. But thev lie truly
in the cranial floor, as is very evident after the flexure
has passed away. The mandibular arch is seen in these
types more distinct from other parts than in many ver-
tebrate forms : its shape is already specialised almost
to the adult condition. The origin of the hyomandibular
element separately in the Skate is noteworthy as com-
pared with its segmentation from the main hyoid bar
elsewhere. In other respects these fishes show to great
perfection the separate elements of which the skull is

Second Stage.

59. In embryos of the Dogfish from 12 to 15 lines
in length the external gills are two or three times as
long as in the preceding stage; the four filaments on
the mandibular arch are only one-third the length of
those on the other arches. The mesocephalic flexure
remains, although rapidly diminishing, and the ** middle
trabecula " is not absorbed ; the brain has become much
more complex.

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60. The trabecitlse (Figs. 4 and 5, tr,) are now fully
<^hondrified, but they are still distinct from one another.
They are relatively longer, and the interti-abecular space is

Fig. 4.'

Head of embryo Dogfish, l|m. long. TV. trabecula ; plpt,qu. palato-
quadrate bar; mck. meokelian cartilage; h,m. hyomandibular ; c.hy,
Ceratohyal; Z3,4,5, labial cartilages; the branchial ardies and extemed
gills are indicated behind the hyoid archv

oblong, with rounded angles. Externally the trabecular edge
is straightened, and so also is tlie anterior margin, which
extends furthest forwards in the middle line. Postero*
externally a crest rises from the hinder half of the trabeculae
in the cranial wall, nearly reaching to the auditory mass^
and arching back over the exit of the trigeminal nerve.
The hinder part of each trabecula has coalesced with
the corresponding parachordal {pa. ch,) ; the trabecula at
the junction forms a low posterior clinoid wall, which
is not continuous across the notochord, but which reaches
to the auditory capsule and fuses with it by its thick
outer extremity. The angle where the trabeculae and
parachordals unite is fast lessening, and they will soon
lie in one plane as a continuous basicranial bar on each

61. The parachordals (Fig. 5, pa. ch.) are well chon-
d rifled, as also is the distinct tubular tract surrounding
the notochord; the parachordals have not united. Thef
apex of the notochord is Hat, the beaded appearance being

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lost; it is wedged in between the two halves of the clinoid
^all just described, and the hinder face of the pituitary
body. The auditory, capsules (au.) are chondrified ; they
are elongated oval in shape, showing enlargements over
the course of the semicircular canals. Most parts of the
roof and walls of the cranium are still membranous.

62. The intemasal element is not fully chondrified ;
it is in the same condition as the nasal roofs, but
distinct from them. Where it lies between the nasal
sacs its relations are substantially as before; but ante-
riorly it has sent forward three prenasal lobes, a median
rounded piece projecting forwards as the prenasal ele-
ment {pn)y and a lateral pair of curved bifoliate lobes
growing round and applying themselves to the antero-
intemal face of the nasal sacs below : these are the commx
trabecjdce (c. tr,).

63. The palato-mandibular arch has undergone trans-
verse segmentation just at its greatest bend : so that the
arch now consists of an upper and anterior portion answer-
ing to the palatopterygoid and quadrate regions of other
vertebrates (Fig. 4, pi pt, qa), and of a lower and hinder
portion, the meckelian or mandibular cartilage. The
metapterygoid ligament is further developed, but retains
its primitive relations ; while a stronger ligamentous union
arises, below the remains of the first cleft (the spiracle),
between the posterior edges of the quadrate and articular
regions of the mandible, and the front edge of the hyo-
mandibular and ceratohyal (see following paragraphs) ; the
upper portion of this ligament may be called sympUctic,
the lower mandibulo-hyoid. In this way by the weakness
of the direct attachment of the jaws to the cranium, and
their suspension from the hyomandibulars^ their great
mobility is provided for.

64. The cartilages of the upper and lower jaw just
mentioned are simple finger-shaped cartilages, those of
each pair becoming rounded towards the middle line,
approaching one another, and being united by ligament.
The palato-quadrate is the larger; it is at first directed

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forwards, then somewhat inwards, and finally almost
abruptly inwards so as to become nearly transverse where

Fig. 6.

Head of embryo Dogfish, second stage ; basal view of craniam from
above, the contents having been removed. 01, forebrain; ol. olfactory
sacs ; an. auditory capsule ; nc. notochord ; py, pituitary body ;
parachordal cartilage ; tr. trabecula ; inf. infundibulum ;, comua
trabeculae ; pn. prenasal element ; sp. spiracular cleft ; hr, external
branchiffi ; cl. 2, 4, visceral clefts.

it lies behind the nasal sac and exactly beneath the
anterior termination of the trabecula. The hinder or
quadrate region is somewhat compressed, and lies in the
cheek above its convex articular condyle. The meckelian
cartilage is thick and solid where it receives the condyle
in a concave surface, and curves but slightly in proceed-
ing forwards and inwards to its. rounded termination in
the chin.

65. The upper part of the hyoid arch, including its
forward process, has been segmented oflf in a position
parallel with the articulation of the jaws. This hyoman-
dibular element [h. m. Fig. 4) developes an oblong articu-

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lating surface in relation to the infero-Iateral region of the
auditory mass at the middle of its length. Below this
facet the cartilage bends forwards so as to be concave on
its hinder margin, while the projecting angle in front
is attached to the quadrate region of the upper jaw by
the symplectic ligament. Below this again the hyoman-
dibular turns a little backwards, and bears a notch and a
posterior angular process for articulation with the simple
arcuate lower piece, the ceratohyal (c. Ay.). Between the
ventral ends of the ceratohyals there intervenes a flat
shield-shaped haaihyal cartilage.

66. The branchial arches are divided into upper and
lower pieces in a manner generally similar to the arches
in fronts forming epihrailchicd and ceratobranchiai seg-
ments; and above each epibranchial a small separate
cartilage is developed, turning inwards and backwards in
the roof of the pharynx : this is the plturyngobranchial.

67. Morphological advance has been signalized in
this stage by opposite processes in the cranial and the
facial regions. In the former the elements which were
primitively distinct, the trabecular and the parachordal,
have coalesced : but the palato-mandibular and the hyoid
bars have undergone segmentation, and in them the
essential relations of the adult condition have already
been established. The nasal region gives as yet only a
mere indication of its future cartilaginous skeleton :
while the cranium itself remains membranous for the
most part.

Third Stage.

68. Embryos of the Dogfish, from an inch and a half
to two inches long, have undergone great metamorphosis.
The external gills are still long, but the internal gills are
functional The mesocephalic flexure is lost ; so that the
buccal cavity is not now behind the first cerebral vesicle,
but Ues below it. . (See Fig. 6.)

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69. The parachordal cartilages (pa. cA.) have grown
round the notochord, above and below, enclosing it com-
pletely, and have coalesced with its cartilaginous sheath ;
thus they unite with each other to form the basilar plate.
The cranial part of the notochord is soon reduced to a
mere posterior cone of gelatinous tissue in the hinder
part of this plate. Behind, it bears a rounded knob on
each side for articulation with the first vertebra ; in front,
although it is fused with the trabecular plate, an emargi-
nation in the middle line is very plain below, the cartilage
becoming very much thinner,

70. Seen from below, the basilar plate is broad and
subquadrate, generally convex, with a groove in the middle
line. It extends widely outwards so as to ^support the
auditory capsules entirely, and come into relatic«i with the
hyomandibular articulation*

71. The lozenge-shaped sub-pituitary space is now
thinly floored by cartilage which is confluent with the
trabecular plate all round, and rises into a low posterior
clinoid ridge behind, directed a little forwards. The
subpituitary cartilage is pierced in its widest part by the
internal carotid arteries (i. car), which, however, lie close
together. The so-called "middle trabecula'' is a mere
fissure between the mid-brain and the hind-brain.

72. The trabeculse have coalesced and now form a
broad flat trabecular plate. This extends most widely out-
wards behind, where a short curved process is sent out
beyond the prominent antero-external angles of the basilar
plate, being not yet perfectly confluent with the latter \
More anteriorly the trabecular, plate not only forms the
floor of the cranial cavity, but also partially supports the
eyeballs. Each antero-external angle is curved outwards
and somewhat backwards, being loosely connected with the

^ The imperfect confluence of these regions, combined with an ex-
aggerated growth external to the auditory capsules, produces a large
fenestra in certain forms, as Carcharias glaucus, and Lamna comubica^
bounded by an extensive curved baud of cartilage.

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palatine cartilage of the same side. This angle may be
termed the antorbitcU or ethmopalatine process.

73. In front of the ethmopalatine region, the tra-
becular plate, which has been gradually rising towards the
level of the nasal roof, is suddenly narrowed, so that it has
a nearly transverse anterior margin except in the middle
line, where it is confluent with the intemasal element
This intemasal plate, which is not very wide, sends for-
wards in the middle line from its transverse anterior
margin a short linear prenasal rostrum.

74. The intemasal plate gives off on its upper aspect
a double ascending lamina, the moieties of which soon
diverge outwardly as broad wings, and coalesce with the
inner wall of each nasal sac. Each of these nasoseptai
hmincB bears a small sigmoid comu (c. tr.) in front, pass-
ing inwards towards the prenasal callage. The roof and
outer walls of the nasal sacs (na,) have very largeljr chon-
drified, forming dome-shaped capsules ; and in addition to
the coalescence of each olfactory cartilage with the corre-
sponding nasoseptai lamina, it is confluent with a cartila-
gmous tract developed in the supraorbital band (§ 41, p. 15).

75. No cartilaginous laminae have arisen in the mul-
tiplicate nasal membranes. The opening through which
each olfactory cms reaches the corresponding nasal cavity
is large, and is situated in the inner side wall of the dome
behind. Owing to the outspreading of the nasoseptai
wings, the opemng is nearly horizontal at this stage ; sub-
sequently it becomes slanted upwards and forwards.

76. Very much of the cranial roof and walls is chon-
drified, the elements being more or less distinct, and more
or less fused with the cartilages already mentioned. There
is a flat cartilaginous cranial roof, or tegmen cranii (Fig. 6),
extending from the supraoccipital or posterior cranial
region {s, o.) to the level of the fore part of the eye-
bflils, and on either side is the curved supraorbital tract,
continued anteriorly to a junction with the olfactory cap-
sules. The front edge of the cranial roof is crescentic and

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incomplete, exposing the olfactory crura as they diverge
to their destinations.

Fig. 6.

Head of embryo Dogfish, third stage; median longitudinal section.
CI, forebrain, (72, midbrain, my. myelon; na, olfactory sac; pa, eh.
parachordal cartilage; s.o. supraoocipital cartilage; pty. pituitary body;
i,car. internal carotid artery ; ctr, cornna trabeculae ;, palate-
pterygoid cartilage; mck. mandibular cartilage; h.hy. basihyal; c.hy.
ceratohyal;, first hypobranchial ; c.fer.l, first ceratobranchial ; extrabranchial cartilage; h. heart;, epicoracoid cartilage;
behind vertebral centra.

77. Between each trabecula below and the supra-
orbital band above, there is a cartilaginous sphenoidal wall
continuous with both, extending from the auditory to the
nasal regions. The supraorbital ridge forms a partial roof
for the orbit, while the outer edge of the trabecula fur-
nishes a partial floor. The optic nerve pierces the sphe-
noidal wall at its anterior third. The trigeminal nerve
passes out in the primary fissure between the sphenoidal
wall and the auditory sac ; when they have coalesced, this
fissure becomes crescentic, and may be called the tri-
geminal foramen. The nasal branch of the ophthalmic
nerve (from the trigeminal) passes forwards in the angle
between the sphenoidal wall and the preorbital part of
the supraorbital band where it goes to join the olfactory
capsule. The facial nerve emerges at the junction between
the auditory mass and the trabecular plate, and curves
upwards and backwards round the former.

78. The cartilaginous capsules of the auditory sacs are
complete on both the inner and outer sides except where
nerves and vessels pass. The capsules have coalesced .with

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the mipraorbital band in firont and above, with the supra-
occipital cartilage and the tegmen cranii on the inner
side above, and with the basiko' plate below. The semi-
circular canals are prominent and large, and the sur-
rounding cartilage is correspondingly bulged outwards.
Over the junction of the anterior and posterior semicircular
canals an unchondrified tract is seen on the upper surfiEtce
of the skull. The foramina for the glossopharyngeal and
the vagus nerves are one behind the other, behind the
auditory capsule, near the postero-extemal angle of the
basilar plate.

79. The palato-quadrate bar has become much larger ;
and it is very like the articulo-meckelian in form and size.
The moieties of each jaw meet in the middle line, where a
strong ligament unites them. At the symphysial ends the
rods are terete and incurved ; proximally they are flat and
sinuous. The articular region is gently scooped to work
on the convex quadrate condyle.

80. The enlarged hyoid arch has undergone no im-
portant change: the hyomandibular is articulated with
the auditory capsule and the edge of the basilar plate
below the horizontal semicircular canaL Between the
ventral ends of the richt and left hyoid bars there is a
broad, flat, basihyal pkte of cartilage, (6. Ay.), which is
like a keystone to tne arch. One more flat keystone
occurs behind, between the two last branchial arches;
but the hypolDranchial pieces of the second and third
arches turn backwards to reach the front edge of this
hasibranchial. The first hypobranchiaJs are but small
lobes (h. br. 1), and do not extend backwards.

81. The branchial arches are very similar, each con-
sisting of four pieces. Their hypobranchials partially floor
the throat cavity; their cerato- and epibranchials form
its lateral boundaries which are very convex outwardly.
The upper segments or pkaryngobranddaU are turned
backwards in the roof of the throat. The fourth pharyngo-
branchial is forked externally, the hinder fork being applied
to the fifth arch so as to be continuous with its epibran-

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chial ; there is a fifth ceratobranchial, from which no hypo*
branchial is cut off.

82. Two other series of cartilages, called labials and
extrabranchials, have now to be described. Three labial
cartilages are related to the anterior and outer edges of
each nasal capsule. The first is pyriform and is already
furrowed above by two or three sUme-glands ; the second
and third are found in valvular folds of the nasal opening :
the second is crescentic, and its lower horn partially floors
the nasal aperture; the third is ear-shaped, and lies on
the outer edge' of the capsule. A fourth labial, lanceolate
in shape, underlies each palato-quadrate bar on the upper
edge of the angle of the mouth; whilst a fifth labial^
converging towards the fourth at the angle, is upon the
external surface of the mandible. Similar cartilages, the
extrabranchials (ex. br.), appear outside each of the first
four branchial arches. They are pointed above and broader
below ; the last is very small.

83. In an embryo of Raia maculata four inches long,
taken from the egg-pouch three months after oviposition,
the tail is twice as long as the body proper, and the
yolk-sac is still of the size of a small walnut. Yet the

Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 3 of 31)