Dissect the skin off the head, neck, and body to back of the pectoral
fins. Observe first the musculature of the dorsal side of the neck and
of the body back of the bases of the pectoral fins, noticing that it is
composed of narrow, zigzag bands, called _myomeres_. Where these are
fully developed they extend from the mid-dorsal to the mid-ventral line.
Note carefully the relation of corresponding myomeres of the two sides,
the exact course of a single myomere, and the direction of the muscle
fibres in a typical myomere. Observe also that the muscles above the
level of the vertebral column form a thick mass, which is frequently
referred to as the dorsal musculature; the muscle below this level may
be correspondingly referred to as the ventral musculature. As the
muscles described below are dissected the mechanical effect of each
should be determined.
MUSCULATURE OF THE HEAD AND NECK. On the lateral and ventral surfaces of
the neck the primary relations of the myomeres are much modified by the
development of numerous special muscles, yet here and there traces of
the metameric arrangement still show. Immediately beneath the skin is a
thin sheet of muscle covering most of the ventral and lateral surfaces
of the throat as far back as the pectoral girdle. On the ventral surface
a triangular space is left in front of the pectoral bar; on the sides of
the neck the sheet extends back to the last gill-cleft; dorsally, it
reaches to the upper extremities of the gill pouches. This is the
_constrictor superficialis_ muscle. It is attached to fasciae dorsally
and ventrally, and to the extra-branchial cartilages.
The constrictor superficialis consists of six metameric segments. The
four posterior ones are distinctly limited by the gill-slits and
extra-branchial cartilages. The second is anterior to the first gill
slit, the largest of all, with distinct dorsal and ventral portions
extending forward above and below the jaws. The first is recognized as
consisting of two distinct parts, on the dorsal and ventral surfaces of
the head. The dorsal portion is a small curved muscle on the anterior
wall of the spiracle, extending from the external surface of the
auditory capsule to the inner surface of the lower jaw. It lies close
against the levator marillae superioris (see below). On the ventral
surface of the throat the posterior constrictor muscles of the two sides
are separated by a large triangular area. In front of this the ventral
portions of the first and second constrictors meet in a median
aponeurosis, from which their fibres extend transversely, those of the
first to the mandibular cartilage, those of the second to the hyoidean
cartilage. The first constrictor lies ventral or superficially to, and
largely covers the second. Reflect the first constrictor from the
aponeurosis outward, and demonstrate the two layers of muscle. It will
be noted that the second to sixth constrictors consist of united dorsal,
lateral and ventral portions, while the first is reduced to widely
separated dorsal and ventral parts.
On each side of the head, just outside the angle of the mouth, is a
large, thick muscle arising from the lateral surface of the cranium, and
inserted upon the outer surface of the mandible, the _adductor
mandibularis_.
In front of the small dorsal constrictor superficialis 1, and scarcely
separated from it, is the strong _levator maxillae superioris_, which
extends from the lateral surface of the auditory capsule to the dorsal
edge of the palato-quadrate cartilage.
Eugaleus. The dorsal portion of the _constrictor superficialis 2_
reaches above the spiracle to the postorbital process. Reflecting it,
the _adductor hyomandibulae_ is seen behind the spiracle, arising from
the upper part of the side of the auditory capsule and inserted on the
end of the hyomandibular cartilage. The _levator palpebrae inferioris_
arises under the origin of the levator hyomandibulae, and passes forward
and downward between the spiracle and postorbital process, to insert in
the posterior end of the lower eyelid. The _depressor palpebrae
superioris_ arises from the fascia dorsal to the spiracle, passes mediad
to the levator palpebrae inferioris, upward and forward, to insert in
the posterior end of the upper eyelid. Remove these muscles. The
infra-orbital canal passes mediad to the muscles of the eyelids. The
_levator maxillae superioris_ lies between the spiracle and the orbit.
Behind it is a small slip of muscle extending from the anterior wall of
the spiracle to the lateral surface of the auditory capsule which seems
to represent the _constrictor superficialis dorsalis 1_.
A thin sheet of muscle covers the anterior face of each inter-branchial
septum. At the surface these pass into the constrictor superficialis,
and are evidently portions of the latter muscle, though they are named
the _musculi interbranchiales_.
Above the constrictor superficialis, lying on the side of the neck
between it and the dorsal musculature, is a broad _trapezius_ muscle. It
arises from the fascia covering the lateral surface of the dorsal
musculature. Its fibres pass obliquely downward and backward, mediad to
the posterior gill pouches, to insert upon the anterior edge of the
scapular portion of the pectoral girdle. The anterior portion of the
trapezius is also inserted upon the epibranchial of the fifth gill arch.
Just in front of the mouth is a pair of strong muscles (_levator
labialis superioris_), each arising from the ventral surface of the
cranium close to the median line. They pass into strong tendons which
are inserted among the fibres of the ventral portion of the adductor
mandibulae. The muscle mass in front of the mouth and the lower part of
the adductor mandibularis thus form the two bellies of a digastric
muscle, with the tendon between them.
[2]Remove the ventral portions of the first and second superficial
constrictors and clear the mass of muscles lying between the coracoid
portion of the pectoral girdle and the mandible. Immediately in front of
the girdle are two large muscles, the _coraco-arcuales communes_, whose
fibres run inward and forward. These muscles cover the ventral surface
of the pericardium, to the wall of which their median fibres are
attached, while the lateral fibres are attached around the ventral ends
of the gill arches.
Footnote 2:
The coraco-mandibularis, coraco-hyodeus, coraco-arcualis communis, and
coraco-branchialis muscles should be dissected as a preliminary
operation to following the ventral aorta and its branches.
In front of the coraco-arcuales communes are three large longitudinal
muscles. The median, unpaired one, arising from the fascia between the
coraco-arcuales communes and inserted upon the posterior surface of the
lower jaw, is the _coraco-mandibularis_. The other two, which lie dorsal
to and outside of the coraco-mandibularis, are the _coraco-hyoidei_.
They arise from the fasciae covering the anterior ends of the
coraco-arcuales communes and posterior parts of the coraco-branchiales,
and insert upon the basihyal.
Dissect out the coraco-mandibularis and coraco-hyoidei, noting
particularly the form and place of origin of the latter. Dorsal to the
coraco-hyoidei are the first divisions of the right and left
_coraco-branchialis_ muscles, which arise from fascia covering the
anterior ends of the coraco-arcuales communes, and are inserted upon the
ventral extremity of the ceratohyal cartilage. Notice that they pass
dorsad to the anterior branches of the aorta, and that the aorta itself
can be exposed between them.
Remove the coraco-arcualis communis by dissecting it from the pectoral
girdle and reflecting it forward. The other four divisions of the
coraco-branchialis are now revealed, attached to the lateral surface of
the pericardium and the lateral portion of the coracoid. The divisions
of the muscle are clearly separated only near their insertions. The
second, third, and fourth coraco-branchials are attached to the
hypobranchial cartilages of the second, third and fourth visceral
arches. The fifth division is inserted upon the lateral portion of the
basibranchial and the expanded medial end of the fifth ceratobranchial.
_The first_ aortic branch passes ventral to the first coraco-branchial.
_The second_ aortic branch passes between the first and second
coraco-branchials. _The third_ aortic branch passes between the second
and third coraco-branchials. _The fourth_ aortic branch passes between
the third and fourth coraco-branchials.
Expose the dorsal ends of the gill arches by clearing away muscles and
other tissues between the gill pouches and the spinal column. Two sets
of four small muscles (interarcuales) will be found connected with the
branchial cartilages. The second, third and fourth _medial
interarcuales_ extend from the posterior surfaces of pharyngo-branchial
cartilages 1, 2, and 3, to the dorsal surfaces of pharyngo-branchials 2,
3, 4, and 5. The first medial interarcuale arises from the under surface
of the cranium and inserts on the upper end of the first
pharyngo-branchial.
The _lateral interarcuales_ lie immediately below the medials. The first
has a double origin, most of the fibres arising from the lower part of
the posterior edge of the first pharyngo-branchial; a smaller bundle
from the anterior edge of the second pharyngo-branchial. Its insertion
is along the dorsal surface of the first epibranchial cartilage. The
second and third lateral interarcuales are like the first, but since the
fourth and fifth pharyngo-branchials are fused, the origin of the fourth
lateral interarcuale is not divided.
The circular muscles of the oesophagus are strongly attached to the last
pharyngo-branchial.
MUSCULATURE OF THE PECTORAL FIN. The dorsal muscle of the fin
(_levator-retractor_) arises mostly from the scapular portion of the
girdle, with a small part arising from the fascia covering the
lateral musculature of the body. It is attached in fasciculi to the
dorsal surfaces of the cartilaginous rays. The ventral muscle
(_depressor-protractor_) arises from the median portion of the
girdle and is inserted upon the cartilaginous rays in similar
fasciculi. A portion of the lateral body muscles is inserted upon
the scapular portion of the girdle.
MUSCULATURE OF THE PELVIC FIN. Ventral surface: An _adductor_ muscle has
origin upon the postero-lateral edge of the girdle; it is inserted upon
the antero-medial surface of the basal cartilage of the fin. The
_depressor_ muscle consists of small fasciculi, each corresponding to a
cartilaginous ray. They arise from the postero-lateral surface of the
basal cartilage and are inserted upon the distal extremities of the
rays.
Dorsal surface: The _abductor_ arises from the fascia covering the trunk
muscles, and inserts upon the fascia covering the intrinsic muscles of
the dorsal side of the fin. These latter (_levatores_) are arranged in
exactly the same manner as the fasciculi of the depressor.
MUSCULATURE OF THE DORSAL FINS. A sheet of muscle is attached to each
side of the anterior dorsal fin, extending nearly up to the bases of the
dermal fin-rays. This muscle passes downward between the dorsal body
musculature of the two sides. Part of the fibres arise from the fasciae
covering the medial surfaces of the body muscles, part from the basal
cartilage of the fin itself. They are inserted upon the lateral surfaces
of the broad cartilaginous fin-rays. The muscles of the posterior dorsal
fin are exactly similar in arrangement.
MUSCULATURE OF THE CAUDAL FIN. There is no special musculature for the
dorsal portion. A narrow, band-like muscle is found on each side of the
ventral portion, widest above the triangular ventral lobe. The fibres of
this muscle arise upon the flattened, expanded ends of the haemal
spines. They pass obliquely backward and upward to be inserted in the
fascia underlying the skin.
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● Transcriber’s Notes:
○ Missing or obscured punctuation was silently corrected.
○ Typographical errors were silently corrected.
○ Inconsistent spelling and hyphenation were made consistent only
when a predominant form was found in this book.
○ Text that was in italics is enclosed by underscores (_italics_).