[Illustration: FIG. 21.--Placoid elements of a male Thorn-back, _Raia
clavata_.]
_Osseous or Bony Skeleton._--The most ancient type of bony skeleton
appears to be represented in the _placoid_ elements such as are seen in
the skin of the Selachian (fig. 21). Each placoid element consists of a
spine with a broadly expanded base embedded in the dermis. The base is
composed of bone: the spine of the somewhat modified bone known as
dentine. Ensheathing the tip of the spine is a layer of extremely hard
enamel formed by the inner surface of the ectoderm which originally
covered it. Such typical placoid scales are well seen on any ordinary
skate. In the groups of fishes above the Selachians, the coating of
placoid elements shows various modifications. The spines disappear,
though they may be present for a time in early development. The bony
basal plates tend to undergo fusion--in certain cases they form a
continuous bony cuirass (various Siluroids, trunk-fishes) formed of
large plates jointed together at their edges. More usually the plates
are small and regular in size. In Crossopterygians and _Lepidosteus_ and
in many extinct forms the scales are of the ganoid type, being
rhomboidal and having their outer layer composed of hard glistening
ganoine. In other Teleostomes the scales are as a rule thin, rounded and
overlapping--the so-called cycloid type (fig. 22, A); where the
posterior edge shows toothlike projections the scale is termed ctenoid
(fig. 22, B). In various Teleosts the scales are vestigial (eel); in
others (as in most electric fishes) they have completely disappeared.
_Teeth._--Certain of the placoid elements belonging to that part of the
skin which gives rise to the lining of the stomodaeum have their spines
enlarged or otherwise modified to form teeth. In the majority of fishes
these remain simple, conical structures: in some of the larger sharks
(_Carcharodon_) they become flattened into trenchant blades with
serrated edges: in certain rays (_Myliobatis_) they form a pavement of
flattened plates suited for crushing molluscan shells. In the young
_Neoceratodus_[31] there are numerous small conical teeth, the bases of
which become connected by a kind of spongework of bony trabeculae. As
development goes on a large basal mass is formed which becomes the
functional tooth plate of the adult, the original separate denticles
disappearing completely. In the other two surviving Dipnoans, similar
large teeth exist, though here there is no longer trace in ontogeny of
their formation by the basal fusion of originally separate denticles. In
the Selachians the bony skeleton is restricted to the placoid elements.
In the Teleostomes and the Dipnoans the original cartilaginous skeleton
becomes to a great extent unsheathed or replaced by bony tissue. It
seems highly probable that the more deeply seated osseous elements
occurring in these as in the higher groups arose in the course of
evolution by the spreading inwards of bony trabeculae from the bases of
the placoid elements. Such a method has been demonstrated as occurring
in individual development in the case of certain of the more
superficially placed bones.[32]
[Illustration: FIG. 22.--A, Cycloid Scale of _Scopelus resplendens_
(magn.). B, Ctenoid Scale of _Lethrinus_ (magn.).]
The placoid element with its cap of enamel secreted by the ectoderm is
probably originally derived from a local thickening of the basement
membrane which with the external cuticle may be looked on as the most
ancient skeletal structure in the Metazoa. The basal plate appears to
have been a later development than the spine; in the palaeozoic
_Coelolepidae_[33] the basal plate is apparently not yet developed.
Only a brief summary can be given here of the leading features in the
osteology of fishes. Care must be taken not to assume that bony elements
bearing the same name in fishes and in other groups, or even in the
various sub-divisions of the fishes, are necessarily strictly
homologous. In all probability bony elements occupying similar positions
and described by the same anatomical name have been evolved
independently from the ancestral covering of placoid elements.
_Teleostei._--It will be convenient to take as the basis of our
description the bony skeleton of such a Teleostean fish as the salmon.
In the vertebral column all the cartilaginous elements are replaced by
bone. The haemal spines of the turned-up tip of the tail are flattened
(hypural bones) and serve to support the caudal fin rays.
In _Argyropelecus_ and in one or two deep-sea forms the vertebral column
remains cartilaginous.
[Illustration: From Parker & Haswell's _Text-book of Zoology_, by
permission of Messrs. Macmillan & Co., Ltd.
FIG. 23.--One of the radialia of the salmon, consisting of three
segments, ptg¹, ptg², ptg³, and supporting a dermal fin ray. _D.F.R._]
Apart from the ossification of the radialia which takes place in the
adults of bony fishes there exist special supporting structures in the
fins (paired as well as median) of all the gnathostomatous fishes and
apparently in nature independent of the cartilaginous skeleton. These
are known as dermal fin-rays.[34] Morphologically they are probably to
be looked on (like placoid elements) as local exaggerations of the
basement membrane.
In their detailed characters two main types of dermal fin-ray may be
recognized. The first of these are horny unjointed rays and occur in
the fins of Selachians and at the edge of the fins of Teleostomes
(well seen in the small posterior dorsal or "adipose" fin,
particularly in Siluroids). The second type of dermal fin-ray is
originally arranged in pairs and forms the main supports of the fin in
the adult Teleost (fig. 23). The members of each pair are in close
contact except proximally where they separate and embrace the tip of
one of the radialia. The fin-rays of this second type are frequently
branched and jointed: in other cases they form unbranched rigid
spines.
In the angler or fishing-frog (_Lophius_) the anterior rays of the
dorsal fin become greatly elongated to form small fishing-rods, from
which depend bait-like lures for the attraction of its prey.
[Illustration: From Wiedersheim, _Verg. Anat. der Wirbeltiere_, by
permission of Gustav Fischer.
FIG. 24.--Chondrocranium of Salmon, seen from the right side.
alsph, Alisphenoid. orbsph, Orbitosphenoid.
basocc, Basioccipital. proot, Prootic.
ekteth, Lateral ethmoid. psph, Parasphenoid.
epiot, Epiotic. ptero, Pterotic.
exocc, Exoccipital. socc, Supra occipital.
fr, Frontal. sphot, Sphenotic.
opisth, Opisthotic. vo, Vomer.]
In the skull of the adult salmon it is seen that certain parts of the
chondrocranium (fig. 24) have been replaced by bone ("cartilage bones")
while other more superficially placed bones ("membrane bones") cover its
surface (fig. 25). Of cartilage bones four are developed round the
foramen magnum--the basioccipital, supraoccipital and two exoccipitals.
In front of the basioccipital is the basisphenoid with an alisphenoid on
each side. The region (presphenoidal) immediately in front of the
basisphenoid is unossified, but on each side of it an orbitosphenoid is
developed, the two orbitosphenoids being closely approximated in the
mesial plane and to a certain extent fused, forming the upper part of
the interorbital septum. In the anterior or ethmoidal portion of the
cranium the only cartilage bones are a pair of lateral ethmoids lying
at the anterior boundary of the orbit. A series of five distinct
elements are ossified in the wall of the auditory or otic capsule, the
prootic and opisthotic more ventrally, and the sphenotic, pterotic and
epiotic more dorsally. The roof of the cranium is covered in by the
following dermal bones--parietals (on each side of the supraoccipital),
frontals, dermal ethmoid and small nasals, one over each olfactory
organ. The floor of the cranium on its oral aspect is ensheathed by the
large parasphenoid and the smaller vomer in front of and overlapping it.
The cartilaginous lower jaw is ossified posteriorly to form the
articular (fig. 25) with a small membrane bone, the angular, ventral to
it, but the main part of the jaw is replaced functionally by a large
membrane bone which ensheaths it--the dentary--evolved in all
probability by the spreading outwards of bony tissue from the bases of
the placoid elements (teeth) which it bears. The original upper jaw
(palatopterygoid bar) is replaced by a chain of bones--palatine in
front, then pterygoid and mesopterygoid, and posteriorly metapterygoid
and quadrate, the latter giving articulation to the articular bone of
the lower jaw. These representatives of the palatopterygoid bar no
longer form the functional upper jaw. This function is performed by
membrane bones which have appeared external to the palatopterygoid
bar--the premaxilla and maxilla--which carry teeth--and the small
scale-like jugal behind them. The quadrate is suspended from the skull
as in the Selachians (hyostylic skull) by the upper portion of the hyoid
arch--here represented by two bones--the hyomandibular and symplectic.
The ventral portion of the hyoid arch is also represented by a chain of
bones (stylohyal, epihyal, ceratohyal, hypohyal and the ventral unpaired
basihyal), as is also each of the five branchial arches behind it. In
addition to the bony elements belonging to the hyoid arch proper a
series of membrane bones support the opercular flap. Ventrally there
project backwards from the ceratohyal a series of ten overlapping
branchiostegal rays, while more dorsally are the broader interopercular,
subopercular and opercular.
[Illustration: From Wiedersheim, _Verg. Anat. der Wirbeltiere_, by
permission of Gustav Fischer.
FIG. 25.--Complete Skull of Salmon from left side.
art, Articular. op, Opercular.
branchiost, Branchiostegal. pal, Palatine.
dent, Dentary. par, Parietal.
epiot, Epiotic. pmx, Premaxilla.
eth, Dermal ethmoid. preop, Preopercular.
fr, Frontal. pt, Pterygoid.
hyom, Hyomandibular. pter, Pterotic.
intop, Interopercular. Quad, Quadrate.
Jug, Jugal. socc, Supraoccipital.
mpt, Mesopterygoid. sphot, Sphenotic.
mtpt, Metapterygoid. subop, Subopercular.
mx, Maxilla. sympl, Symplectic.
nas, Nasal. Zunge, Tongue.]
In addition to the bones already enumerated there is present a ring of
circumorbital bones, a preopercular, behind and external to the
hyomandibular and quadrate, and squamosal, external to the hinder end of
the auditory capsule.
In the salmon, pike, and various other Teleosts, extensive regions of
the chondrocranium persist in the adult, while in others (e.g. the
cod) the replacement by bone is practically complete. Bony elements
may be developed in addition to those noticed in the salmon.
In the sturgeon the chondrocranium is ensheathed by numerous membrane
bones, but cartilage bones are absent. In the Crossopterygians[35] the
chondrocranium persists to a great extent in the adult, but portions
of it are replaced by cartilage bones--the most interesting being a
large sphenethmoid like that of the frog. Numerous membrane bones
cover the chondrocranium externally. In the Dipneusti[36] the
chondrocranium is strengthened in the adult by numerous bones. One of
the most characteristic is the great palatopterygoid bone which
develops very early by the spreading of ossification backwards from
the tooth bases, and whose early development probably accounts for the
non-development of the palatopterygoid cartilage.
_Appendicular Skeleton._--The primitive pectoral girdle, which in the
Dipneusti is strengthened by a sheath of bone, becomes in the
Teleostomes reduced in size (small scapula and coracoid bones) and
replaced functionally by a secondary shoulder girdle formed of
superficially placed membrane bones (supraclavicular and cleithrum or
"clavicle," with, in addition in certain cases, an infraclavicular and
one or two postclavicular elements), and connected at its dorsal end
with the skull by a post-temporal bone.
The pelvic girdle is in Teleostomes completely absent as a rule.
The skeleton of the free limb undergoes ossification to a less or
greater extent in the Teleostomes.
In _Polypterus_ the pectoral fin (fig. 18, B) shows three
ossifications in the basal part of the fin--pro-, meso- and
metapterygium. Of these the metapterygium probably represents the
ossified skeletal axis: while the propterygium and also the numerous
diverging radials probably represent the lateral rays of one side of
the archipterygium.
In the _Teleostomes_ the place of the pelvic girdle is taken
functionally by an element apparently formed by the fusion of the
basal portions of several radials.
_Vascular System._--The main components of the blood vascular system in
the lower vertebrates are the following: (1) a single or double dorsal
aorta lying between the enteron and notochord; (2) a ventral vessel
lying beneath the enteron; and (3) a series of paired hoop-like aortic
arches connecting dorsal and ventral vessels round the sides of the
pharynx. The blood-stream passes forwards towards the head in the
ventral vessel, dorsalwards through the aortic arches, and tailwards in
the dorsal aorta.
The dorsal aorta is single throughout the greater part of its extent,
but for a greater or less extent at its anterior end (_circulus
cephalicus_) it consists of two paired aortic roots. It is impossible to
say whether the paired or the unpaired condition is the more primitive,
general morphological conditions being in favour of the latter, while
embryological evidence rather supports the former. The dorsal aorta,
which receives its highly oxygenated blood from the aortic arches, is
the main artery for the distribution of this oxygenated blood.
Anteriorly the aortic roots are continued forwards as the dorsal carotid
arteries to supply the head region. A series of paired,
segmentally-arranged arteries pass from the dorsal aorta to supply the
muscular body wall, and the branches which supply the pectoral and
pelvic fins (subclavian or brachial artery, and iliac artery) are
probably specially enlarged members of this series of segmental vessels.
Besides these paired vessels a varying number of unpaired branches pass
from dorsal aorta to the wall of the alimentary canal with its glandular
diverticula (coeliac, mesenteric, rectal).
The ventral vessel undergoes complicated changes and is represented in
the adults of existing fishes by a series of important structures. Its
post-anal portion comes with the atrophy of the post-anal gut to lie
close under the caudal portion of the dorsal aorta and is known as the
caudal vein. This assumes a secondary connexion with, and drains its
blood into, the posterior cardinal veins (see below). In the region
between cloaca and liver the ventral vessel becomes much branched or
even reticular and--serving serving to convey the food-laden blood from
the wall of the enteron to the capillary network of the liver--is known
as the hepatic portal vein. The short section in front of the liver is
known as the hepatic vein and this conveys the blood, which has been
treated by the liver, into a section of the ventral vessel, which has
become highly muscular and is rhythmically contractile. This enlarged
muscular portion, in which the contractility--probably once common to
the main vessels throughout their extent--has become concentrated,
serves as a pump and is known as the heart. Finally the precardiac
section of the ventral vessel--the ventral aorta--conveys the blood from
heart to aortic arches.
In addition to the vessels mentioned a large paired vein is developed in
close relation to the renal organ which it serves to drain. This is the
posterior cardinal. An anterior prolongation (anterior cardinal) serves
to drain the blood from the head region. From the point of junction of
anterior and posterior cardinal a large transverse vessel leads to the
heart (_ductus Cuvieri_).
[Illustration: From Boas, _Lehrbuch der Zoologie_, by permission of
Gustav Fischer.
Fig. 26.--Diagram to illustrate the condition of the Conus in an
Elasmobranch (A), _Amia_ (B) and a typical Teleost (C).
a, Atrium. v,v´, Valves.
b.a, Bulbus aortae. v.a, Ventral aorta.
c.a, Conus arteriosus. vt, Ventricle.]
s.v, Sinus venosus.
_Heart._--Originally a simple tube curved into a somewhat S-shape, the
heart, by enlargements, constrictions and fusions of its parts, becomes
converted into the complex, compact heart of the adult. In this we
recognize the following portions--(1) _Sinus venosus_, (2) _Atrium_, (3)
_Ventricle_. A fourth chamber, the _conus arteriosus_, the enlarged and
contractile hinder end of the ventral aorta, is also physiologically a
part of the heart. The sinus venosus receives the blood from the great
veins (ductus Cuvieri and hepatic veins). It--like the atrium which it
enters by an opening guarded by two lateral valves--has thin though
contractile walls. The atrium is as a rule single, but in the Dipnoans,
in correlation with the importance of their pulmonary breathing, it is
incompletely divided into a right and a left auricle. In Neoceratodus
the incomplete division is effected by the presence of a longitudinal
shelf projecting into the atrial cavity from its posterior wall. The
opening of the sinus venosus is to the right of this shell, that of the
pulmonary vein to the left. In _Prototerus_ and _Lepidosiren_ a nearly
complete septum is formed by the fusion of trabeculae, there being only
a minute opening in it posteriorly. The atrium opens by a wide opening
guarded by two or more flap valves provided with chordae tendineae into
the ventricle.
The ventricle, in correspondence with it being the main pumping
apparatus, has its walls much thickened by the development of muscular
trabeculae which, in the lower forms separated by wide spaces in which
most of the blood is contained, become in the Teleostomes so enlarged as
to give the wall a compact character, the spaces being reduced to small
scattered openings on its inner surface. In the Dipnoans the ventricle,
like the atrium, is incompletely divided into a right and left
ventricle. In _Ceratodus_ this is effected by an extension of the
interauricular shelf into the ventricle. In _Lepidosiren_ the separation
of the two ventricles is complete but for a small perforation
anteriorly, the heart in this respect showing a closer approximation to
the condition in the higher vertebrates than is found in any Amphibians
or in any reptiles except the Crocodilia. The conus arteriosus is of
interest from the valvular arrangements in its interior to prevent
regurgitation of blood from ventral aorta into ventricle. In their
simplest condition, as seen e.g. in an embryonic Selachian, these
arrangements consist of three, four or more prominent longitudinal
ridges projecting into the lumen of the conus, and serving to obliterate
the lumen when jammed together by the systole of the conus. As
development goes on each of these ridges becomes segmented into a row of
pocket valves with their openings directed anteriorly so that
regurgitation causes them to open out and occlude the lumen by their
free edges meeting. Amongst the Teleostomes the lower ganoids show a
similar development of longitudinal rows of valves in the conus. In
_Amia_ (fig. 26, B), however, the conus is shortened and the number of
valves in each longitudinal row is much reduced. This leads to the
condition found in the Teleosts (fig. 26, O), where practically all
trace of the conus has disappeared, a single circle of valves
representing a last survivor of each row (save in a few exceptional
cases, e.g. _Albula_, _Tarpen_, _Osteoglossum_, where two valves of each
row are present).
[Illustration: After Newton Parker, from _Trans. of the Royal Irish
Academy_, vol. xxx.
FIG. 27.--Venous System of _Protopterus_, as seen from ventral side.
a, Atrium. k, Kidney.
ac, Anterior cardinal. l, Liver.
an.v, Anastomotic vein. ov.v, Ovarian veins.
c, Intestine. p, Pericardium.
c.v, Caudal vein. p.c.v, Left posterior cardinal.
f.v, Femoral vein. p.v´, Parietal veins.
g.b, Gall-bladder. r.p.v, Renal portal.
h.v, Hepatic vein. s, Stomach.
i.j.v, Inferior jugular vein. s.b.v, Subclavian.]
i.v.c, Posterior vena cava.
In Front of the conus vestige of the Teleost there is present a thick
walled _bulbus aortae_ differing from the conus in not being
rhythmically contractile, its walls being on the contrary richly
provided with elastic tissue.
The Dipnoans[37] show an important advance on the conus as in atrium and
ventricle. The conus has a characteristic spiral twist. Within it in
_Neoceratodus_ are a number of longitudinal rows of pocket valves. One
of these rows is marked out by the very large size of its valves and by
the fact that they are not distinct from one another but even in the
adult form a continuous, spirally-running, longitudinal fold. This ridge
projecting into the lumen of the conus divides it incompletely into two
channels, the one beginning (i.e. at its hinder end) on the _left_ side
and ending in front _ventrally_, the other beginning on the _right_ and
ending _dorsally_. In _Protopterus_ a similar condition occurs, only in
the front end of the conus a second spiral fold is present opposite the
first and, meeting this, completes the division of the conus cavity into
two separate parts. The rows of pocket valves which do not enter into
the formation of the spiral folds are here greatly reduced.
These arrangements in the conus of the Dipnoans are of the highest
morphological interest, pointing in an unmistakable way towards the
condition found in the higher lung-breathing vertebrates. Of the two
cavities into which the conus is partially divided in the Dipneusti the
one which begins posteriorly on the right receives the (venous) blood
from the right side of the heart, and ending up anteriorly dorsal to the
other cavity communicates only with aortic arches V. and VI. In the
higher vertebrates this cavity has become completely split off to form
the root of the pulmonary arteries, and a result of aortic arch V.
receiving its blood along with the functionally much more important VI.
(the pulmonary arch) from this special part of the conus has been the
almost complete disappearance of this arch (V.) in all the higher
vertebrates.
_Arterial System._--There are normally six aortic arches laid down
corresponding with the visceral arches, the first (mandibular) and
second (hyoidean) undergoing atrophy to a less or greater extent in
post-embryonic life. Where an external gill is present the aortic arch
loops out into this, a kind of short-circuiting of the blood-stream
taking place as the external gill atrophies. As the walls of the clefts
assume their respiratory function the aortic arch becomes broken into a
network of capillaries in its respiratory portion, and there is now
distinguished a ventral afferent and a dorsal efferent portion of each
arch. Complicated developmental changes, into which it is unnecessary to
enter,[38] may lead to each efferent vessel draining the two sides of a
single cleft instead of the adjacent walls of two clefts as it does
primitively. In the Crossopterygians and Dipnoans as in the higher
vertebrates the sixth aortic arch gives off the pulmonary artery to the
lung. Among the Actinopterygians this, probably primitive, blood-supply
to the lung (swimbladder) persists only in _Amia_.
[Illustration: FIG. 28.--Venous System of Polypterus 30 mm. larva
(dorsal view).
a.c.v, Anterior cardinal vein.
d.C, Ductus Cuvieri.
h.v, Hepatic vein.
i.j.v, Inferior jugular vein.
ir.v, Inter-renal vein.
l.v, Lateral cutaneous vein.
p.c.v, Posterior cardinal vein.
p.n, Pronephros.
p.v, Pulmonary vein.
s, Subclavian vein.
s.v, Sinus venosus.
th, Thyroid.
v, Vein from pharyngeal wall.
* Anterior portion of left posterior cardinal vein.]
_Venous System._--The most interesting variations from the general plan
outlined have to do with the arrangements of the posterior cardinals. In
the Selachians these are in their anterior portion wide and sinuslike,
while in the region of the kidney they become broken into a sinusoidal
network supplied by the postrenal portion now known as the renal portal
vein. In the Teleostomes the chief noteworthy feature is the tendency to
asymmetry, the right posterior cardinal being frequently considerably
larger than the left and connected with it by transverse anastomotic
vessels, the result being that most of the blood from the two kidneys
passes forwards by the right posterior cardinal. The Dipnoans (fig. 27)
show a similar asymmetry, but here the anterior end of the right
posterior cardinal disappears, being replaced functionally by a new
vessel which conveys the blood from the right posterior cardinal direct
to the sinus venosus instead of to the outer end of the ductus Cuvieri.
This new vessel is the posterior vena cava which thus in the series of
vertebrates appears for the first time in the Dipneusti.
_Pulmonary Veins._--In _Polypterus_ (fig. 28) the blood is drained from
the lungs by a pulmonary vein on each side which unites in front with
its fellow and opens into the great hepatic vein behind the heart. In
the Dipnoans the conjoined pulmonary veins open directly into the left
section of the atrium as in higher forms. In the Actinopterygians with
their specialized air-bladder the blood passes to the heart via
posterior cardinals, or hepatic portal, or--a probably more primitive
condition--directly into the left ductus Cuvieri (_Amia_).
_Lymphatics._--More or less irregular lymphatic spaces occur in the
fishes as elsewhere and, as in the Amphibia, localized muscular
developments are present forming lymph hearts.
_Central Nervous System._--The neural tube shows in very early stages an
anterior dilated portion which forms the rudiment of the brain in
contradistinction to the hinder, narrower part which forms the spinal
cord. This enlargement of the brain is correlated with the increasing
predominance of the nerve centres at the anterior end of the body which
tend to assume more and more complete control over those lying behind.
_Spinal Cord._--A remarkable peculiarity occurs in the sun fishes
(_Molidae_), where the body is greatly shortened and where the spinal
cord undergoes a corresponding abbreviation so as to be actually shorter
than the brain.
_Brain._--It is customary to divide the brain into three main regions,
fore-, mid-, and hind-brain, as in the most familiar vertebrates there
is frequently seen in the embryo a division of the primitive brain
dilatation into three vesicles lying one behind the other. A
consideration of the development of the brain in the various main groups
of vertebrates shows that these divisions are not of equal importance.
In those archaic groups where the egg is not encumbered by the presence
of a large mass of yolk it is usual for the brain to show in its early
stages a division into two main regions which we may term the primitive
fore-brain or cerebrum and the primitive hind-brain or rhombencephalon.
Only later does the hinder part of the primitive fore-brain become
marked off as mid-brain. In the fully developed brain it is customary to
recognize the series of regions indicated below, though the boundaries
between these regions are not mathematical lines or surfaces any more
than are any other biological boundaries:--
Rhombencephalon (Hind-brain) / Myelencephalon (Medulla oblongata).
\ Metencephalon (Cerebellum).
/ Mesencephalon (Mid-brain).
Cerebrum (Primitive Fore-brain) < Thalamencephalon (Diencephalon).
\ [Hemispheres (Telencephalon).]
The myelencephalon or medulla oblongata calls for no special remark,
except that in the case of _Torpedo_ there is a special upward bulging
of its floor on each side of the middle line forming the electric lobe
and containing the nucleus of origin of the nerves to the electric
organ.
[Illustration: A and B from Wiedersheim, by permission of Gustav
Fischer.
FIG. 29.--Brain of _Scyllium_ (A), _Salmo_ (B) and _Lepidosiren_ (C).
The three figures are not drawn to the same scale.
cer, Cerebellum.
c.h, Cerebral hemisphere.
th, Thalamencephalon.
f.b, Primitive fore-brain (in B the line points to the thickened
wall of the fore-brain, the so-called "basal ganglia").
G.p, Pineal body.
m.b, Roof of mid-brain, optic lobes, _tectum opticum_.
o.l, Olfactory lobe.
IV.v, Fourth ventricle.]
The cerebellum occurs in its simplest form in lampreys and Dipnoans
(fig. 29, C), where it forms a simple band-like thickening of the
anterior end of the roof of the hind-brain. In Selachians it is very
large and bulges upwards, forming a conspicuous organ in a dorsal view
of the brain (fig. 29, A). In Teleosts (fig. 29, B) the cerebellum is
also large. It projects back as a great tongue-like structure over the
roof of the fourth ventricle, while in front it dips downwards and
projects under the roof of the mid-brain forming a highly characteristic
_valvula cerebelli_. A _valvula cerebelli_ occurs also in ganoids, while
in the Crossopterygians a similar extension of the cerebellum projects
backwards into the IV. ventricle or cavity of the hind-brain (fig. 30).
The mesencephalon is a conspicuous structure in the fishes from its
greatly developed roof (_tectum opticum_) which receives the end pencils
of the optic nerve. Normally it projects upwards as a pair of large
optic lobes, but in the Dipnoans (fig. 29, C) the lateral thickening is
not sufficiently great to cause obvious lateral swellings in external
view.
[Illustration: FIG. 30.--Median Longitudinal Section through the brain
of _Lepidosiren_ and _Polypterus_. In the upper figure (_Lepidosiren_)
the habenular ganglion and hemisphere are shown in outline though not
actually present in a median section.
a.c, Anterior commissure. par, Paraphysis.
cer, Cerebellum. pin, Pineal body.
d.s, Dorsal sac. p.c, Posterior commissure.
g.h, Habenular ganglion. s.v, Saccus vasculosus.
h.c, Habenular commissure. t.o, Tectum opticum.
i.g, Infundibular gland. v.III, Third ventricle.
l.p, Lateral plexus. v.IV, Fourth ventricle.
o.c, Optic chiasma. vel, Velum transversum.]
pall, Pallium.
The thalamencephalon is one of the most interesting parts of the brain
from its remarkable uniformity throughout the Vertebrata. Even in
_Amphioxus_ the appearance of a sagittal section strongly suggests
vestiges of a once present thalamencephalon.[39] The roof--like that of
the myelencephalon--remains to a great extent membranous, forming with
the closely applied _pia mater_ a vascular roof to the III. ventricle.
Frequently a transverse fold of the roof dips down into the III.
ventricle forming the _velum transversum_ (fig. 30).
The side walls of the thalamencephalon are greatly thickened forming the
_thalamus_ (epithalamus and hypothalamus), while a ganglionic thickening
of the roof posteriorly on each side forms the _ganglia habenulae_ which
receive olfactory fibres from the base of the hemisphere. The habenular
ganglia are unusually large in the lampreys and are here strongly
asymmetrical, the right being the larger.
The floor of the thalamencephalon projects downwards and backwards as
the infundibulum. The side walls of this are thickened to form
characteristic _lobi inferiores_, while the blind end develops glandular
outgrowths (infundibular gland, fig. 30) overlaid by a rich development
of blood sinuses and forming with them the _saccus vasculosus_. The
optic chiasma, where present, is involved in the floor of the
thalamencephalon and forms a large, upwardly-projecting ridge. Farther
forwards on the floor or anterior wall is the anterior commissure (see
below).
Passing forwards from the mid-brain (cf. fig. 30) a series of
interesting structures are found connected with the roof of the
primitive fore-brain, viz.--posterior commissure (intercalary region),
pineal organ, habenular commissure with anterior parietal organ, dorsal
sac (= pineal cushion), _velum transversum_, paraphysis. The posterior
commissure is situated in the boundary between thalamencephalon and
mid-brain. It is formed of fibres connecting up the right and left
sides of the tectum opticum (?). The habenular or superior commissure
situated farther forwards connects the two ganglia habenulae. In the
immediate neighbourhood of these ganglia there project upwards two
diverticula of the brain-roof known as the pineal organ and the
parapineal (or anterior parietal) organ. The special interest of these
organs[40] lies in the fact that in certain vertebrates one (parapineal
in _Sphenodon_ and in lizards) or both (_Petromyzon_) exhibit
histological features which show that they must be looked on as visual
organs or eyes. In gnathostomatous fishes they do not show any definite
eye-like structure, but in certain cases (_Polyodon_, _Callichthys_,
&c.) the bony plates of the skull-roof are discontinuous over the pineal
organ forming a definite parietal foramen such as exists in lizards
where the eye-like structure is distinct. It is also usual to find in
the epithelial wall of the pineal organ columnar cells which show
club-shaped ends projecting into the lumen (exactly as in the young
visual cells of the retina[41]) and are prolonged into a root-like
process at the other end. Definite nerve fibres pass down from these
parietal organs to the brain. It is stated that the fibres from the
pineal organ pass into the posterior commissure, those of the parapineal
organ into the habenular commissure.
The facts mentioned render it difficult to avoid the conclusion that
these organs either have been sensory or are sensory. Possibly they
represent the degenerate and altered vestiges of eye-like organs present
in archaic vertebrates, or it may be that they represent the remains of
organs not eye-like in function but which for some other reason lay
close under the surface of the body. It would seem natural that a
diverticulum of brain-tissue exposed to the influence of light-rays
should exhibit the same reaction as is shown frequently elsewhere in the
animal kingdom and tend to assume secondarily the characters of a visual
organ. The presence of the rod-like features in the epithelial cells is
perhaps in favour of the latter view. In evolution we should expect
these to appear before the camera-like structure of a highly developed
eye, while in the process of degeneration we should expect these fine
histological characters to go first.
Selachians.--No parapineal organ is present. The pineal body (except
in _Torpedo_ where it is absent) is in the form of a long slender tube
ending in front in a dilated bulb lying near the front end of the
brain in close contact with, or enclosed in, a definite foramen in the
cranial roof.
Holocephali and Crossopterygii.--Here also the pineal body is long and
tubular: at its origin it passes dorsalwards or slightly backwards
behind the large dorsal sac.
Actinopterygian Ganoids resemble Selachians on the whole. In _Amia_ a
parapineal organ is present, and it is said to lie towards the left
side and to be connected by a thick nerve with the _left_ habenular
ganglion (cf. _Petromyzon_, article CYCLOSTOMATA). This is adduced to
support the view that the pineal and parapineal bodies represent
originally paired structures.
Teleostei.--A parapineal rudiment appears in the embryo of some forms,
but in the adult only the pineal organ is known to exist. This is
usually short and club-shaped, its terminal part with much folded wall
and glandular in character. In a few cases a parietal foramen occurs
(_Callichthys_, _Loricaria_, &c.).
Dipneusti.--The pineal organ is short and simple. No parapineal organ
is developed.
The dorsal sac is formed by that part of the roof of the
thalamencephalon lying between the habenular commissure and the region
of the velum. In some cases a longitudinal groove is present in which
the pineal organ lies (Dipneusti). In the Crossopterygians the dorsal
sac is particularly large and was formerly mistaken for the pineal
organ.
The _velum transversum_ is a transverse, inwardly-projecting fold of the
roof of the primitive fore-brain in front of the dorsal sac. To those
morphologists who regard the hemisphere region or telencephalon as a
primitively unpaired structure the velum is an important landmark
indicating the posterior limit of the telencephalon. Those who hold the
view taken in this article that the hemispheres are to be regarded as
paired outpushings of the side wall of the primitive fore-brain
attribute less morphological importance to the velum. Physiologically
the velum is frequently important from the plexus of blood-vessels which
passes with it into the III. ventricle.
In _Petromyzon_ and _Chimaera_ the velum is not developed. In Dipnoans
there are present in its place _paired_ transverse folds which are
probably merely extensions backwards of the lateral plexuses.
The Paraphysis is a projection from the roof of the primitive fore-brain
near its anterior end. It is well seen in Dipnoans[42] (_Lepidosiren_
and _Protopterus_) where in the larva (exactly as in the urodele larva)
it forms a blindly ending tube sloping upwards and forwards between the
two hemispheres. In the adult it becomes mixed with the two lateral
plexuses and is liable to be confused with them. In the other
groups--except the Teleosts where it is small (_Anguilla_) or absent
(most Teleosts)--the paraphysis is by no means such a definite
structure, but generally there is present a more or less branched and
divided diverticulum of the brain wall, frequently glandular, which is
homologized with the paraphysis. The morphological significance of the
paraphysis is uncertain. It may represent the remains of an ancient
sense organ, or it may simply represent the last connexion between the
brain and the external ectoderm from which it was derived.
An important derivative of the primitive fore-brain is seen in the pair
of cerebral hemispheres which in the higher vertebrates become of such
relatively gigantic dimensions. The hemispheres appear to be primitively
associated with the special sense of smell, and they are prolonged
anteriorly into a pair of olfactory lobes which come into close relation
with the olfactory organ. From a consideration of their adult relations
and of their development--particularly in those groups where there is no
disturbing factor in the shape of a large yolk sac--it seems probable
that the hemispheres are primitively paired outpushings of the lateral
wall of the primitive fore-brain[43]--in order to give increased space
for the increased mass of nervous matter associated with the olfactory
sense. They are most highly developed in the Dipneusti amongst fishes.
They are there (cf. fig. 29, C) of relatively enormous size with thick
nervous floor (corpus striatum) and side walls and roof (pallium)
surrounding a central cavity (lateral ventricle) which opens into the
third ventricle. At the posterior end of the hemisphere a small area of
its wall remains thin and membranous, and this becomes pushed into the
lateral ventricle by an ingrowth of blood-vessel to form the huge
lateral plexus ( = _plexus hemisphaerium_). In this great size of the
hemispheres[44] and also in the presence of a rudimentary cortex in the
Dipnoi we see, as in many other features in these fishes, a distinct
foreshadowing of conditions occurring in the higher groups of
vertebrates. The Cyclostomes possess a distinct though small pair of
hemispheres. In the Selachians the relatively archaic _Notidanidae_[45]
possess a pair of thick-walled hemispheres, but in the majority of the
members of the group the paired condition is obscured (fig. 29, A).
In the Teleostomes the mass of nervous matter which in other groups
forms the hemispheres does not undergo any pushing outwards except as
regards the small olfactory lobes. On the contrary, it remains as a
great thickening of the lateral wall of the thalamencephalon (the
so-called basal ganglia), additional space for which, however, may be
obtained by a considerable increase in length of the fore-brain region
(cf. fig. 30, A) or by actual involution into the third ventricle
(_Polypterus_).[46] The great nervous thickenings of the
thalamencephalic wall bulge into its cavity and are covered over by the
thin epithelial roof of the thalamencephalon which is as a consequence
liable to be confused with the pallium or roof of the hemispheres with
which it has nothing to do: the homologue of the pallium as of other
parts of the hemisphere is contained within the lateral thickening of
the thelamencephalic wall, not in its membranous roof.[47]
Associated with the parts of the fore-brain devoted to the sense of
smell (especially the corpora striata) is the important system of
bridging fibres forming the anterior commissure which lies near the
anterior end of the floor, or in the front wall, of the primitive
fore-brain. It is of great interest to note the appearance in the
_Dipnoans_ (_Lepidosiren_ and _Protopterus_) of a corpus callosum (cf.
fig. 30 B) lying dorsal to the anterior commissure and composed of
fibres connected with the pallial region of the two hemispheres.
_Sense Organs._--The olfactory organs are of special interest in the
Selachians, where each remains through life as a widely-open, saccular
involution of the ectoderm which may be prolonged backwards to the
margin of the buccal cavity by an open oronasal groove, thus retaining a
condition familiar in the embryo of the higher vertebrates. In Dipnoans
the olfactory organ communicates with the roof of the buccal cavity by
definite posterior nares as in the higher forms--the communicating
passage being doubtless the morphological equivalent of the oronasal
groove, although there is no direct embryological evidence for this. In
the Teleostomes the olfactory organ varies from a condition of great
complexity in the Crossopterygians down to a condition of almost
complete atrophy in certain Teleosts (Plectognathi).[48]
The _eyes_ are usually of large size. The lens is large and spherical
and in the case of most Teleostomes accommodation for distant vision is
effected by the lens being pulled bodily nearer the retina. This
movement is brought about by the contraction of smooth muscle fibres
contained in the _processus falciformis_, a projection from the choroid
which terminates in contact with the lens in a swelling, the _campanula
Halleri_. In _Amia_ and in Teleosts a network of capillaries forming the
so-called choroid gland surrounds the optic nerve just outside the
retina. As a rule the eyes of fishes have a silvery, shining appearance
due to the deposition of shining flakes of guanin in the outer layer of
the choroid (_Argentea_) or, in the case of Selachians, in the inner
layers (_tapetum_). Fishes which inhabit dark recesses, e.g. of caves or
of the deep sea, show an enlargement, or, more frequently, a reduction,
of the eyes. Certain deep-sea Teleosts possess remarkable telescopic
eyes with a curious asymmetrical development of the retina.[49]
The otocyst or auditory organ agrees in its main features with that of
other vertebrates. In Selachians the otocyst remains in the adult open
to the exterior by the _ductus endolymphaticus_. In _Squatina_[50] this
is unusually wide and correlated; with this the calcareous otoconia are
replaced by sand-grains from the exterior. In Dipnoans (_Lepidosiren_
and _Protopterus_) curious outgrowths arise from the ductus
endolymphaticus and come to overlie the roof of the fourth ventricle,
recalling the somewhat similar condition met with in certain Amphibians.
In various Teleosts the swimbladder enters into intimate relations
with the otocyst. In the simplest condition these relations consist in
the prolongation forwards of the swimbladder as a blindly ending tube
on either side, the blind end coming into direct contact either with
the wall of the otocyst itself or with the fluid surrounding it
(perilymph) through a gap in the rigid periotic capsule. A wave of
compression causing a slight inward movement of the swimbladder wall
will bring about a greatly magnified movement of that part of the wall
which is not in relation with the external medium, viz. the part in
relation with the interior of the auditory capsule. In this way the
perception of delicate sound waves may be rendered much more perfect.
In the Ostariophysi (Sagemehl), including the _Cyprinidae_, the
_Siluridae_, the _Characinidae_ and the _Gymnotidae_, a
physiologically similar connexion between swimbladder and otocyst is
brought about by the intervention of a chain of auditory ossicles
(Weberian ossicles) formed by modification of the anterior
vertebrae.[51]
_Lateral Line Organs._[52]--Epidermal sense buds are scattered about in
the ectoderm of fishes. A special arrangement of these in lines along
the sides of the body and on the head region form the highly
characteristic sense organs of the lateral line system. In _Lepidosiren_
these organs retain their superficial position; in other fishes they
become sunk beneath the surface into a groove, which may remain open
(some Selachians), but as a rule becomes closed into a tubular channel
with openings at intervals. It has been suggested that the function of
this system of sense organs is connected with the perception of
vibratory disturbances of comparatively large wave length in the
surrounding medium.
_Peripheral Nerves._--In the Cyclostomes the dorsal afferent and ventral
efferent nerves are still, as in _Amphioxus_, independent, but in the
gnathostomatous fishes they are, as in the higher vertebrates, combined
together into typical spinal nerves.
As regards the cranial nerves the chief peculiarities of fishes relate
to (1) the persistence of the branchial clefts and (2) the presence of
an elaborate system of cutaneous sense organs supplied by a group of
nerves (_lateralis_) connected with a centre in the brain which develops
in continuity with that which receives the auditory nerve. These points
may be exemplified by the arrangements in Selachians (see fig. 31). I.,
II., III., IV. and VI. call for no special remark.
[Illustration: From Bridge, _Cambridge Natural History_, vol. vii.
"Fishes" (by permission of Macmillan & Co., Ltd.). After Wiedersheim,
_Grundriss der vergleichenden Anatomie_ (by permission of Gustav
Fischer).
FIG. 31.--Diagram of Cranial nerves of a Fish. Cranial nerves and
branchial clefts are numbered with Roman figures. Trigeminus black;
Facialis dotted; Lateralis oblique shading; Glossopharyngeal
cross-hatched; Vagus white.
bucc, Buccal.
c, Commissure between pre- and postauditory parts of lateralis
system.
d.r, Dorsal roots of spinal nerves.
g.g, Gasserian ganglion.
gn.g, (Geniculate) ganglion of VII.
hy, Hyomandibular.
l.n.X, Lateralis vagi.
m, Motor branches of hy.
md, Mandibular.
md.ex, External mandibular.
mk.c, Meckel's cartilage.
mx, Maxillary.
oc, Occipitospinal.
ol.o, Olfactory organ.
op.p, Ophthalmicus profundus.
op.s, Ophthalmicus superficialis.
pn, Palatine.
pq., Palatopterygo-quadrate cartilage.
s, Spiracle.
st, Supra-temporal branch of lateralis system.
t.a, Lateralis centre in brain.
v.n, Visceral nerve.
v.r, Ventral roots.]
_Trigeminus_ (V.).--The _ophthalmicus profundus_ branch (op.p.)--which
probably is morphologically a distinct cranial nerve--passes forwards
along the roof of the orbit to the skin of the snout. As it passes
through the orbit it gives off the long ciliary nerves to the eyeball,
and is connected with the small ciliary ganglion (also connected with
III.) which in turn gives off the short ciliary nerves to the eyeball.
The _ophthalmicus superficialis_ (cut short in the figure) branch passes
from the root ganglion of V. (Gasserian ganglion), and passes also over
the orbit to the skin of the snout. It lies close to, or completely
fused with, the corresponding branch of the lateralis system.
The main trunk of V. branches over the edge of the mouth into the
_maxillary_ (mx.) and _mandibular_ (md.) divisions, the former, like the
two branches already mentioned, purely sensory, the latter
mixed--supplying the muscles of mastication as well as the teeth of the
lower jaw and the lining of the buccal floor.
The main trunk of the _Facialis_ (VII.) bifurcates over the spiracle
into a pre-spiracular portion--the main portion of which passes to the
mucous membrane of the palate as the palatine (pnVII.)--and a
postspiracular portion, the hyomandibular (hy.) trunk which supplies the
muscles of the hyoid arch and also sends a few sensory fibres to the
lining of the spiracle, the floor of mouth and pharynx and the skin of
the lower jaw. Combined with the main trunk of the facial are branches
belonging to the _lateralis_ system.
_Lateralis Group of Nerves._--The _lateralis_ group of nerves are
charged with the innervation of the system of cutaneous sense organs and
are all connected with the same central region in the medulla. A special
sensory area of the ectoderm becomes involuted below the surface to form
the otocyst, and the nerve fibres belonging to this form the auditory
nerve (VIII.). Other portions of the _lateralis_ group become mixed up
with various other cranial nerves as follows:
(a) Facial portion.
(1) _Ophthalmicus superficialis_ (op.s.VII.): passes to lining of nose
or to the lateral line organs of the dorsal part of snout.
(2) _Buccal_ (bucc.VII): lies close to maxillary division of V. and
passes to the sensory canals of the lower side of the snout.
(3) _External mandibular_ (md.ex.): lies in close association with the
mandibular division of V., supplies the sensory canals of the lower jaw
and hyoid region.
_Lateralis vagi_ (l.n.X.) becomes closely associated with the vagus. It
supplies the lateral line organs of the trunk.
In the lamprey and in Dipnoans the _lateralis vagi_ loses its
superficial position in the adult and comes into close relation with the
notochord.
In Actinopterygians and at least some Selachians a _lateralis_ set of
fibres is associated with IX., and in the former fishes a conspicuous
trunk of _lateralis_ fibres passes to some or all (_Gadus_) of the fins.
This has been called the _lateralis accessorius_ and is apparently
connected with V., VII., IX., X. and certain spinal nerves.[53]
_Vagus Group_ (IX., X., XI.).--The _glossopharyngeus_ (IX.) forks over
the first branchial cleft (pretrematic and post-trematic branches) and
also gives off a palatine branch (pn.IX.). In some cases (various
Selachians, Ganoids and Teleosts) it would seem that IX. includes a few
fibres of the _lateralis_ group.
Vagus (X.) is shown by its multiple roots arising from the medulla and
also by the character of its peripheral distribution to be a compound
structure formed by the fusion of a number of originally distinct
nerves. It consists of (1) a number of branchial branches (X.¹ X.² &c.),
one of which forks over each gill cleft behind the hyobranchial and
which may (Selachians) arise by separate roots from the medulla; (2) an
intestinal branch (v.n.X.) arising behind the last branchial and
innervating the wall of the oesophagus and stomach and it may be even
the intestine throughout the greater part of its length (_Myxine_).
The _accessorius_ (XI.) is not in fishes separated as a distinct nerve
from the vagus.
With increased development of the brain its hinder portion, giving rise
to the vagus system, has apparently come to encroach on the anterior
portion of the spinal cord, with the result that a number of spinal
nerves have become reduced to a less or more vestigial condition. The
dorsal roots of these nerves disappear entirely in the adult, but the
ventral roots persist and are to be seen arising ventrally to the vagus
roots. They supply certain muscles of the pectoral fins and of the
visceral arches and are known as spino-occipital nerves.[54]
These nerves are divisible into an anterior more ancient set--the
occipital nerves--and a posterior set of more recent
origin--(occipito-spinal nerves). In Selachians 1-5 pairs of occipital
nerves alone are recognizable: in Dipnoans 2-3 pairs of occipital and
2-3 pairs of occipito-spinal: in Ganoids 1-2 pairs occipital and 1-5
pairs occipito-spinal; in Teleosts finally the occipital nerves have
entirely disappeared while there are 2 pairs of occipito-spinal. In
Cyclostomes no special spino-occipital nerves have been described.
The fibres corresponding with those of the _Hypoglossus_ (XII.) of
higher vertebrates spring from the anterior spinal nerves, which are
here, as indeed in Amphibia, still free from the cranium.
_Sympathetic._--The sympathetic portion of the nervous system does not
in fishes attain the same degree of differentiation as in the higher
groups. In Cyclostomes it is apparently represented by a fine plexus
with small ganglia found in the neighbourhood of the dorsal aorta and on
the surface of the heart and receiving branches from the spinal nerves.
In Selachians also a plexus occurs in the neighbourhood of the cardinal
veins and extends over the viscera: it receives visceral branches from
the anterior spinal nerves. In Teleosts the plexus has become condensed
to form a definite sympathetic trunk on each side, extending forwards
into the head and communicating with the ganglia of certain of the
cranial nerves. (J. G. K.)