THE SKELETON OF THE DOG[139] (_Canis familiaris_).
I. EXOSKELETON.
The exoskeleton of the dog includes three sets of structures: 1.
hairs, 2. claws, 3. teeth. =Hairs= and =claws= are epidermal
exoskeletal structures, while =teeth= are partly of dermal, and partly
of epidermal origin.
1. =Hairs= are delicate epidermal structures which grow imbedded in
little pits or follicles in the dermis. Specially large hairs forming
the =vibrissae= or =whiskers= grow attached to the upper lip.
2. =Claws= are horny epidermal sheaths, one of which fits on to the
pointed distal phalanx of each digit. They are sharply curved
structures, and being in the dog non-retractile, their points are
commonly much blunted by friction with the ground. The claws of the
pollex, and of the hallux when it is present, however do not meet the
ground, and therefore remain comparatively sharp.
3. =Teeth=[140]. Although as regards their mode of origin, teeth are
purely exoskeletal or tegumentary structures, they become so
intimately connected with the skull that they appear to belong to the
endoskeleton.
Each tooth, as has been already described, consists of three distinct
tissues, dentine and cement of dermal origin, and enamel of epidermal
origin.
[Illustration FIG. 68. DENTITION OF A DOG (_Canis familiaris_) × 1/2.
(Camb. Mus.)
_i_ 2. second incisor.
_c._ canine.
_pm_ 1, _pm_ 4. first and fourth premolars.
_m_ 1. first molar.]
The teeth of the dog (fig. 68) form a regular series arranged along
the margins of both upper and lower jaws, and imbedded in pits or
=alveoli= of the maxillae, premaxillae, and mandibles. They are all
fixed in the bone by tapering roots, and none of them grow from
persistent pulps.
They are divisible into four distinct groups, the =incisors=,
=canines=, =premolars= and =molars=. There are three incisors, one
canine and four premolars on each side of each jaw. But while there
are three molars on each side of the lower jaw, the last is wanting in
the upper jaw. The dentition of the dog may then be represented by the
formula
_i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 2/3 × 2 = 42.
In each jaw there is one large specially modified tooth called the
=carnassial=, the teeth in front of this are more or less pointed and
compressed, while those behind it are more or less flattened and
tuberculated.
=Teeth of the upper jaw.=
The first and second =incisors= are small teeth with long conical
roots and somewhat chisel-shaped crowns. Surrounding the base of the
crown there is a rather prominent ridge, terminated laterally by a
pair of small cusps. This ridge, the =cingulum=, serves to protect the
edge of the gums from injury by the hard parts of food. The third
incisor is a good deal like the others but larger, and has the
cingulum well developed though not terminated by lateral cusps. All
the incisors are borne by the premaxillae, the remaining teeth by the
maxillae.
The =canine= is a large pointed tooth, slightly recurved and with a
long tapering root.
The =premolars= are four in number, and in all the cingulum is fairly
well seen. The first is a very small tooth with a single tapering
root, the second and third are larger and have two roots, while the
fourth, the =carnassial=, is much the largest and has three roots.
Each of the second, third and fourth premolars has a stout blade, the
middle portion of which is drawn out into a prominent cone; the
posterior part of the fourth premolar forms a compressed ridge, and
at the antero-internal edge of the tooth there is a small inner
tubercle.
The two =molar= teeth are of very unequal size. The first, which has
two anterior roots and one posterior, is wider than it is long, its
outer portion being produced into two prominent cusps, while its inner
portion is depressed. The second molar is a small tooth resembling the
first in its general appearance, but with much smaller outer cusps.
=Teeth of the lower jaw.=
The three =incisors= of the lower jaw have much the same character as
the first two of the upper jaw; while the =canine= is identical in
character with that of the upper jaw.
The four =premolars= gradually increase in size from the first to the
last, but none are very large. The first premolar is a single-rooted
tooth resembling that of the upper jaw; the second, third and fourth
are two-rooted, like the second and third of the upper jaw, which they
closely resemble in other respects.
The first =molar= forms the =carnassial= (fig. 84, V), and with the
exception of the canine, is much the largest tooth of the lower jaw;
it is a two-rooted tooth, with a long compressed bilobed blade, and a
posterior tuberculated talon or heel. The second molar is much
smaller, though likewise two-rooted, while the third molar is very
small and has only a single root. All the teeth except the molars are
preceded in the young animal by temporary =milk teeth=. These milk
teeth, though smaller, are very similar to the permanent teeth by
which they are ultimately replaced.
II. ENDOSKELETON.
1. THE AXIAL SKELETON.
This includes the vertebral column, the skull, and the ribs and
sternum.
A. THE VERTEBRAL COLUMN.
This consists of a series of about forty vertebrae arranged in
succession so that their centra form a continuous rod, and their
neural arches a continuous tube, surrounding a cavity, the =neural
canal=.
The vertebrae may be readily divided into five groups:--
1. The =cervical= or neck vertebrae.
2. The =thoracic= or chest vertebrae which bear ribs.
3. The =lumbar= vertebrae which are large and ribless.
4. The =sacral= vertebrae which are fused with one another and united
with the pelvis.
5. The =caudal= or tail vertebrae which are small.
Except in the sacral region the vertebrae are movably articulated to
one another, while their centra are separated from one another by
cartilaginous =intervertebral discs=.
GENERAL CHARACTERS OF A VERTEBRA.
Take as a type the =fourth lumbar vertebra=. It may be compared to a
short tube whose inner surface is smooth and regular, and whose outer
surface is thickened and drawn out in a variety of ways. The basal
part of the vertebra is the =centrum= or body which forms the
thickened floor of the neural canal. Its two ends are slightly convex
and are formed by the =epiphyses=, two thin plates of bone which are
at first altogether distinct from the main part of the centrum, but
fuse with it as the animal grows older; its sides are drawn out into a
pair of strong =transverse processes=, which project forwards,
outwards, and slightly downwards. The =neural arch= forms the sides
and roof of the neural canal, and at each end just above the centrum
bears a pair of =intervertebral notches= for the passage of the spinal
nerves, the posterior notches being considerably deeper than the
anterior. The neural arch is drawn out into a series of processes.
Arising from the centre of the dorsal surface is a prominent median
=neural spine= or =spinous process=, which projects upwards and
slightly forwards; its anterior edge is vertical, while its posterior
edge slopes gradually. At the two ends of the neural arch arise the
two pairs of =zygapophyses= or articulating surfaces, which
interlock with those of the adjacent vertebrae. The anterior or
=prezygapophyses= look inwards, and are large and concave; they are
borne upon a pair of large blunt outgrowths of the neural arch, the
=metapophyses=. The posterior or =postzygapophyses= are slightly
convex and look outwards and downwards; they are borne upon backwardly
projecting outgrowths of the neural arch. Lastly there are a pair of
minute projections arising from the posterior end of the neural arch,
below the postzygapophyses. These are the =anapophyses=. In young
individuals the development of all the processes of the various
vertebrae is less marked, and the epiphyses are obviously distinct.
[Illustration FIG. 69. A, ATLAS AND B, AXIS VERTEBRA OF A DOG (_Canis
familiaris_) (after VON ZITTEL).
1. transverse process of atlas.
2. vertebrarterial canal.
3. foramen for exit of spinal nerve.
4. neural spine.
5. odontoid process.
6. anterior articulating surface of centrum.
7. centrum.
8. transverse process of axis.
9. postzygapophysis.]
THE CERVICAL VERTEBRAE.
These are seven in number, as in almost all mammals. They are
characterised by the fact that they have small ribs fused with them,
forming transverse processes perforated by canals through which the
vertebral arteries run.
The first, or =atlas= vertebra (fig. 69, A), differs much from all the
others; it is drawn out into a pair of wide wing-like transverse
processes (fig. 69, A, 1), and forms a ring surrounding a large
cavity. This cavity is during life divided into two parts by a
transverse ligament; the upper cavity is the true neural canal, while
the lower lodges the =odontoid process= of the second vertebra, which
is the detached centrum of the atlas. The neural arch is broad and
regular; it has no spinous process, and is perforated in front by a
pair of foramina for the passage of the first spinal nerves. The
mid-ventral portion of the atlas is rather thick, and bears a minute
backwardly-projecting hypapophysis. The bases of the broad transverse
processes are perforated by the =vertebrarterial canals= (fig. 69, A,
2). The atlas bears at each end a pair of large articulating surfaces;
those at the anterior end articulate with the condyles of the skull,
and are very deeply concave; those at the posterior end for
articulation with the axis, are nearly as large, but are flattened.
The atlas ossifies from three centres, one forming the mid-ventral
portion, the others the two halves of the remainder.
The second, or =axis= vertebra (fig. 69, B), also differs much from
the other cervicals. The long and broad centrum has a very flat dorsal
surface, and is produced in front into the conical =odontoid process=
(fig. 69, B, 5), and bears a pair of very large convex outwardly
directed surfaces for articulation with the atlas. At its posterior
end it is drawn out into a pair of small backwardly-directed spines,
the transverse processes; these are perforated at their bases by the
vertebrarterial canals. The neural arch is deeply notched in front and
behind for the passage of the spinal nerves, and is drawn out above
into a very long compressed neural spine (fig. 69, B, 4), which
projects a long way forwards, and behind becomes bifid and thickened,
bearing a pair of flat downwardly directed postzygapophyses. In the
young animal the odontoid process is readily seen to ossify from a
centre anterior to that forming the anterior epiphysis of the axis.
The remaining five cervical vertebrae, the third to the seventh
inclusive, have rather flattened wide centra, obliquely truncated at
either end. The neural spine progressively increases in size as the
vertebrae are followed back. The transverse processes vary
considerably; those of the third are divided into a thicker
backwardly-, and a more slender forwardly-projecting portion; those of
the fourth and fifth mainly extend downwards, and that of the sixth is
divided into a horizontal portion and a downwardly-projecting
=inferior lamella=. All the cervical vertebrae except the seventh have
the bases of the transverse processes perforated by the
vertebrarterial canals. The prezygapophyses in each case look upwards
and slightly inwards, while the postzygapophyses look downwards and
slightly outwards.
THE THORACIC VERTEBRAE.
The =thoracic vertebrae= are twelve or thirteen in number, and all
bear movably articulated ribs. As a group they are characterised by
their comparative shortness, and in the case of the first eight or
nine by the great length of the backwardly-sloping neural spine. The
posterior thoracic vertebrae approach in character the succeeding
lumbar vertebrae.
As type of the anterior thoracic vertebrae, take any one between the
second and sixth inclusive. The centrum is short, and has its
terminations vertically truncated. At the top of the centrum, at both
anterior and posterior ends on each side, is a demi-facet (fig. 70, A,
4), which, together with that on the adjacent vertebra, forms an
articulating surface for the capitulum of the rib. The neural arch is
small and deeply notched behind for the passage of the spinal nerve.
It is drawn out above into a very long neural spine (fig. 70, A, 1),
whose base extends back over the succeeding vertebra and bears the
downwardly-directed postzygapophyses (fig. 70, A, 6). The summit of
the neural arch is deeply notched in front, and on each side of the
notch are the prezygapophyses, which look almost vertically upwards.
The transverse processes are short and blunt, and are flattened below
(fig. 70, A, 3) for the articulation of the tubercula of the ribs.
[Illustration FIG. 70. A, SECOND THORACIC, AND B, SECOND LUMBAR
VERTEBRA OF A DOG (_Canis familiaris_) SEEN FROM THE RIGHT SIDE (after
VON ZITTEL).
1. neural spine.
2. centrum.
3. transverse process bearing in A the facet for articulation with
the tuberculum of the rib.
4. facet for articulation with the capitulum of the rib.
5. metapophysis.
6. postzygapophysis.]
The posterior three or four thoracic vertebrae differ much from the
others. The centra are longer, the neural spines short and not
directed backwards, the articular facets for the heads of the ribs are
confined to the anterior end of the centrum of each vertebra, not
overlapping on to the preceding vertebra. The transverse processes are
small and irregular, and metapophyses and anapophyses are developed.
The prezygapophyses also look more inwards, and the postzygapophyses
more outwards than in the more typical thoracic vertebrae.
THE LUMBAR VERTEBRAE.
The =lumbar vertebrae= are seven in number, and their general
characteristics have been already described. As a group they are
characterised by their large size, and the great development of the
transverse processes, metapophyses and neural spines.
THE SACRAL VERTEBRAE.
Three vertebrae are commonly found fused together, forming the
=sacrum=; the divisions between the three being indicated by the
foramina for the exit of the spinal nerves.
Of these three vertebrae, the first is much the largest, and is firmly
united to the ilium on each side by a structure formed by the
transverse processes and expanded ribs. In the adult this structure
forms one continuous mass, but in the young animal a ventral portion
formed by the rib is clearly distinguishable from a dorsal portion
formed by the transverse process. All three have low neural spines.
The anterior sacral vertebra bears a large pair of prezygapophyses,
while the posterior one bears a small pair of postzygapophyses.
THE CAUDAL VERTEBRAE.
The =caudal vertebrae= are about nineteen in number. The earlier ones
have well-developed neural arches, transverse processes, and
zygapophyses, but as the vertebrae are followed back they gradually
lose all their processes, and the neural arch as well, becoming at
about the thirteenth from the end reduced to simple cylindrical
centra.
B. THE SKULL.
The =skull= consists of the following three parts: (_a_) the cranium,
with which are included the skeletal supports of the various special
sense organs, and the bones of the face and upper jaw; (_b_) the lower
jaw or mandible, which is movably articulated to the cranium, and
(_c_) the hyoid.
(_a_) THE CRANIUM.
The cranium is a compact bony box, forming the anterior expanded
portion of the axial skeleton. It has a longitudinal axis, the
=craniofacial= axis around which the various parts are arranged, and
this axis is a direct continuation of that of the vertebral column.
Similarly the cavity of the cranium is a direct continuation of the
spinal canal. The posterior part of the craniofacial axis, which has
relations only with the cranium, is called the =basicranial axis=.
In the dog as in the other types previously described, the skull in
its earliest stages is cartilaginous, containing no bone. In the
adult, however, the cartilage is to a great extent replaced by bone,
and in addition to this cartilage bone, membrane bone is largely
developed, and intimately united with the cartilage bone to form one
complete whole.
In the description of the dog's skeleton, as in those of the previous
types, the names of the membrane bones are printed in italics, while
those of the cartilage bones are printed in thick type.
Most of the numerous foramina perforating the skull walls will be
described after the bones have been dealt with.
For purposes of description the cranium may be further subdivided
into:--
1. The cranium proper or brain case.
2. The sense capsules.
3. The upper jaw.
1. THE CRANIUM PROPER OR BRAIN CASE.
Taking the membrane and cartilage bones together, they are seen to be
more or less arranged in three segments, which however must not be
regarded as homologous with the segments forming the vertebral column.
[Illustration FIG. 71. DIAGRAM OF THE RELATIONS OF THE PRINCIPAL BONES
IN THE MAMMALIAN SKULL (modified after FLOWER).
Cartilage is dotted. Cartilage bones are marked by dots and dashes,
membrane bones are left white.
1. basi-occipital.
2. exoccipital.
3. supra-occipital.
4. basisphenoid.
5. alisphenoid.
6. parietal.
7. presphenoid.
8. orbitosphenoid.
9. frontal.
10. periotic, immediately below which is the tympanic.
11. lachrymal.
12. ethmo-turbinal.
13. maxillo-turbinal.
14. nasal.
15. mesethmoid.
16. vomer.
17. pterygoid.
18. palatine.
19. maxillae.
20. premaxillae.
21. squamosal.
22. mandible.
23. tympano-hyal.
24. stylo-hyal.
25. epi-hyal.
26. basi-hyal. Between this and the epi-hyal is the cerato-hyal.
27. thyro-hyal.
28. jugal.
Nerve exits are indicated by Roman numerals.]
The =occipital segment= is the most posterior of the three, and
consists of four cartilage bones, which in the adult are commonly
completely fused together. They surround the great =foramen magnum=
(fig. 75, 2) through which the brain and spinal cord communicate.
Forming the lower margin of the foramen magnum is a large flat
unpaired bone, the =basi-occipital= (fig. 75, 5). Above this on each
side are the =exoccipitals=, whose sides are drawn out into a pair of
downwardly-directed =paroccipital processes=, which are applied to the
tympanic bullae[141]. The inner side of each exoccipital is converted
into the large rounded =occipital condyle= (fig. 72, 13) by which the
skull articulates with the atlas vertebra. The dorsal boundary of the
foramen magnum is formed by a large unpaired flat bone, the
=supra-occipital= (figs. 72 and 75, 1), which is continuous with a
small bone, the _interparietal_, prolonged forwards between the
parietal bones of the next segment.
In old animals the interparietal forms the hind part of a prominent
ridge running along the mid-dorsal surface of the skull and called the
=sagittal crest=, while the junction line of the occipital and
parietal segments forms a prominent =occipital crest=.
The plane in which the bones of the occipital segment lie is called
the occipital plane; the angle that it makes with the basicranial axis
varies much in different mammals.
The =parietal segment= consists of both cartilage and membrane bones.
It is formed of five bones, which are in contact with those of the
occipital segment on the dorsal and ventral surfaces, while laterally
they are separated by the interposition of the auditory bones, and to
some extent of the squamosal. The =basisphenoid= (fig. 75, 6), an
unpaired bone forming the ventral member of this segment, is the
direct continuation of the basi-occipital. It tapers anteriorly, but
is rather deep vertically, its upper or dorsal surface bearing a
depression, the =sella turcica=, which lodges the pituitary body of
the brain. From the sides of the basisphenoid arise the =alisphenoids=
(fig. 75, 11) a pair of bones of irregular shape generally described
as wing-like; each gives off from its lower surface a =pterygoid
plate=, which is united in front with the palatine, and below with the
pterygoid. The alisphenoids are united above with a pair of large
nearly square bones, the _parietals_ (fig. 73, 2), which meet one
another in the mid-dorsal line. The line of junction is frequently
drawn out into a strong ridge, which forms the anterior part of the
=sagittal crest=.
[Illustration FIG. 72. VERTICAL LONGITUDINAL SECTION TAKEN A LITTLE TO
THE LEFT OF THE MIDDLE LINE THROUGH THE SKULL OF A DOG (_Canis
familiaris_) × 3/5. (Camb. Mus.)
1. supra-occipital.
2. interparietal.
3. parietal.
4. frontal.
5. cribriform plate.
6. nasal.
7. mesethmoid.
8. maxillae.
9. vomer.
10. ethmo-turbinal.
11. maxillo-turbinal.
12. premaxillae.
13. occipital condyle.
14. basi-occipital.
15. tympanic bulla.
16. basisphenoid.
17. pterygoid.
18. palatine.
19. alisphenoid.
20. internal auditory meatus.
21. tentorium.
22. foramen lacerum posterius.
23. floccular fossa.
24. coronoid process.
25. condyle.
26. angle.
27. mandibular symphysis.
28. inferior dental foramen.
29. stylo-hyal.
30. epi-hyal.
31. cerato-hyal.
32. basi-hyal.
33. thyro-hyal.
XII. condylar foramen.]
The =frontal segment=, which surrounds the anterior part of the brain,
is closely connected along almost its whole posterior border with the
parietal segment.
Its base is formed by the =presphenoid= (fig. 75, 12), a very deep
unpaired bone, narrow and compressed ventrally, and with an irregular
dorsal surface. The presphenoid is continuous with a second pair of
wing-like bones, the =orbitosphenoids=. Each orbitosphenoid meets the
alisphenoid behind, but the relations of the parts in this region are
somewhat obscured by a number of large foramina piercing the bones,
and also by an irregular vacuity, the =foramen lacerum anterius= or
=sphenoidal fissure=, which lies between the orbitosphenoid and
alisphenoid, separating the lateral parts of the parietal and frontal
segments, in the same way as the space occupied by the auditory bones
separates the lateral parts of the occipital and parietal segments.
The orbitosphenoids pass obliquely forwards and upwards, and are
united above with a second pair of large membrane bones, the
_frontals_ (fig. 73, 3). The outer side of each frontal is drawn out
into a rather prominent rounded =postorbital process= (fig. 73, 10),
from which a ridge converges backwards to meet the sagittal crest. The
anterior part of the frontal is produced to form the long nasal
process, which is wedged in between the nasal and maxillae.
[Illustration FIG. 73. DORSAL VIEW OF THE CRANIUM OF A DOG (_Canis
familiaris_) × 2/3.
1. supra-occipital.
2. parietal.
3. frontal.
4. nasal.
5. maxillae (facial portion).
6. premaxillae.
7. squamosal.
8. jugal.
10. postorbital process of frontal.
11. infra-orbital foramen.
12. anterior palatine foramen.
13. lachrymal foramen.
_i_ 1. first incisor.
_c._ canine.
_pm_ 4. fourth premolar.]
The cranial cavity is continuous in front with the =nasal= or
=olfactory cavities=, but the passage is partially closed by a screen
of bone, the =cribriform plate= (fig. 72, 5), which is placed
obliquely across the anterior end of the cranial cavity, and is
perforated by a number of holes through which the olfactory nerves
pass. The plane of the cribriform plate is called the =ethmoidal
plane=, and as was the case also with the occipital plane, the angle
that it makes with the basicranial axis varies much in different
mammals, and is of importance. The =olfactory fossa= in which lie the
olfactory lobes of the brain, is partially separated from the
=cerebral fossa=, or cavity occupied by the cerebral hemispheres, by
ridges on the orbitosphenoids and frontals. The presphenoid is
connected in front with a vertical plate formed partly of bone, partly
of unossified cartilage; this plate, the =mesethmoid= (fig. 72, 7),
separates the two olfactory cavities which lodge the olfactory organs.
Its anterior end always remains unossified, and forms the septal
cartilage of the nose.
The brain case may then, to use the words of Sir W.H. Flower, be
described as a tube dilated in the middle and composed of three bony
rings or segments, with an aperture at each end, and a fissure or
space at the sides between each of them.
2. THE SENSE CAPSULES.
Each of the three special sense organs, of hearing, of sight, and of
smell, is in the embryo provided with a cartilaginous or membranous
protecting capsule; and two of these, the auditory and olfactory
capsules, become afterwards more or less ossified, and intimately
related to the cranium proper.
(1) =Bones in relation to the Auditory capsules.=
These bones lie on each side wedged into the vacuity between the
lateral parts of the occipital and parietal segments; they are three
in number, the =periotic=, the _tympanic_ and the _squamosal_.
The =periotic= is the most important of them, as it replaces the
cartilaginous auditory capsule of the embryo, and encloses the
essential organ of hearing. It commences to ossify from three centres
corresponding to the pro-otic, epi-otic and opisthotic of lower
skulls, such as those of the Turtle and Crocodile.
These ossifications however very early combine to form a single bone,
the =periotic=, which nevertheless consists of two portions, the
=petrous= and the =mastoid=, differing considerably from one another.
[Illustration FIG. 74. DIAGRAM OF THE MAMMALIAN TYMPANIC CAVITY AND
ASSOCIATED PARTS (modified from LLOYD MORGAN).
1. external auditory meatus.
2. tympanic membrane.
3. malleus.
4. incus.
5. lenticular.
6. stapes.
7. fenestra ovalis.
8. fenestra rotunda.
9. Eustachian tube.
10. cavity occupied by the cochlea.
11. cavity occupied by the membranous labyrinth.]
The =petrous portion= lies dorsally and anteriorly, and is much the
more important of the two, as it encloses the essential part of the
auditory organ. It forms an irregular mass of hard dense bone,
projecting into the cranial cavity, and does not appear on the
external surface at all. The =mastoid portion= lies ventrally and
posteriorly, is smaller, and formed of less dense bone than is the
petrous portion, from which it differs also in the fact that it
appears on the surface of the skull, just external to the exoccipital.
The petrous portion bears a ridge, which together with a ridge on the
supra-occipital, and the =tentorium= (fig. 72, 21), a transverse fold
of the dura mater[142], separates the large cerebral fossa from the
=cerebellar fossa=, which is much smaller than the cerebral fossa and
lies behind and partly beneath it. The plane of the tentorium is
called the =tentorial plane=, and the angles that it makes with the
basicranial axis and with the occipital and ethmoidal planes vary much
in different mammals.
The periotic has its inner surface marked by important depressions,
while both inner and outer surfaces are pierced by foramina. At about
the middle of its inner surface are seen two deep pits, one lying
immediately above the other. Of these the more ventral is a foramen,
the =internal auditory meatus= (fig. 72, 20), through which the VIIth
(facial) and VIIIth (auditory) nerves leave the cranial cavity, the
facial nerve passing through the bone and afterwards leaving the skull
by the =stylomastoid foramen= (fig. 75, VII), while the auditory
passes to the inner ear. The more dorsal of the two pits is not a
foramen but the =floccular fossa= (fig. 72, 23) which lodges the
floccular lobe of the cerebellum. In some skulls another wide and
shallow but fairly prominent depression is seen dorsal to and slightly
behind the floccular fossa, this also lodges part of the cerebellum.
Behind the internal auditory meatus, between the periotic and
exoccipital is seen the internal opening of the =foramen lacerum
posterius= (fig. 72, 22). The shape of this opening varies. The
ventro-anterior border of the periotic is marked by a deep notch, the
sides of which sometimes unite, converting it into a foramen.
On the outer side of the periotic, and clearly seen only after the
removal of the tympanic, are two holes, the =fenestra ovalis= and the
=fenestra rotunda=.
The _tympanic_ (figs. 72, 15 and 75, 4) is a greatly expanded
boat-shaped bone, which forms the auditory bulla and lies immediately
ventral to the periotic; it is separated from the periotic by the
=tympanic cavity= into which the fenestra rotunda and the fenestra
ovalis open.
There are several other openings into the tympanic cavity.
(_a_) On the external surface is a large oval opening, the =external
auditory meatus= bounded by a thickened rim.
(_b_) Into the outer and anterior part of the cavity the outer end of
the =Eustachian tube= opens; while the inner end passes through a
foramen (fig. 75, 22) just external to the foramen lacerum medium, on
its way to open into the pharynx.
(_c_) The internal carotid artery also enters the tympanic cavity by a
canal which commences in the foramen lacerum posterius, and passes
forwards to open on the inner side of the bulla. The artery then
passes forwards, and barely appearing on the ventral surface of the
cranium, enters the brain cavity through the foramen lacerum medium
(fig. 75, 9).
Immediately behind the tympanic, between it and the mastoid process of
the periotic and the paroccipital process of the exoccipital is the
=stylomastoid foramen= (fig. 75, VIII).
Within the tympanic cavity are four small bones, the =auditory
ossicles= (cp. fig. 74), called respectively the =malleus=, =incus=,
=lenticular= and =stapes=; these together form a chain extending from
the fenestra ovalis to the tympanic membrane.
The =malleus= has a somewhat rounded head (fig. 100, B, 1) which
articulates with the incus, while the other end of the bone is drawn
out into a long process, the =manubrium=, which lies in relation to
the tympanic membrane. The head is also more or less connected by a
thin plate of bone, the =lamella=, to another outgrowth, the
=processus longus=. The =incus= (fig. 100, B, 3) is somewhat
anvil-shaped, and is drawn out into a process which is connected with
the =lenticular=, a nodule of bone interposed between the incus and
the stapes, with which it early becomes united. The =stapes= (fig.
100, B, 2) is stirrup-shaped, consisting of a basal portion from which
arise two =crura=, which meet and enclose a space, the =canal=.
The _squamosal_ (fig. 73, 7) is a large bone occupying much of the
side wall of the cranial cavity, and articulating above with the
parietal, and behind with the supra-occipital, while in front it
overlaps the frontal and alisphenoid. But though it occupies so large
a space on the outer wall, it forms very little of the internal wall
of the skull, but is really like a bony plate attached to the outer
surface of the cranial wall. The squamosal is drawn out into a strong
forwardly-directed =zygomatic process= which meets the jugal or malar.
The ventral side of the zygomatic process is hollowed out, forming the
=glenoid fossa= (fig. 75, 8), a smooth laterally elongated surface
with which the lower jaw articulates, while the hinder edge of the
glenoid fossa is drawn out into a rounded =postglenoid process= (fig.
75, 23). The articulation is such as to allow but little lateral play
of the lower jaw.
(2) =Bones in relation to the Optic capsules.=
The only bone developed in relation to the optic capsule on each side
is the _lachrymal_. This is a small membrane bone lying between the
frontal and palatine behind, and the maxillae and jugal in front. It is
perforated by a prominent =lachrymal foramen= (fig. 73, 13) which
opens within the orbit.
(3) =Bones in relation to the Olfactory capsules.=
In connection with the =olfactory capsules=, five pairs of bones are
developed, two pairs being membrane bones, and three pairs cartilage
bones.
Of membrane bones, the _nasals_ (fig. 73, 4) are a pair of long narrow
bones, lying closely side by side, and forming the main part of the
roof of the olfactory chamber. Their posterior ends overlap the
frontals, and the outer margin of each is in contact with the nasal
process of the frontal, and with the maxillae and premaxillae.
Lying immediately ventral to the nasals, and on each side of the
perpendicular mesethmoid, are the =ethmoid= or =turbinal= bones,
which have a curious character, being formed of a number of delicate
plates intimately folded on one another. The posterior pair of these
bones, the =ethmo-turbinals= (fig. 72, 10), are the larger, and form a
mass of intricately folded lamellae attached behind to the cribriform
plate, and passing laterally into two thin plates of bone, which abut
on the maxillae. The uppermost lamella of each ethmo-turbinal is larger
than the others and more distinct. It is sometimes distinguished as
the =naso-turbinal=, and forms an imperfect lower boundary to a canal,
which is bounded above by the nasals. In front of and somewhat below
the ethmo-turbinals, lie another pair of bones of similar character,
the =maxillo-turbinals= (fig. 72, 11).
The last bone to be mentioned in connection with the olfactory
capsules is a membrane bone, the _vomer_ (fig. 72, 9). This is a
slender vertically-placed bone, whose anterior part lies between the
maxillo-turbinals, while behind it extends beyond the mesethmoid, so
as to underlie the anterior part of the presphenoid. The anterior part
of the vomer forms a kind of trough, while further back in the region
of the ethmo-turbinals it sends out a pair of strong lateral plates,
each of which, passing below the ethmo-turbinal, joins the side wall
of the nasal cavity, and forms a partition dividing the nasal cavity
into a lower =narial passage= and an upper =olfactory chamber=.
THE JAWS.
In the embryo both upper and lower jaws are formed of cartilaginous
bars, but in the adult not only has the cartilage entirely
disappeared, but even cartilage bone is absent, the jaws being formed
of membrane bone.
3. THE UPPER JAW.
[Illustration FIG. 75. VENTRAL VIEW OF THE CRANIUM OF A DOG (_Canis
familiaris_) × 3/5. (Camb. Mus.)
1. supra-occipital.
2. foramen magnum.
3. occipital condyle.
4. tympanic bulla.
5. basi-occipital.
6. basisphenoid.
7. external auditory meatus.
8. glenoid fossa.
9. foramen lacerum medium and anterior opening of carotid canal.
10. postglenoid foramen.
11. alisphenoid.
12. presphenoid.
13. vomer.
14. jugal.
15. pterygoid.
16. palatal process of palatine.
17. maxillae (palatal portion).
18. posterior palatine foramina.
19. anterior palatine foramen.
20. premaxillae.
21. alisphenoid canal.
22. Eustachian foramen.
23. postglenoid process of squamosal.
II. optic foramen.
III, IV, V{1}, VI. foramen lacerum anterius.
V{2}. foramen rotundum.
V{3}. foramen ovale.
VII. stylomastoid foramen.
IX, X, XI. foramen lacerum posterius.
XII. condylar foramen.
_i_ 2. second incisor.
_c._ canine.
_pm_ 1, _pm_ 4. first and fourth premolars.
_m_ 1. first molar.]
The bones of the upper jaw are closely connected with those of the
cranium proper and olfactory capsules. The most posterior of them is
the _pterygoid_ (fig. 75, 15), a thin vertically placed plate of bone,
which articulates above with the basisphenoid, the presphenoid, and
the strong pterygoid process of the alisphenoid. The ventral end of
the pterygoid is drawn out into a small backwardly-projecting =hamular
process=. In front the pterygoid articulates with the _palatine_, a
much larger bone, consisting of (1) a vertical portion, which passes
up to meet the orbitosphenoid and frontal, and sends inwards a plate
which meets the presphenoid and vomer, forming much of the roof of the
posterior part of the narial passage; and (2) a strong horizontal
portion, the =palatal process= (fig. 75, 16), which passes inwards and
meets its fellow in the middle line, forming the posterior part of the
bridge of bone supporting the hard palate. The palatal process is
continuous in front, with a large bone, the _maxillae_, which, like
the palatine, consists of vertical and horizontal portions. The
vertical, or =facial portion= (fig. 73, 5), is the largest, and
constitutes the main part of the side of the face in front of the
orbit, forming also the chief part of the outer wall of the nasal
cavity. It is continuous in front with the premaxillae, above with the
nasal and frontal, and behind with the lachrymal, jugal, and palatine.
The horizontal, or =palatal portion= (fig. 75, 17), forms the anterior
part of the bony plate supporting the hard palate, and meets its
fellow in a long straight symphysis. The junction line between the
palatal and facial portions is called the =alveolar border=, and
along it are attached the canine, premolar, and molar teeth.
The anterior part of the upper jaw on each side is formed by a small
bone, the _premaxillae_, which bears the incisor teeth. It, like the
maxillae, has a palatal portion (fig. 75, 20), which meets its fellow
in the middle line, and an ascending portion, which passes backwards
as the =nasal process=, tapering regularly and lying between the nasal
and the maxillae. The two premaxillae form the outer and lower borders
of the anterior nares. The last bone to be mentioned in connection
with the upper jaw and face is the _jugal_ or _malar_ (figs. 73, 8,
and 75, 14), a strong bone which forms the anterior half of the
zygomatic arch. It is firmly united in front to the maxillae, and
behind meets the zygomatic process of the squamosal, being drawn out
dorsally into a short =postorbital process= at the point of meeting.
This process lies immediately below the postorbital process of the
frontal, and if the two met, as they do in some mammals, they would
partially shut off the orbit from a larger posterior cavity, the
=temporal fossa=. In the living animal a ligament unites the two
postorbital processes.
(_b_) THE LOWER JAW OR MANDIBLE.
This consists of two elongated symmetrical halves, the =rami=, which
are united to one another at the median symphysis in front, while
behind they diverge considerably, and each articulates with the
glenoid surface of the corresponding squamosal. In young animals the
rami are united at the symphysis by fibrous tissue, but in old animals
they sometimes become fused together. The upper or alveolar border
bears the teeth, and behind them is drawn out into a high laterally
compressed =coronoid process= (fig. 72, 24), which is hollowed on its
outer surface. Immediately behind the coronoid process is the
transversely elongated =condyle= (fig. 72, 25), which fits into the
glenoid cavity in such a way as to allow free up and down movement of
the jaw, with but little rolling motion. The posterior end of the jaw
below the condyle forms a short rounded process, the =angle= (fig.
72, 26). Two prominent foramina are to be seen in the lower jaw. These
are firstly the =inferior dental foramen= (fig. 72, 28), which lies on
the inner surface below the coronoid process; through it an artery and
a branch of the fifth nerve enter to supply the teeth, and secondly
the =mental foramen=, which lies on the outer side near the anterior
end, and through which a branch of the same nerve emerges.
(_c_) THE HYOID.
The =Hyoid= of the dog consists of a transverse median piece, the
=basi-hyal=[1] (fig. 72, 32), from which arise two pairs of =cornua=.
The =anterior cornu= is much the longer of the two, and consists
principally of three short separate ossifications, placed end to end
and called respectively the =cerato-hyal=[143], =epi-hyal=, and
=stylo-hyal=. All of them are short rods of bone, contracted in the
middle, and expanded at the ends, where they are tipped with
cartilage. The cerato-hyal (fig. 72, 31) lies next to the basi hyal.
The stylo-hyal is terminated by a much smaller bone, the
=tympano-hyal=, which lies in a canal between the tympanic and
periotic, and is ankylosed to the periotic just to the anterior and
inner side of the stylomastoid foramen.
The =posterior cornu= of the hyoid is much smaller than the anterior;
it consists of a short bone, the =thyro-hyal= (fig. 72, 33), which
connects the basi-hyal with the thyroid cartilage of the larynx.
FORAMINA OF THE SKULL.
The foramina, or apertures perforating the walls of the skull, are
very numerous, and may either be due to holes actually penetrating the
bone, or may be small vacuities between the margins of two elsewhere
contiguous bones.
They may be divided into two groups, the first including
I. The holes through which the =twelve cranial nerves= leave the
cranial cavity.
_a._ The most anterior of these nerves, the olfactory, leaves the
skull by a number of small holes piercing the =cribriform plate= (fig.
72, 5).
_b._ The second, or optic, passes out by a large hole, the =optic
foramen= (fig. 75, II) piercing the orbitosphenoid. The optic foramen
is the most anterior of the three prominent holes seen within and
immediately behind the orbit.
_c._ The third, fourth, and sixth nerves, i.e. those supplying the eye
muscles, and with them the first or ophthalmic branch of the large
fifth or trigeminal nerve, pass out by a large hole, the =foramen
lacerum anterius= (fig. 75, III, IV, V{1}, VI), which, as has been
already mentioned, lies between the orbitosphenoid and alisphenoid.
_d._ Immediately behind the foramen lacerum anterius, the alisphenoid
is perforated by a prominent round hole, the =foramen rotundum= (fig.
75, V{2}), through which the second branch of the trigeminal nerve
passes out.
_e._ A quarter of an inch further back there is another prominent
hole, the =foramen ovale= (fig. 75, V{3}), through which the third
branch of the trigeminal nerve leaves the cranium.
_f._ The seventh or facial nerve, as already mentioned, leaves the
cranial cavity and enters the auditory capsule, through an opening in
the periotic called the =internal auditory meatus=, while it finally
leaves the skull by the =stylomastoid foramen= (fig. 75, VII), which
lies between the tympanic bulla, the paroccipital process, and the
mastoid portion of the periotic.
_g._ The eighth or auditory nerve on leaving the cranial cavity,
passes with the facial straight into the auditory capsule through the
=internal auditory meatus= (fig. 72, 20). It is then distributed to
the organ of hearing.
_h._ The ninth, tenth and eleventh nerves leave the skull through the
=foramen lacerum posterius= (fig. 75, IX, X, XI), a large space lying
between the auditory bones and the exoccipital.
_i._ Finally, the twelfth nerve, the hypoglossal, passes out through
the prominent =condylar foramen= (fig. 75, XII), which perforates the
exoccipital just behind the foramen lacerum posterius.
II. OTHER OPENINGS IN THE SKULL.
_a._ The =anterior narial opening= lies at the anterior end of the
skull, and is bounded by the premaxillae and nasals. In the natural
condition it is divided into two by a vertical partition, formed by
the =narial septum=, the anterior unossified part of the mesethmoid.
_b._ Penetrating the middle of the maxillae at the side of the face is
the rather large =infra-orbital foramen= (fig. 73, 11), through which
part of the second branch of the trigeminal nerve passes out from the
orbit to the side of the face.
_c._ Several foramina are seen perforating the anterior part of the
orbit. The most dorsal of these, perforating the lachrymal bone, is
the =lachrymal foramen= (fig. 73, 13). Lying below and slightly
external to this is a large foramen, through which part of the second
branch of the trigeminal enters on its way to the infra-orbital
foramen and so to the side of the face. Lastly, lying below these, and
perforating the palatine, are two closely apposed foramina, the
=internal orbital foramina=, through which part of the first or
ophthalmic branch of the trigeminal nerve leaves the orbit, passing
into the nasal cavity.
_d._ The anterior part of the palate between the premaxillae and the
maxillae is perforated by a pair of long closely apposed apertures, the
=anterior palatine foramina= (fig. 75, 19). They transmit part of the
trigeminal nerve.
_e._ Towards the posterior part of the palate are two pairs of small
=posterior palatine foramina= (fig. 75, 18). These perforate the
palatine and transmit branches of the trigeminal nerve and certain
blood-vessels.
_f._ The =posterior narial opening= is bounded chiefly by the
palatines.
_g._ The =alisphenoid canal= (fig. 75, 21) is a short canal
penetrating the base of the alisphenoid bone, and transmitting the
external carotid artery. It lies between the foramen rotundum and the
foramen ovale.
_h._ Between the auditory bulla and the foramen ovale are seen two
openings. The more external of these is the opening of the =Eustachian
canal= (fig. 75, 22), which communicates with the tympanic cavity. The
more internal is the =foramen lacerum medium= (fig. 75, 9), through
which the internal carotid enters the cranial cavity.
_i._ The =external auditory aperture= (fig. 75, 7) is a large opening
with rough edges at the outer side of the tympanic bulla.
_j._ Between it and the glenoid surface of the squamosal is the
=postglenoid= foramen (fig. 75, 10) through which a vein passes out.
_k._ Lastly, there is the great =foramen magnum= (fig. 75, 2), between
the occipital condyles. Through it the brain and spinal cord
communicate.
C. THE RIBS AND STERNUM.
[Illustration FIG. 76. STERNUM AND STERNAL RIBS OF A DOG (_Canis
familiaris_) × 1/2.
1. presternum.
2. first sternebra of mesosternum.
3. last sternebra of mesosternum.
4. xiphisternum. The flattened cartilaginous plate terminating the
xiphisternum is not shown.
5. first sternal rib.]
These, together with the thoracic vertebrae, form the skeletal
framework of the thorax. Each rib is a curved rod, which at its dorsal
end is movably articulated to the vertebra, and at its ventral end is
either connected with the sternum, or ends freely. In the dog there
are thirteen pairs of ribs, nine pairs of which are directly connected
with the sternum, while the remaining four end freely and are known as
=floating ribs=. Each rib is obviously divided into two parts, a
dorsal or =vertebral part=, and a ventral or =sternal part=. The
vertebral portion, which forms about two-thirds of the whole rib, is a
flattened, regularly curved rod, completely ossified. Its dorsal end
is rounded, forming the =head= or =capitulum=, which articulates with
a concave surface furnished partly by the corresponding vertebra and
partly by the vertebra next in front. The last three or four however
articulate with one vertebra only. A short way behind the capitulum on
the dorsal side of the rib is a rounded outgrowth, the =tubercle= or
=tuberculum=, by means of which the rib articulates with the
transverse process. The portion of the rib between the head and the
tubercle is known as the =neck=. The =sternal portion= of the rib
(fig. 76) is a short bar of calcified or imperfectly ossified
cartilage, about one-third of the length of the corresponding bony
portion. The anterior sternal ribs are somewhat more cartilaginous
than the posterior ones. The vertebral portions increase in length
from the first which is very stout, and has the capitulum and
tuberculum very distinct, to about the eighth or ninth; afterwards
they gradually diminish in size. The first nine to eleven have the
capitula and tubercula separate, afterwards they gradually merge
together.
THE STERNUM.
This is an elongated cylindrical structure lying in the mid-ventral
wall of the thorax, and is divided into eight segments or
=sternebrae=. The anterior segment, the =presternum= (fig. 76, 1) or
=manubrium sterni= is expanded in front; the next six segments, which,
together form the =mesosternum= are elongated, somewhat contracted in
the middle and expanded at the ends. The last segment or
=xiphisternum= (fig. 76, 4) is long and narrow, and terminates in a
flattened expanded plate of cartilage. The first pair of sternal ribs
articulate with the sides of the presternum, and the remaining pairs
between the successive sternebrae. Between the last sternebra and the
xiphisternum two pairs articulate. Development shows that the sternum
is formed by the union in the middle line of two lateral portions;
this can be well seen in the presternum and xiphisternum of the puppy,
but no traces of this median division remain in the adult dog.
2. THE APPENDICULAR SKELETON.
The appendicular skeleton consists of the bones of the anterior and
posterior limbs, and of their respective supports, the pectoral and
pelvic girdles.
THE PECTORAL GIRDLE.
The =pectoral girdle= lies external to the ribs, and has no bony
attachment to the axial skeleton. In almost all Mammalia it is, as
compared with that in Sauropsids, very incomplete; and in the dog it
is even more reduced than in the majority of Mammalia. The dorsal
portion or =scapula= is well developed, but the ventral portion is
almost entirely absent.
The =scapula= is somewhat triangular in shape, the apex being
directed downwards and forwards, and being expanded to form the
shallow =glenoid cavity= with which the head of the humerus
articulates. The inner surface of the scapula is nearly flat, while
the outer is drawn out into a very prominent ridge, the =spine=,
which, arising gradually near the dorsal end, runs downwards, dividing
the surface into two nearly equal parts, the =prescapular= and
=postscapular fossae=, and ends in a short blunt process, the
=acromion=. The anterior border of the scapula is somewhat curved, and
is called the =coracoid border=; it is terminated ventrally by a
slight blunt swelling, the =coracoid process=, which ossifies from a
different centre from the rest of the scapula, and is probably the
sole representative of the =coracoid=. The dorsal or =suprascapular
border= of the scapula is rounded, while the posterior or =glenoid
border= is nearly straight. The clavicle[144] or collar bone, which in
a large proportion of mammals is well seen, in the dog is very
imperfectly developed; it is short and broad, and is suspended in the
muscles, not reaching either the scapula or sternum.
THE ANTERIOR LIMB.
The anterior limb of the dog is divisible into the usual three
portions, the =brachium= or =upper arm=, the =antibrachium= or
=fore-arm=, and the =manus= or =wrist= and =hand=.
The =brachium= or =upper arm= includes only a single bone, the
=humerus=.
The =humerus= is a stout elongated bone, articulating by its large
proximal =head= (fig. 77, 1) with the glenoid cavity of the scapula,
and at its distal end by the =trochlea= with the bones of the
fore-arm. The head passes on its inner side into an area roughened for
the attachment of muscles and called the =lesser tuberosity= (fig. 77,
2); while in front it is divided by the shallow =bicipital groove=
from a large roughened area, the =greater tuberosity= (fig. 77, 3),
which is continued as a slight roughened ridge, extending about
one-third of the way down the outer side of the shaft. This ridge,
which in many animals is much more strongly developed than it is in
the dog, is called the =deltoid ridge=. The =trochlea= (fig. 77, 5) at
the distal end of the bone is a pulley-like surface, elevated at the
sides and grooved in the middle. It articulates with the radius and
ulna of the fore-arm. On each side of it are slight roughened
projections, the =internal= and =external condyles= (fig. 77, 7). In
the cat and many other animals there's a foramen, the =ent-epicondylar
foramen= above the internal condyle, but in the dog this is not
developed. Passing up the shaft from the external condyle is a slight
ridge, the =supinator= or =ectocondylar ridge=; this is better
developed in many mammals. Immediately above the trochlea in front and
behind are the deep =supra-trochlear fossae=, which communicate with
one another through the =supra-trochlear foramen= (fig. 77, 8). The
posterior of these, the =olecranon fossa=, is much the deeper, and
receives the olecranon process of the ulna when the arm is extended.
The head and tuberosities of the humerus ossify from one centre, the
shaft from a second, and the trochlea and condyles from a third.
The =fore-arm= or =antibrachium= contains two bones, the =radius= and
=ulna=; they are immovably articulated with one another, but not
fused. The pre-axial bone, the =radius= (fig. 77, B), which lies more
or less in front of the ulna, is external to the ulna at its proximal
end, and at its distal end is internal to that bone. It articulates
with the external portion of the trochlea, while the ulna articulates
with the internal portion. It is a straight bone with its distal end
slightly larger than its proximal end. The proximal end articulates
with the trochlea, the distal end with the bones of the carpus.
[Illustration FIG. 77. BONES OF THE LEFT UPPER ARM AND FORE-ARM OF A
DOG (_Canis familiaris_) × 1/2.
A, humerus (seen from the posterior side); B, radius, C, ulna, both
seen from the anterior side.
1. head.
2. lesser tuberosity.
3. greater tuberosity.
4. shaft of the humerus.
5. trochlea.
6. internal condyle.
7. external condyle.
8. supra-trochlear foramen.
9. proximal end of the radius.
10. shaft of the radius.
11. olecranon.
12. surface for articulation with the trochlea.
13. surface for articulation with the radius.
14. distal end of the ulna.]
The postaxial bone, the =ulna= (fig. 77, C), has the proximal end much
enlarged, forming the =olecranon= (fig. 77, 11), and tapers gradually
to the distal end. Near its proximal end the ulna is marked by a deep
=sigmoid notch=, which bears on its inner side a concave surface
(fig. 77, 12) for articulation with the trochlea. The pointed proximal
end of the sigmoid notch is called the =coronoid process=. Somewhat
in front of and below the sigmoid notch is a smaller hollow (fig. 77,
13), with which the radius articulates.
In the young animal the ends of both radius and ulna are seen to
ossify from centres different from those forming the shafts. The
epiphyses forming both ends of the radius, and the distal end of the
ulna are large, while that at the proximal end of the ulna is small,
and forms only the end of the olecranon.
The =Manus= is divided into
_a._ The =carpus= or =wrist=, formed of a group of small bones.
_b._ The =hand=, which includes firstly some elongated bones, the
=metacarpals=, forming what corresponds to the palm of the hand, and
secondly the phalanges, which form the =fingers=.
The =Carpus= or =wrist=. The carpus of the dog consists of seven small
bones, arranged in a proximal row of three, and a distal row of four.
It differs much from the simpler type met with in the newt. The
largest bone of the proximal row is the =scapho-lunar= (fig. 80, 1),
formed by the fused =scaphoid= (radiale), =lunar= (intermedium), and
=centrale=; it has a large convex proximal surface for articulation
with the radius, and articulates distally with the trapezium,
trapezoid, and magnum, and internally with the cuneiform. The
=cuneiform= (ulnare) (fig. 80, 2) has a posterior rounded surface
articulating with the ulna; it articulates in front with the unciform,
and internally with the =pisiform= (fig. 80, 7), which is a
comparatively large sesamoid bone on the ulnar side of the carpus.
Frequently also there is a small sesamoid bone on the radial side of
the carpus. The =trapezium= (carpale 1), =trapezoid= (carpale 2), and
=magnum= (carpale 3) (fig. 80, 5) are all small bones, and support
respectively the first, second, and third metacarpals. The =unciform=
(carpalia 4 and 5) (fig. 80, 6) is larger, and supports the fourth and
fifth metacarpals.
The hand has five =digits=, each consisting of an elongated
=metacarpal=, followed by =phalanges=, the last of which, the =ungual
phalanx=, is pointed and curved, and bears the claw. Each of the
metacarpals is seen in the young animal to have its distal end formed
by a prominent epiphysis, and each of the phalanges, except those
bearing the claws, has a similar epiphysis at its proximal end.
The =pollex= (fig. 80, A, I ) is far shorter than the other digits,
and normally does not touch the ground in walking. It has only two
phalanges, while each of the other digits has three. A pair of small
sesamoid bones are developed on the ventral or flexor side of the
metacarpo-phalangeal articulations of all the digits except the
pollex. Frequently similar sesamoid bones occur also on the dorsal
side of the phalangeal articulations.
THE PELVIC GIRDLE.
The =pelvic girdle= consists of two halves, which lie nearly parallel
to the vertebral column.
Each half is firmly united to its fellow in a ventral symphysis
behind, and is in front expanded and united to the sacrum. Each half
or =innominate bone= is seen in the young animal to consist of four
distinct parts, the =ilium= or dorsal element, the =pubis= or anterior
ventral element, the =ischium= or posterior ventral element, and a
small fourth part, the =acetabular= or =cotyloid= bone, wedged in
between the three others. These parts, though all distinct in the
young animal, are in the adult so completely fused that their
respective boundaries cannot be distinguished. At about the middle of
the outer surface of the innominate bone is a very deep cavity, the
=acetabulum= (fig. 78, A, 1) with which the head of the femur
articulates; all the bones except the pubis take part in its
formation.
The =ilium= is a rather long bone, expanded in front and contracted
behind; it forms about half the acetabulum. On its inner or =sacral
surface= (fig. 78, 4) is a large roughened patch for articulation
with the sacrum; its outer or =gluteal surface= is concave. The
posterior part of the bone is flattened below, forming the narrow
=iliac surface= (fig. 78, A, 5).
[Illustration FIG. 78. RIGHT INNOMINATE BONE, A, OF A FULL-GROWN
TERRIER, B, OF A COLLIE PUPPY. × 1.
A is seen from the ventral side, B from the inner or sacral side.
1. acetabulum.
2. thyroid foramen.
3. supra-iliac border of ilium.
4. sacral surface.
5. iliac surface.
6. acetabular border.
7. pubic border.
8. ischial border.
9. ischium.
10. tuberosity of ischium.
11. ischial symphysis.
12. pubis.
13. pubic symphysis.
14. cotyloid or acetabular bone.]
The =ischium= (fig. 78, 9) is a wide flattened bone forming the
posterior part of the innominate bone. It meets the pubis ventrally,
but is separated from it for the greater part of its length by the
large =obturator= or =thyroid foramen= (fig. 78, 2). At its posterior
end externally it bears a rather prominent roughened =ischial
tuberosity= (fig. 78, A, 10). The ischium meets its fellow in a
ventral symphysis, and forms about one-third of the acetabulum.
[Illustration FIG. 79. FRONT VIEW OF THE LEFT LEG BONES OF A DOG
(_Canis familiaris_) × 1/2.
A femur, B tibia, C fibula, D patella.
1. head of femur.
2. neck.
3. great trochanter.
4. shaft.
5. external condyle.
6. internal condyle.
7. fabella.
8. cnemial crest.]
The =pubis= (fig. 78, 12) is smaller than either the ischium or ilium;
it does not take part in the formation of the acetabulum, and like the
ischium, meets its fellow in a ventral symphysis. The =acetabular
bone= (fig, 78, B, 14) is small and triangular, and is wedged in
between the other three. It forms about one-sixth of the acetabulum.
THE POSTERIOR LIMB.
The =posterior limb=, like the anterior, is divisible into three
parts; these are the =thigh=, the =crus= or =shin=, and the =pes=.
The =thigh= contains only a single bone, the =femur=.
The =femur= is a long straight bone with a nearly smooth shaft and
expanded ends. The proximal end bears on its inner side the large
rounded =head= (fig. 79, A, 1) which articulates with the acetabulum.
External to the head and divided from it by a deep pit is a large
rough outgrowth, the =great trochanter= (fig. 79, 3). The deep pit is
the =trochanteric= or =digital fossa=. On the inner side below the
head is a smaller roughened surface, the =lesser trochanter=. The
lower or distal end of the bone bears two prominent rounded surfaces,
the =condyles=, which articulate with the tibia. They are separated
from one another by the deep =intercondylar notch=, which is continued
above and in front as a shallow groove, lodging a large sesamoid bone,
the =patella= or =knee-cap=. At the back of the knee-joint are a pair
of smaller sesamoids, the =fabellae= (fig. 79, 7).
In the young animal there are three epiphyses to the shaft of the
femur, one forming the head, one the great trochanter, and one the
distal end.
The =crus= or =shin= contains two bones, the =tibia= and =fibula=. The
=tibia= is a fairly thick straight bone, expanded at both ends,
especially at the head or proximal end. The proximal end is triangular
in cross section, and bears two facets for articulation with the
condyles of the femur. The anterior surface of the proximal end of the
tibia is marked by the strong =cnemial crest= (fig. 79, 8), which runs
some way down the shaft. The distal end of the tibia articulates with
the astragalus by an irregular, somewhat square surface.
The shaft of the tibia ossifies from one centre, the distal end from a
second, and the proximal end from two more.
[Illustration FIG. 80. A, RIGHT MANUS, B, RIGHT PES OF A DOG (_Canis
familiaris_) × 1/2 (after VON ZITTEL).
1. bone representing the fused scaphoid, lunar and centrale.
2. cuneiform.
3. trapezium.
4. trapezoid.
5. magnum.
6. unciform.
7. pisiform.
8. first metacarpal.
9. fifth metacarpal.
10. astragalus.
11. calcaneum.
12. navicular.
13. middle cuneiform.
14. external cuneiform.
15. cuboid.
16. first metatarsal.
The digits are numbered with Roman numerals.]
The =fibula= (fig. 79, C) is a distinct but very slender bone,
somewhat expanded at both ends. It lies external to the tibia and
articulates by its proximal end with the head of the tibia, and by its
distal end with the calcaneum. Its shaft and proximal end ossify from
one centre, and its distal end from a second.
The =Pes=.
The structure of the =pes= corresponds closely with that of the manus.
It is divided into:--
_a._ The =tarsus= or =ankle= formed of a group of small bones.
_b._ The =foot=, which includes, firstly, some elongated bones, the
=metatarsals=, forming what corresponds to the sole of the foot, and
secondly the =phalanges=, which form the toes.
The =Tarsus=. The tarsus of the dog consists of seven bones arranged
in two rows, of two and four respectively, with a =centrale= between
them. The two bones of the proximal row are the =astragalus= and
=calcaneum=.
The =astragalus= (fig. 80, 10) corresponds to the fused =tibiale= and
=intermedium= of the typical tarsus. Its proximal end is much wider
than its distal end, and forms a large rounded =condyle= articulating
with the tibia, while its posterior end meets the navicular. It lies
to the dorsal side of the foot.
The =calcaneum= (fibulare) (fig. 80, 11), the thickest bone in the
pes, lies somewhat behind, and to the outer side of the astragalus. It
articulates with the astragalus and fibula, and is drawn out behind
into a long rounded process, which forms the heel, and is in the young
animal terminated by an epiphysis. Between the proximal and distal
rows of tarsals is the =navicular= (centrale) (fig. 80, 12), a
somewhat flattened and square bone articulating with the astragalus.
The distal row of tarsals consists of four bones. The =internal
cuneiform= (tarsale 1) is a smooth flattened bone lying to the inner
side of the foot; it articulates with the first metatarsal and with
the navicular. The =middle cuneiform= (tarsale 2) (fig. 80, 13) is a
still smaller bone, lying external to the internal cuneiform. It
articulates with the second metatarsal and with the navicular. The
=external cuneiform= (tarsale 3) (fig. 80, 14) is a larger, somewhat
square bone lying external to the middle cuneiform. It articulates
with the third metatarsal and with the navicular. The =cuboid=
(tarsalia 4 and 5) (fig. 80, 15) is a considerably larger bone lying
to the outer side of the foot. It articulates with the fourth and
fifth metatarsals and with the calcaneum.
The pes has sometimes five digits, sometimes four, the hallux being
absent. Even when present the =hallux= (fig. 80, =B, I=) is commonly
much reduced, and may be quite vestigial, and represented only by a
small nodular metatarsal.
Each of the other digits consists of a long metatarsal, which in the
young animal has a prominent epiphysis at its distal end, and of three
phalanges. The proximal and middle phalanges have epiphyses at their
proximal ends, while the distal phalanx is without epiphyses and is
claw-shaped.
FOOTNOTES:
[139] W. Ellenberger and H. Baum, _Anatomie des Hundes_, Berlin, 1891.
[140] T.H. Huxley, "Dental and cranial characters of the Canidae,"
_P.Z.S._ 1880.
[141] See p. 392.
[142] The dura mater is a membrane which lines the cranial cavity and
is formed of tough connective tissue.
[143] These are not strictly homologous with the basi-hyal and
cerato-hyal of the Dogfish.
[144] See note to p. 25.