The anatomical structure of the horse has been described in detail in
several works mentioned in the bibliography at the end of this section,
though these have generally been written from the point of view of the
veterinarian rather than of the comparative anatomist. The limits of the
present article will only admit of the most salient points being
indicated, particularly those in which the horse differs from other
Ungulata. Unless otherwise specified, it must be understood that all
that is stated here, although mostly derived from observation upon the
horse, applies equally well to the other existing members of the group.
_Skeleton._--The skull as a whole is greatly elongated, chiefly in
consequence of the immense size of the face as compared with the
hinder or true cranial portion. The basal line of the cranium from the
lower border of the foramen magnum to the incisor border of the palate
is nearly straight. The orbit, of nearly circular form, though small
in proportion to the size of the whole skull, is distinctly marked,
being completely surrounded by a strong ring of bone with prominent
edges. Behind it, and freely communicating with it beneath the osseous
bridge (the post-orbital process of the frontal) forming the boundary
between them, is the small temporal fossa occupying the whole of the
side of the cranium proper, and in front is the great flattened
expanse of the "cheek," formed chiefly by the maxilla, giving support
to the long row of cheek-teeth, and having a prominent ridge running
forward from below the orbit for the attachment of the masseter
muscle. The lachrymal occupies a considerable space on the flat
surface of the cheek in front of the orbit, and below it the jugal
does the same. The latter sends a horizontal or slightly ascending
process backwards below the orbit to join the under surface of the
zygomatic process of the squamosal, which is remarkably large, and
instead of ending as usual behind the orbit, runs forwards to join the
greatly developed post-orbital process of the frontal, and even forms
part of the posterior and inferior boundary of the orbit, an
arrangement not met with in other mammals. The closure of the orbit
behind distinguishes the skull of the horse from that of its allies
the rhinoceros and tapir, and also from all of the perissodactyles of
the Eocene period. In front of the brain cavity, the great tubular
nasal cavities are provided with well-developed turbinal bones, and
are roofed over by large nasals, broad behind, and ending in front in
a narrow decurved point. The opening of the anterior nostrils is
prolonged backwards on each side of the face between the nasals and
the elongated slender premaxillae. The latter expand in front, and are
curved downwards to form the semicircular alveolar border which
supports the large incisor teeth. The palate is narrow in the interval
between the incisor and molar teeth, in which are situated the large
anterior palatine foramina. Between the molar teeth it is broader, and
it ends posteriorly in a rounded excavated border opposite the hinder
border of the penultimate molar tooth. It is mainly formed by the
maxillae, as the palatines are very narrow. The pterygoids are
delicate slender slips of bone attached to the hinder border of the
palatines, and supported externally by, and generally welded with, the
rough pterygoid plates of the alisphenoid, with no pterygoid fossa
between. They slope obliquely forwards, and end in curved, compressed,
hamular processes. There is a distinct alisphenoid canal for the
passage of the internal maxillary artery. The base of the cranium is
long and narrow; the alisphenoid is very obliquely perforated by the
foramen rotundum, but the foramen ovale is confluent with the large
foramen lacerum medium behind. The glenoid surface for the
articulation of the mandible is greatly extended transversely, concave
from side to side, convex from before backwards in front, and hollow
behind, and is bounded posteriorly at its inner part by a prominent
post-glenoid process. The squamosal enters considerably into the
formation of the temporal fossa, and, besides sending the zygomatic
process forwards, it sends down behind the meatus auditorius a
post-tympanic process which aids to hold in place the otherwise loose
tympano-periotic bone. Behind this the exoccipital gives off a long
paroccipital process. The periotic and tympanic are welded together,
but not with the squamosal. The former has a wide but shallow
floccular fossa on its inner side, and sends backwards a considerable
"pars mastoidea," which appears on the outer surface of the skull
between the post-tympanic process of the squamosal and the
exoccipital. The tympanic forms a tubular meatus auditorius externus
directed outwards and slightly backwards. It is not dilated into a
distinct bulla, but ends in front in a pointed rod-like process. It
completely embraces the truncated cylindrical tympanohyal, which is of
great size, corresponding with the large development of the whole
anterior arch of the hyoid. This consists mainly of a long and
compressed stylohyal, expanded at the upper end, where it sends off a
triangular posterior process. The basi-hyal is remarkable for the
long, median, pointed, compressed "glossohyal" process, which it sends
forward from its anterior border into the base of the tongue. A
similar but less developed process is found in the rhinoceros and
tapir. The lower jaw is large, especially the region of the angle,
which is expanded and flattened, giving great surface for the
attachment of the masseter muscle. The condyle is greatly elevated
above the alveolar border; its articular surface is very wide
transversely, and narrow and convex from before backwards. The
coronoid process is slender, straight, and inclined backwards. The
horizontal ramus, long, straight, and compressed, gradually narrows
towards the symphysis, where it expands laterally to form with the
ankylosed opposite ramus the wide, semicircular, shallow alveolar
border for the incisor teeth.
[Illustration: FIG. 1.--Side view of Skull of Horse, with the bone
removed so as to expose the whole of the teeth.
PMx, Premaxilla.
Mx, Maxilla.
Na, Nasal bone.
Ma, Jugal or malar bone.
L, Lacrymal bone.
Fr, Frontal bone.
Sq, Squamosal bone.
Pa, Parietal bone.
oc, Occipital condyle.
pp, Paroccipital process.
i^1, i^2, and i^3, The three incisor teeth.
c, The canine tooth.
pm^1, The situation of the rudimentary first premolar, which has been
lost in the lower, but is present in the upper jaw.
pm^2, pm^3, and pm^4, The three fully developed premolar teeth.
m^1, m^2, and m^3, The three true molar teeth.]
The vertebral column consists of seven cervical, eighteen dorsal, six
lumbar, five sacral, and fifteen to eighteen caudal vertebrae There
may be nineteen rib-bearing vertebrae, in which case five only will be
reckoned as belonging to the lumbar series. The odontoid process of
the axis is wide, flat, and hollowed above, as in the ruminants. The
bodies of the cervical vertebrae are elongated, strongly keeled, and
markedly opisthocoelous, or concave behind and convex in front. The
neural laminae are broad, the spines almost obsolete, except in the
seventh, and the transverse processes not largely developed. In the
trunk vertebrae the opisthocoelous character of the centrum gradually
diminishes. The spinous processes of the anterior thoracic region are
high and compressed. To these is attached the powerful elastic
ligament (_ligamentum nuchae_, or "paxwax") which, passing forwards in
the middle line of the neck above the neural arches of the cervical
vertebrae--to which it is also connected--is attached to the occiput
and supports the weight of the head. The transverse processes of the
lumbar vertebrae are long, flattened, and project horizontally
outwards or slightly forward from the arch. The metapophyses are
moderately developed, and there are no anapophyses. The caudal
vertebrae, except those quite at the base, are slender and
cylindrical, without processes and without chevron bones beneath. The
ribs are eighteen or nineteen in number on each side, flattened, and
united to the sternum by short, stout, tolerably well ossified sternal
ribs. The sternum consists of six pieces; the anterior or presternum
is compressed and projects forwards like the prow of a boat. The
segments which follow gradually widen, and the hinder part of the
sternum is broad and flat.
As in all other ungulates, there are no clavicles. The scapula is long
and slender, the supra-scapular border being rounded, and slowly and
imperfectly ossified. The spine is very slightly developed; rather
above the middle its edge is thickened and somewhat turned backwards,
but it gradually subsides at the lower extremity without forming any
acromial process. The coracoid is a prominent rounded nodule. The
humerus is stout and rather short. The ulna is rudimentary, being
represented by little more than the olecranon. The shaft gradually
tapers below and is firmly welded to the radius. The latter bone is of
nearly equal width throughout. The three bones of the first row of the
carpus (scaphoid, lunar and cuneiform) are subequal in size. The
second row consists of a broad and flat magnum, supporting the great
third metacarpal, having to its radial side the trapezoid, and to its
ulnar side the unciform, which are both small, and articulate
inferiorally with the rudimentary second and fourth metacarpals. The
pisiform is large and prominent, flattened and curved; it articulates
partly with the cuneiform and partly with the lower end of the radius.
The large metacarpal is called in veterinary anatomy "cannon bone";
the small lateral metacarpals, which gradually taper towards their
lower extremities, and lie in close contact with the large one, are
called "splint bones." The single digit consists of a moderate-sized
proximal (_os suffraginis_, or large pastern), a short middle (_os
coronae_, or small pastern), and a wide, semi-lunar, ungual phalanx
(_os pedis_, or coffin bone). There is a pair of large nodular
sesamoids behind the metacarpo-phalangeal articulation, and a single
large transversely-extended sesamoid behind the joint between the
second and third phalanx, called the "navicular bone."
The carpal joint, corresponding to the wrist of man, is commonly
called the "knee" of the horse, the joint between the metacarpal and
the first phalanx the "fetlock," that between the first and second
phalanges the "pastern," and that between the second and third
phalanges the "coffin joint."
In the hinder limb the femur is marked, as in other perissodactyles,
by the presence of a "third trochanter," a flattened process, curving
forwards and arising from the outer side of the bone, about one-third
of the distance from the upper end. The fibula is reduced to a mere
rod-like rudiment of the upper end. The lower part is absent or
completely fused with the tibia. The calcaneum has a long and
compressed calcaneal process. The astragalus has a large flat
articular surface in front for the navicular, and a small one for the
cuboid. The navicular and the external cuneiform bones are broad and
flat. The cuboid is small, and the internal and middle cuneiform bones
are small and united together. The metapodals and phalanges resemble
very closely those of the fore limb, but the principal metatarsal is
more laterally compressed at its upper end than is the corresponding
metacarpal. The joint between the femur and tibia, corresponding to
the knee of man, is called the "stifle-joint"; that between the tibia
and tarsus, corresponding to the ankle of man, the "hock." The bones
and joints of the foot have the same names as in the fore limb. The
horse is eminently "digitigrade," standing on the extremity of the
single digit of each foot, which is kept habitually in a position
approaching to vertical.
The muscles of the limbs are modified from those of the ordinary
mammalian type in accordance with the reduced condition of the bones
and the simple requirements of flexion and extension of the joints, no
such actions as pronation and supination, or opposition of digits,
being possible or needed. The muscles therefore which perform these
functions in other quadrupeds are absent or rudimentary.
Below the carpal and tarsal joints, the fore and hind limbs correspond
almost exactly in structure as well as function. On the anterior or
extensor surface of the limb a powerful tendon (7 in fig. 2), that of
the anterior extensor of the phalanges (corresponding to the _extensor
communis digitorum_ of the arm and _extensor longus digitorum_ of the
foot of man) passes down over the metacarpal bone and phalanges, to be
inserted mainly into the upper edge of the anterior surface of the
last phalanx or pedal bone. There is also a much smaller second
extensor on the outer side of this in each limb, the lateral extensor
of the phalanges. In the fore-leg the tendon of this muscle (which
corresponds with the _extensor minimi digiti_ of man) receives a slip
from that of the principal extensor, and is inserted into the first
phalanx. In the hind-leg (where it is the homologue apparently of the
_peroneus brevis_ of man) the tendon becomes blended with that of the
large extensor.
[Illustration: FIG. 2.--Section of Foot of Horse.
1, Metacarpal bone.
2, First phalanx (_os suffraginis_).
3, Second phalanx (_os coronae_).
4, Third or ungual phalanx (_os pedis_, or coffin bone).
5, One of the upper sesamoid bones.
6, Lower sesamoid or navicular bone.
7, Tendon of anterior extensor of the phalanges.
8, Tendon of superficial flexor (_fl. perforatus_).
9, Tendon of deep flexor (_fl. perforans_).
10, Suspensory ligament of fetlock.
11, Inferior or short sesamoid ligament.
12, Derma or skin of the foot, covered with hair, and continued into
13, The coronary cushion,
14, The podophyllous or laminar membrane, and
15, The keratogenous membrane of the sole.
16, Plantar cushion.
17, Hoof.
18, Fatty cushion of fetlock.]
A strong ligamentous band behind the metapodium, arising from near the
upper extremity of its posterior surface, divides into two at its
lower end, and each division, being first connected with one of the
paired upper sesamoid bones, passes by the side of the first phalanx
to join the extensor tendon of the phalanges. This is called in
veterinary anatomy the "suspensory ligament of the sesamoids," or of
the "fetlock" (10 in fig. 2); but its attachments and relations, as
well as the occasional presence of muscular fibres in its substance,
show that it is the homologue of the interosseous muscles of other
mammals, modified in structure and function, to suit the requirements
of the horse's foot. Behind or superficial to this are placed the two
strong tendons of the flexor muscles, the most superficial, or _flexor
perforatus_ (8) dividing to allow the other to pass through, and then
inserted into the middle phalanx. The _flexor perforans_ (9) is as
usual inserted into the terminal phalange. In the fore-leg these
muscles correspond with those similarly named in man. In the hind-leg,
the perforated tendon is a continuation of that of the plantaris,
passing pulley-wise over the tuberosity of the calcaneum. The
perforating tendon is derived from the muscle corresponding with the
long flexor of man, and the smaller tendon of the oblique flexor
(_tibialis porticus_ of man) is united with it.
The hoof of the horse corresponds to the nail or claw of other
mammals, but is so constructed as to form a complete and solid case to
the expanded termination of the toe, giving a firm basis of support
formed of a non-sensitive substance, which is continually renewed by
the addition of material from within, as its surface wears away by
friction. The terminal phalange of the toe is greatly enlarged and
modified in form to support this hoof, and the size of the internal
framework of the foot is increased by a pair of lateral
fibro-cartilaginous masses attached on each side to the hinder edges
of the bone, and by a fibro-cellular and fatty plantar cushion in the
median part. These structures are all enclosed in the middle
subcorneous integument, a continuation of the ordinary skin of the
limb, but extremely vascular, and having its superficial extent
greatly increased by being developed into papillae or laminae. From
this the horny material which constitutes the hoof is exuded. A
thickened ring encircling the upper part, called coronary cushion (13)
and the sole (15), are covered with numerous thickly-set papillae or
villi, and take the greatest share in the formation of the hoof; the
intermediate part constituting the front and side of the foot (14),
corresponding with the wall of the hoof, is covered with parallel,
fine longitudinal laminae, which fit into corresponding depressions in
the inner side of the horny hoof.
The horny hoof is divided into a wall or crust consisting of the front
and sides, the flattened or concave sole, and the frog, a triangular
median prominence, notched posteriorly, with the apex turned forwards,
situated in the hinder part of the sole. It is formed of pavement
epithelial cells, mainly grouped in a concentric manner around the
vascular papillae of the subcorneous integument, so that a section
near the base of the hoof, cut transversely to the long axis of these
papillae, shows a number of small circular or oval orifices, with
cells arranged concentrically round them. The nearer the surface of
the hoof, or farther removed from the seat of growth, the more
indistinct the structure becomes.
Small round or oval plates of horny epithelium called "chestnuts,"
callosities growing like the hoof from enlarged papillae of the skin,
are found on the inner face of the fore-arm, above the carpal joint in
all species of Equidae, and in the horse (_E. caballus_) similar
structures occur near the upper extremity of the inner face of the
metatarsus. They are evidently rudimentary structures which it is
suggested may represent glands (Lydekker, _Proc. Zool. Soc. London_,
1903, vol. i.).
[Illustration: FIG. 3.--Longitudinal and Transverse Section of Upper
Incisor of Horse.
p, Pulp cavity.
d, Dentine or ivory.
e, Enamel.
c, Outer layer of cementum or crusta petrosa.
c', Inner layer of cementum, lining a, the pit
or cavity of the crown of the tooth.]
_Dentition._--The dentition of the horse, when all the teeth are in
place, is expressed by the formula _i._ 3/3, _c._ 1/1, _p._ 4/4 _m._
3/3 = 44. The incisors of each jaw are placed in close contact,
forming a semicircle. The crowns are broad, somewhat awl-shaped, and
of nearly equal size. They have all the great peculiarity, not found
in the teeth of any other mammal, and only in the Equidae of
comparatively recent geological periods (see also PALAEONTOLOGY), of
an involution of the external surface of the tooth (see fig. 3), by
which what should properly be the apex is carried deeply into the
interior of the crown, forming a pit, the bottom of which becomes
partially filled with cement. As the tooth wears, the surface, besides
the external enamel layer as in an ordinary simple tooth, shows in
addition a second inner ring of the same hard substance surrounding
the pit, which adds greatly to the efficiency of the tooth as an organ
for biting tough, fibrous substances. This pit, generally filled in
the living animal with particles of food, is conspicuous from its dark
colour, and constitutes the "mark" by which the age of the horse is
judged, as in consequence of its only extending to a certain depth in
the crown it becomes obliterated as the latter wears away, and then
the tooth assumes the character of that of an ordinary incisor,
consisting only of a core of dentine, surrounded by the external
enamel layer. It is not quite so deep in the lower as in the upper
teeth. The canines are either rudimentary or absent in the female. In
the male they are compressed, pointed, and smaller than the incisors,
from which they are separated by a slight interval. The teeth of the
cheek series are all in contact with each other, but separated from
the canines by a considerable toothless space. The anterior premolars
are quite rudimentary, sometimes not developed at all, and generally
fall by the time the animal attains maturity, so that there are but
six functional cheek teeth,--three that have predecessors in the
milk-dentition, and hence are considered as premolars, and three
molars, but otherwise, except the first and last of the series, not
distinguishable in form or structure. These teeth in both upper and
lower jaws are extremely long-crowned or hypsodont, successive
portions being pushed out as the surface wears away, a process which
continues until the animal becomes advanced in age. The enamelled
surface is infolded in a complex manner (a modification of that found
in other perissodactyles), the folds extending quite to the base of
the crown, and the interstices being filled and the surface covered
with a considerable mass of cement, which binds together and
strengthens the whole tooth. As the teeth wear, the folded enamel,
being harder than the other constituents, the dentine and cement,
forms projecting ridges on the surface arranged in a definite
pattern, which give it great efficiency as a grinding instrument (see
fig. 2, in article EQUIDAE). The free surfaces of the upper teeth are
quadrate, except the first and last, which are nearly triangular. The
lower teeth are much narrower than the upper.
The milk-dentition consists of i. 3/3, c. 0/0, m. 3/3 = 24,--the
canines and first or rudimentary premolars having apparently no
predecessors. In form and structure the milk-teeth much resemble the
permanent ones, having the same characteristic enamel-foldings. Their
eruption commences a few days after birth, and is complete before the
end of the first year, the upper teeth usually appearing somewhat
earlier than the lower. The first teeth which appear are the first and
second milk-molars (about five days), then the central incisor (from
seven to ten days); this is followed by the second incisor (at one
month), then the third molar, and finally the third incisor. Of the
permanent teeth the first molar appears a little after the end of the
first year, followed by the second molar before the end of the second
year. At about two and a half years the first premolar replaces its
predecessor. Between two and a half and three years the first incisor
appears. At three years the second and third premolars, and the third
molar have appeared, at from three and a half to four years the second
incisor, at four to four and a half years the canine, and, finally, at
five years, the third incisor, completing the permanent dentition. Up
to this period the age of the horse is clearly shown by the condition
of dentition, and for some time longer indications can be obtained
from the wear of the incisors, though this depends to a certain extent
upon the hardness of the food or other circumstances. As a general
rule, the depression caused by the infolding of the surface of the
incisor (the "mark") is obliterated in the first or central incisor at
six years, in the second at seven years, and in the third at eight
years. In the upper teeth, as the depressions are deeper, this
obliteration does not take place until about two years later. After
this period no certain indications can be obtained of the age of the
horse from the teeth.
_Digestive Organs._--The lips are flexible and prehensile; and the
membrane that lines them and the cheeks smooth. The palate is long and
narrow; its mucous surface has seventeen pairs of not very sharply
defined oblique ridges, extending as far back as the last molar tooth,
beyond which the _velum palati_ extends for about 3 in., having a soft
corrugated surface, and ending posteriorly in an arched border without
a uvula. This embraces the base of the epiglottis, and, except while
swallowing food, shuts off all communication between the cavity of the
mouth and the pharynx, respiration being, under ordinary
circumstances, exclusively through the nostrils. Between the mucous
membrane and the bone of the hard palate is a dense vascular and
nervous plexus. The membrane lining the jaws is soft and corrugated.
An elongated raised glandular mass, 3 in. long and 1 in. from above
downwards, extending backwards from the root of the tongue along the
side of the jaws, with openings on the surface leading into crypts
with glandular walls, represents the tonsil. The tongue, corresponding
to the form of the mouth, is long and narrow. It consists of a
compressed intermolar portion with a flat upper surface, broad behind
and becoming narrower in front, and of a depressed anterior part
rather shorter than the former, which is narrow behind and widens
towards the evenly rounded apex. The dorsal surface generally is soft
and smooth. There are two large circumvallate papillae near the base,
rather irregular in form, about a quarter of an inch in diameter and
half an inch apart. The conical papillae are small and close set,
though longer and more filamentous on the intermolar portion. There
are no fungiform papillae on the dorsum, but a few inconspicuous ones
scattered along the sides of the organ.
Of the salivary glands the parotid is by far the largest, elongated in
the vertical direction, and narrower in the middle than at either end.
Its upper extremity embraces the lower surface of the cartilaginous
ear-conch; its lower end reaches the level of the inferior margin of
the mandible, along the posterior margin of which it is placed. Its
duct leaves the inferior anterior angle, at first descends a little,
and runs forward under cover of the rounded inferior border of the
lower jaw, then curves up along the anterior margin of the masseter
muscle, becoming superficial, pierces the buccinator, and enters the
mouth by a simple aperture opposite the middle of the crown of the
third premolar tooth. It is not quite so thick as a goose-quill when
distended, and nearly a foot in length.
The submaxillary gland is of very similar texture to the last, but
much smaller; it is placed deeper, and lies with its main axis
horizontal. It is elongated and slender, and flattened from within
outwards. Its posterior end rests against the anterior surface of the
transverse process of the atlas, from which it extends forwards and
downwards, slightly curved, to beneath the ramus of the jaw. The duct
which runs along its upper and internal border passes forwards in the
usual course, lying in the inner side of the sublingual gland, to open
on the outer surface of a distinct papilla, situated on the floor of
the mouth, half an inch from the middle line, and midway between the
lower incisor teeth and the attachment of the fraenum linguae. The
sublingual is represented by a mass of glands lying just beneath the
mucous membrane of the floor of the mouth on the side of the tongue,
causing a distinct ridge, extending from the fraenum backwards, the
numerous ducts opening separately along the summit of the ridge. The
buccal glands are arranged in two rows parallel with the molar teeth.
The upper ones are the largest, and are continuous anteriorly with the
labial glands, the ducts of which open on the mucous membrane of the
upper lip.
The stomach of the horse is simple in its external form, with a
largely developed right _cul de sac_, and is a good deal curved on
itself, so that the cardiac and pyloric orifices are brought near
together. The _antrum pyloricum_ is small and not very distinctly
marked. The interior is divided by the character of the lining
membrane into two distinct portions, right and left. Over the latter
the dense white smooth epithelial lining of the oesophagus is
continued, terminating abruptly by a raised crenulated border. Over
the right part the mucous membrane has a greyish-red colour and a
velvety appearance, and contains numerous peptic glands, which are
wanting in the cardiac portion. The oesophageal orifice is small, and
guarded by a strong crescentic or horseshoe-like band of muscular
fibres, supposed to be the cause of the difficulty of vomiting in the
horse. The small intestine is of great length (80 to 90 ft.), its
mucous membrane being covered with numerous fine villi. The caecum is
of conical form, about 2 ft. long and nearly a foot in diameter; its
walls are sacculated, especially near the base, having four
longitudinal muscular bands; and its capacity is about twice that of
the stomach. It lies with its base near the lower part of the abdomen,
and its apex directed towards the thorax. The colon is about one-third
the length of the small intestine, and very capacious in the greater
part of its course. As usual it may be divided into an ascending,
transverse, and descending portion; but the middle or transverse
portion is folded into a great loop, which descends as low as the
pubis; so that the colon forms altogether four folds, generally
parallel to the long axis of the body. The descending colon is much
narrower than the rest, and not sacculated, and, being considerably
longer than the distance it has to traverse, is thrown into numerous
folds.
The liver is tolerably symmetrical in general arrangement, being
divided nearly equally into segments by a well-marked umbilical
fissure. Each segment is again divided by lateral fissures, which do
not extend quite to the posterior border of the organ; of the central
lobes thus cut off, the right is rather the larger, and has two
fissures in its free border dividing it into lobules. The extent of
these varies, however, in different individuals. The two lateral lobes
are subtriangular in form. The Spigelian lobe is represented by a flat
surface between the postal fissure and the posterior border, not
distinctly marked off from the left lateral by a fissure of the ductus
venosus, as this vessel is buried deep in the hepatic substance, but
the caudate lobe is distinct and tongue-shaped, its free apex reaching
nearly to the border of the right lateral lobe. There is no
gall-bladder, and the biliary duct enters the duodenum about 6 in.
from the pylorus. The pancreas has two lobes or branches, a long one
passing to the left and reaching the spleen, and a shorter right lobe.
The principal duct enters the duedenum with the bile-duct, and there
is often a second small duct opening separately.
_Circulatory and Respiratory Organs._--The heart has the form of a
rather elongated and pointed cone. There is one anterior vena cava,
formed by the union of the two jugular and two axillary veins. The
aorta gives off a large branch (the anterior aorta) very near its
origin, from which arise--first, the left axillary, and afterwards the
right axillary and the two carotid arteries.
Under ordinary circumstances the horse breathes entirely by the nasal
passages, the communication between the larynx and the mouth being
closed by the velum palati. The nostrils are placed laterally, near
the termination of the muzzle, and are large and dilatable, being
bordered by cartilages upon which several muscles act. Immediately
within the opening of the nostril, the respiratory canal sends off on
its upper and outer side a blind pouch ("false nostril") of conical
form, and curved, 2 to 3 in. in depth, lying in the notch formed
between the nasal and premaxillary bones. It is lined by mucous
membrane continuous with that of the nasal passage; its use is not
apparent. It is longer in the ass than in the horse. Here may be
mentioned the guttural pouches, large air-sacs from the Eustachian
tubes, and lying behind the upper part of the pharynx, the function of
which is also not understood. The larynx has the lateral sacculi well
developed, though entirely concealed within the alae of the thyroid
cartilage. The trachea divides into two bronchi.
_Nervous System._--The brain differs little, except in details of
arrangement of convolutions, from that of other ungulates. The
hemispheres are rather elongated and subcylindrical, the olfactory
lobes are large and project freely in front of the hemispheres, and
the greater part of the cerebellum is uncovered. The eye is provided
with a nictitating membrane or third eyelid, at the base of which open
the ducts of the Harderian gland.
_Reproductive System._--The testes are situated in a distinct sessile
or slightly pedunculated scrotum, into which they descend from the
sixth to the tenth month after birth. The accessory generative glands
are the two vesiculae seminales, with the median third vesicle, or
_uterus masculinus_, lying between them, the single bilobed prostate,
and a pair of globular Cowper's glands. The penis is very large,
cylindrical, with a truncated, expanded, flattened termination. When
in a state of repose it is retracted, by a muscle arising from the
sacrum, within the prepuce, a cutaneous fold attached below the
symphysis pubis.
The uterus is bicornuate. The vagina is often partially divided by a
membraneous septum or hymen. The teats are two, inguinally placed. The
surface of the chorion is covered evenly with minute villi,
constituting a diffuse non-deciduate placenta. The period of gestation
is eleven months.
AUTHORITIES.--R. I. Pocock, "The Species and Subspecies of Zebras,"
_Ann. Mag. Nat. Hist._ ser. 6, vol. xx., 1897, and "A New Arrangement
of the Existing Species of Equidae," Op. cit. ser. 7, vol. x., 1902;
R. Lydekker, "Notes on the specimens of Wild Asses in English
Collections," _Novitates Zoologicae_, vol. xi., 1904; B. Salensky, "On
Equus przewalskii," _Mem. Acad. St Petersburg_, 1902; M. S. Arloing,
"Organisation du pied chez le cheval," _Ann. Sci. Nat._, 1867, viii.
55-81; H. Burmeister, _Los caballos fosiles de la Pampa Argentina_
(Buenos Aires, 1875); Chauveau and Arloing, _Traite d'anatomie
comparee des animaux domestiques_ (Paris, 1871), and English edition
by G. Fleming (1873); A. Ecker, "Das Europaische Wildpferd und dessen
Beziehungen zum domesticirten Pferd," _Globus_, Bd. xxxiv. (Brunswick,
1878); Major Forsyth, "Beitrage zur Geschichte der fossilen Pferde
besonders Italiens," _Abh. Schw. Pal. Ges._ iv. 1-16, pt. iv.; George,
"Etudes zool. sur les Hemiones et quelques autres especes chevalines,"
_Ann. Sci. Nat._, 1869, xii. 5; E. F. Gurlt, _Anatomische Abbildungen
der Haussaugethiere_ (1824), and _Hand. der vergleich. Anat. der
Haussaugethiere_ (2 vols., 1822); Huet, "Croisement des diverses
especes du genre cheval," _Nouv. Archives du Museum_, 2nd ser., tom.
ii. p. 46, 1879; Leisering, _Atlas der Anatomie des Pferdes_ (Leipzig,
1861); O. C. Marsh, "Notice of New Equine Mammals from the Tertiary
Formation," _Am. Journ. of Science and Arts_, vol. vii., March 1874;
_Id._, "Fossil Horses in America," _Amer. Naturalist_, vol. viii., May
1874; _Id._, "Polydactyle Horses," _Am. Journ. Sci. and Arts_, vol.
xvii., June 1879; Franz Muller, _Lehrbuch der Anatomie des Pferdes_
(Vienna, 1853); R. Owen, "Equine Remains in Cavern of Bruniquel,"
_Phil. Trans._ vol. clix., 1870, p. 535; W. Percivall, _The Anatomy of
the Horse_ (1832); G. Stubbs, _Anatomy of the Horse_ (1766); W. H.
Flower, _The Horse_ (London, 1891); Ridgeway, _Origin of the
Thoroughbred Horse_ (1905). (W. H. F; R. L.*)