These (Vertebræ thoracales) are usually eighteen in number in the horse,
but there are sometimes nineteen, rarely seventeen. As regional
characters we note the surfaces for articulation with the ribs and the
length and form of the spinous processes. Those in the middle of the
series are the most typical and present the following features:
1. The =bodies= are short and constricted in the middle. The ends are
expanded and have articular surfaces which are not strongly curved. On
the upper part of each side are anterior and posterior =costal facets=
(Fovea costalis cranialis, caudalis), which, with those of adjacent
vertebræ, form sockets for the heads of the ribs.
2. The =arches= are small. Their posterior notches are relatively large
and are often converted into foramina.
[Illustration:
FIG. 13.—THIRD, FOURTH, AND FIFTH THORACIC VERTEBRÆ OF HORSE, LATERAL
VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]
3. The =articular processes= are small. The anterior pair are in fact
represented only by two oval facets on the anterior part of the laminæ
which face almost directly upward. The posterior pair spring from the
base of the spinous process; their facets face almost directly downward.
4. The =transverse processes= are short, thick, and tuberous at the free
end. Each has a =facet= (Fovea transversaria) for articulation with the
tuberosity of the rib which has the same serial number.
5. The =spinous process= is large, narrow, and slopes upward and
backward. The anterior border is thin, the posterior wider and furrowed.
The summit is expanded and rough.
The =first thoracic vertebra= is easily recognized by the following
specific characters: The body is wide and flattened dorso-ventrally. In
front it has a head like the cervical vertebræ, and behind a cavity
somewhat deeper than any other thoracic vertebra. Two =costal facets=
are found on either side, and a well-marked spine ventrally. The =arch=
is large and strong, and has large notches. The =articular processes=
are much larger than those of other thoracic vertebræ, and resemble a
good deal those of the seventh cervical in form. The =transverse
processes= are short and thick, and each has on its ventral aspect a
large concave facet for articulation with the tubercle of the first rib.
The spinous process is curved backward and tapers to a point. Its length
is usually about three or four inches (ca. 8 to 10 cm.). It may be
mistaken at first glance for the last cervical, but is promptly
identified by the three costal facets on each side and the length of the
spine.
The =last thoracic vertebra= is distinguished by the absence of the
posterior pair of costal facets, and the confluence of the anterior pair
with those on the transverse processes.
[Illustration:
FIG. 14.—THIRD THORACIC VERTEBRÆ AND UPPER PART OF RIB OF HORSE,
ANTERIOR VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]
[Illustration:
FIG. 15.—LOWER PART OF NINTH THORACIC VERTEBRA OF HORSE, POSTERIOR
VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]
The serial position of others may be determined at least approximately
by the following data: (1) The bodies gradually diminish in length and
width to the middle of the region and then increase slightly. Their
costal facets become smaller and less concave from first to last. The
ventral crest is distinct on three or four vertebræ at either end of the
region. (2) The transverse processes diminish in size and are placed
lower down as they are traced backward. Their costal facets become
smaller and lower in position; on the last (and sometimes on its
predecessor also) it fuses with the costal facet of the body. The upper
non-articular part of the process gradually becomes more sharply
defined, and in the last four or five separates to form the =mammillary
process=. (3) The spinous processes increase in length to the third and
fourth, and then gradually diminish to the fifteenth, beyond which they
have about the same length. The backward inclination is most pronounced
in the second, the sixteenth is vertical, and the last two are directed
a little forward. The longest spines (_i. e._, those of the withers) are
the thickest and have expanded summits which remain more or less
cartilaginous; the others are more plate-like, and are surmounted by a
thick lip. The second spine is more than twice as large as the first.
The summits of the fourth and fifth usually form the highest point of
the withers.
=Development.=—There are =six= or =seven centers=, three for the body,
two for the arch, and one for the spinous process; some of the latter
have an additional center for the summit.