There are usually eighteen pairs of ribs in the horse, but a nineteenth
rib on one side or both is not at all rare. Eight are =sternal= or
“true” ribs, the remainder =asternal= or “false.” Ribs from different
parts of the series vary much in length, curvature, and other
characters. We will therefore consider as a type a rib from the middle
of the series first, and afterward note the chief serial differences. A
=typical rib= has the following characters:
The =shaft= or =body= (Corpus costæ) is elongated, relatively very
narrow, and strongly curved; the curvature is most pronounced in the
dorsal third, and the ventral part is twisted and inclined inward, so
that when a rib is laid with its outer surface on the table, the ventral
end is raised. The =external surface= is convex in its length and also
transversely; its anterior part is, however, grooved longitudinally. A
distinct angle, _i. e._, a point at which the curve of the rib changes
rather suddenly, as in man, can scarcely be said to exist in the horse.
The term is often applied, however, to a corresponding rough elevation
which gives attachment to the ilio-costalis muscle; it is most distinct
on the fourth to the eighth inclusive. The =internal surface= is smooth,
concave in its length, and rounded from side to side; the costal groove,
situated posteriorly, is very distinct above and fades out about the
middle. The =anterior border= is concave, the =posterior= convex.
The =vertebral extremity= (Extremitas vertebralis) consists of the head,
neck, and tubercle. The =head= (Capitulum costæ) has an articular
surface (Facies articularis capituli costæ), composed of two convex
facets, anterior and posterior, separated by a groove for the attachment
of the conjugal ligament. It articulates with the cavity formed by
facets on the bodies of two adjacent thoracic vertebræ and the
intervertebral fibro-cartilage. The _neck_ (Collum costæ) is roughened
above and in front. The =tubercle= (Tuberculum costæ) is placed above
and behind the junction of neck and shaft; it has a small surface
(Facies articularis tuberculi costæ) for articulation with the
transverse process of the corresponding thoracic vertebra.
The =sternal extremity= (Extremitas sternalis) is somewhat expanded, and
is continued by the costal cartilage.
The =first rib= is easily distinguished. It is the shortest and least
curved. At the lower part of the anterior border there is a smooth
impression where the brachial vein curves around it; above this is
usually the scalene tubercle. The costal groove is absent. The head is
large and has two facets of unequal extent which meet at an acute angle
in front; the smaller one faces forward and articulates with the last
cervical vertebra; the larger one is directed inward and articulates
with the first thoracic vertebra. The neck is thick and very short. The
tubercle is larger than that of any other rib and has an extensive
articular surface which is convex in its length. The sternal end is
larger than that of any other rib; it is thick and very wide, and is
turned a little forward.
[Illustration:
FIG. 22.—LEFT SEVENTH RIB OF HORSE, ANTERO-EXTERNAL VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]
The =last rib= is the most slender and regularly curved. It is usually
but little longer than the second. The facet on the tubercle is
confluent with that of the head. (This feature, however, is usual on the
seventeenth also, and not rare on the sixteenth.)
The serial position of the other ribs may be determined approximately by
the following considerations: The =length= increases from the first to
the tenth and eleventh and then diminishes. The =width= increases
somewhat to the sixth and then diminishes. The =anterior border= is thin
and sharp from the second to the eighth, and behind this becomes thick
and rounded. The =external groove= is distinct on the fourth to the
eighth inclusive. The =curvature= increases in degree rapidly from the
first to the seventh, remains about the same to the sixteenth, and then
decreases very noticeably. In regard to dorso-ventral =direction=, the
first rib inclines a little forward, the second is about vertical, while
behind this they slope backward in increasing degree, so that a plane
tangent to the ventral ends of the last pair cuts the third lumbar
vertebra. The =head= and =tubercle= diminish in size from first to last.
Their relative positions change in that the tubercle of the first rib
lies almost directly outward from the head, while further back it
gradually comes to lie behind it. The =neck= is longest on the longest
ribs, and is absent on the last two or three. A costo-transverse foramen
is formed between the neck and the transverse process.
=Development.=—The ribs ossify in cartilage from =three centers=, one
each for the shaft, head, and tubercle; the third center is absent in
some of the posterior ribs.
=Variations.=—A nineteenth rib on one side or both is not rare. It is
usually imperfectly developed and quite variable. In some cases it is
a mere strip of cartilage connected by ligament with the first lumbar
transverse process; in other cases it is ossified, and may be fused
with the process; in others again it is connected with an additional
presacral vertebra which may be thoracic or lumbar or ambiguous in
character. It is commonly floating, but may be attached to the
eighteenth. Reduction in number is uncommon. Fusion of adjacent ribs
sometimes occurs.