The =thoracic nerves= (Nn. thoracales) number eighteen on either side in
the horse. They are designated numerically according to the vertebræ
behind which they emerge. Most of them are arranged in a very similar
manner and therefore do not require separate description. Each divides
into a dorsal and a ventral branch, the latter being the larger.
The =dorsal branches= (Rami dorsales) emerge behind the levatores
costarum and divide into internal and external branches. The internal
branches ascend on the multifidus and supply the dorsal spinal muscles.
The external branches run outward under the longissimus and emerge
between that muscle and the transversalis costarum; after giving twigs
to these muscles they pass through the latissimus dorsi and the
lumbo-dorsal fascia and ramify as dorsal cutaneous nerves under the skin
of the back (Fig. 523). In the region of the withers they give branches
to the serratus anticus and rhomboideus, and their cutaneous terminals
pass through these muscles and the dorso-scapular ligament to supply the
skin over the ligamentum nuchæ and the scapular cartilage.
The =ventral branches= or =intercostal nerves= (Nn. intercostales) are
much larger than the preceding, and are connected with the sympathetic
by rami communicantes. The first goes almost entirely to the brachial
plexus, but sends a fine branch downward in the first intercostal space
which is expended in the muscle there without reaching the lower end of
the space. The second ventral branch furnishes a considerable root to
the brachial plexus, but its intercostal continuation is typical. The
intercostal nerves (Fig. 184) descend in the intercostal spaces with the
vessels of like name, at first between the intercostal muscles, and
lower down chiefly between the pleura and the internal intercostal
muscle. In the anterior spaces the artery lies along the posterior
border of the rib, with the nerve in front of it; further back the nerve
lies behind the border of the rib, with the artery in front of it. They
supply the intercostal muscles, give off lateral perforating branches,
and terminate in the following manner: The second to the sixth inclusive
emerge through the spaces between the costal cartilages and concur in
supplying the pectoral muscles. The second to the eighth give branches
to the transversus thoracis. The succeeding ones give branches to the
diaphragm, pass between the transverse and internal oblique muscles,
give twigs to these, and end in the rectus abdominis. There are three
series of cutaneous nerves given off by the intercostal nerves. The
upper nerves emerge through the serratus magnus and the external
intercostals about parallel with the digitations of the external
oblique. The middle set perforate the origin of the latter muscle. The
lower ones appear through the abdominal tunic. They supply branches to
the abdominal muscles, the panniculus, and the skin. Some of the
anterior ones anastomose with the posterior thoracic branches of the
brachial plexus. The posterior three supply in part the skin of the
flank. The ventral branch of the last thoracic nerve runs outward behind
the last rib across the dorsal surface of the psoas major and divides
into superficial and deep branches. The superficial branch passes
between the obliquus internus and transversus abdominis, perforates the
obliquus externus, and ramifies under the skin of the flank (Fig. 525).
The deep branch descends on the inner face of the internal oblique to
the rectus abdominis, in which it ends.