820 NERVOUS SYSTEM OF THE HORSE 
distal end of the metacarpus it is joined by the oblique branch from the medial 
nerve, and beyond this is arranged like the latter. Below the carpus it detaches a 
deep branch to the suspensory ligament and the interossei, and also supplies twigs 
to the skin.! 
THE THORACIC NERVES 
The thoracic nerves (Nn. thoracales) number eighteen on either side in the 
horse. They are designated numerically according to the vertebra 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 medial and lateral branches. The medial branches ascend on the 
multifidus and supply the dorsal spinal muscles. The lateral branches run outward 
under the longissimus dorsi and emerge between that muscle and the longissimus 
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. 655). In the region of the withers they give branches to the 
serratus dorsalis and rhomboideus, and their cutaneous terminals pass through 
these muscles and the dorso-scapular ligament to supply the skin over the liga- 
mentum nuch 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 communi- 
cantes. The first goes almost entirely to the brachial plexus, but sends a small 
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. 273) 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 abdominal 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 dorsal nerves emerge through the serratus ventralis and the external 
intercostals about parallel with the digitations of the external oblique. The middle 
ones perforate the origin of the latter muscle. The ventral ones appear through 
the abdominal tunic. They supply branches to the abdominal muscles, the cu- 
taneus, 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 over the superficial 
face of the transversus abdominis, perforates the obliquus externus, and ramifies 
under the skin of the flank (Fig. 657). The deep branch descends on the deep face 
of the internal oblique to the rectus abdominis, in which it ends. 
1 Anastomoses are established between the digital branches, and the areas innervated by 
them are not well defined, but really overlap each other. In certain diseased conditions, however, 
in which the lesions are confined to the volar structures, relief from pain may be afforded by sec- 
tion of the volar branches only. 
