702 NERVOUS SYSTEM OF THE HORSE 



THE THORACIC NERVES 



The thoracic nerves (Nn. thoracales) number eighteen on either side in the 

 horse. They are designated numerically according to the vertebrae 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 trans- 

 versalis 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 nuchse 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 fine branch 

 downward in the first intercostal space which is expended in the muscle there with- 

 out 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 ])art 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 ab- 

 dominis, perforates the obliquus externus, and ramifies under the skin of the flank 

 (Fig. 525). The deep branch desc(>nds on the inner face of the internal oblique 

 to the rectus abdominis, in which it ends. 



THE LUMBAR NERVES 

 There are six pairs of lumbar nerves (Nn. lum])ales) in the horse, the last of 

 which emerge between the last lumbar vertebra and the sacrum. The anterior two 

 or three are about the same size as the thoracic nerves, but the others are much 

 larger. * 



