THE FASCIA AND MUSCLES OF THE BACK AND LOINS 



237 



Blood-supply. — Intercostal arteries. 



Nerve-supphj. — Dorsal Ijranches of the thoracic nerves. 



7. Longissimus/ — This is the largest and longest muscle in the body. It 

 extends from the sacrum and ilium to the neck, filling up the space between the 

 spinous processes medially and the lumbar transverse processes and the upper 

 ends of the ribs below; consequently it has the form of a three-sided prism. 



Origin. — (1) The internal angle, crest, and adjacent part of the ventral surface 

 of the ilium; (2) the first three sacral spines; (3) the lumbar and thoracic spines 

 and the supraspinous ligament. 



Insertion. — (1) The lumbar transverse and articular processes; (2) the thoracic 

 transverse processes; (3) the spinous and transverse processes of the last four 

 cervical vertebrae; (4) the outer surfaces of 

 the ribs, except the first. 



Action. — Acting with its fellow, it is 

 the most powerful extensor of the back 

 and loins; by its cervical attachment it 

 assists in extending the neck. By its 

 costal attachment it may also assist in 

 expiration. Acting singly, it flexes the 

 spine laterally. 



Structure. — This is quite complex. The 

 posterior part of the muscle is greatly de- 

 veloped and constitutes the common mass 

 of the loins. This is covered by a strong 

 aponeurosis which blends with the supra- 

 spinous and sacro-iliac ligaments, and is 

 attached to the crest and inner (sacral) 

 angle of the ilium and the first and second 

 sacral spines; it furnishes origin to the lum- 

 bar portion of the middle gluteus. In its 

 course further forward the muscle receives 

 fasciculi from the lumbar and thoracic 

 spines, but diminishes somewhat in vol- 

 ume. At the withers it divides into two 

 parts. The dorsal division (spinalis et 

 semispinalis), reinforced by bundles from 

 the first four thoracic spines, passes for- 

 ward under the complexus to be inserted 

 into the spines of the last four cervical 

 vertebrae. The ventral division passes 

 forward and downward underneath the 

 serratus magnus to be inserted into the 

 ribs and the transverse processes of the 



last four cervical vertebrae. Three sets of fasciculi may be distinguished, viz.: (1) 

 spinal, which are superficial and internal; (2) transverse, attached to the transverse 

 and articular processes, which are internal and deep; (3) costal, w^hich are external. 



Relations. — Superficially, the middle gluteus, the lumbo-dorsal fascia, the 

 latissimus dorsi, dorsal serrati, serratus magnus, and complexus; deeply, the 

 multifidus, intertransversales, external intercostals, levatores costarum, the liga- 

 mentum nuchse, and its fellow of the opposite side (in the neck). 



Blood-supply. — Dorsal, superior cervical, intercostal, and lumbar arteries. 



Nerve-supply. — Dorsal branches of the thoracic and lumbar nerves. 



^The muscle as here described includes the spinalis and semispinalis components, as the 

 separation of these is largely artificial in the horse. 



Fig. 181. — Right Poktion of Cross-section op 

 Back of Horse. Section is Cut Through 

 Seventh Thob.'VCic Vertebra. 

 a, Ligamentum nuch»; b, trapezius mus- 

 cle; c, cartilage of scapula; d, latissimus dorsi; e, 

 panniculus carnosus; /, rhomboideus ilorsi; o, ser- 

 ratus magnus; h, serratus anticus; h' , lumbo- 

 dorsal fascia, which divides below into three 

 layers; i, transversalis costarum; k, levator costa>; 

 k' , internal intercostal muscle; I, longissimus dor- 

 si; m, m' , seventh thoracic vertebra; «, head of 

 eighth thoracic vertebra; o, head of eighth rib; 

 p, seventh rib; r, intercostal artery and ner\'e; s, 

 skin. The fascicp are indicated by dotted lines. 

 (After Ellenberger, in Leisering's Atlas.) 



