THORAX AND ABDOMEN OF THE HORSE 3 



longissimus dorsi muscle close to its lateral border or emerging from the 

 interval between this muscle and the iliocostalis. These are derived 

 from the dorsal branches of the thoracic nerves. That part of the 

 longissimus dorsi which lies over the lumbar vertebrae is pierced by 

 cutaneous branches of the lumbar spinal nerves. Small blood vessels, 

 branches of the intercostal and lumbar vessels, accompany these nerves. 



M. ILIOCOSTALIS.^ — The composite ilio-costalis muscle has origin 

 from the lumbo-dorsal fascia, the first two or three (or more) lumbar 

 transverse processes, and the cranial border and lateral surface of the 

 last fifteen ribs. The oblique bundles of fibres of which the mu.scle is 

 composed, more or less blended with each other, terminate in long, 

 narrow, flattened tendons. The tendons of the more superficial and 

 lateral bundles cross three or four ribs, with a slight lateral inclination, 

 to end on the caudal border of the first fourteen or fifteen ribs, close to 

 their angles, and the transverse process of the seventh (possibly also of 

 the sixth) cervical vertebra. The slenderer tendons of the deeper and 

 medial bundles are directed in a caudal direction. They cross one or 

 two ribs and end on the cranial border of the ribs from the fourth to the 

 last. Small synovial bursse may intervene between some of the tendons 

 and the ribs over which they pass. 



M. LONGISSIMUS DORSI. — This very long and powerful muscle is of 

 great complexity, and extends from the sacrum to the last cervical 

 vertebra. Close to the ilium it forms a shoaling depression into which 

 the middle gluteal muscle extends, and from which a considerable 

 number of the fibres of the muscle take origin. In the loins the 

 longissimus is at its thickest, and entirely fills the space between the 

 spinous and transverse processes of the vertebrae. Indeed, it overhangs 

 the tips of the lumbar transverse processes. In the thoracic region the 

 muscle gradually becomes narrower and of less thickness, until it 

 terminates at the last cervical vertebra as a single flattened tendon. A 

 strong, glistening tendinous sheet, particularly noticeable in the lumbar 

 region, covers the surface of the muscle, blends with the supraspinous 

 and sacro-iliac ligaments, and is firmly attached to the crest and sacral 

 tuber of the ilium and the spinous processes of the first two segments of 

 the sacrum. 



The longissimus dorsi has, naturally, extensive attachments. It 

 arises from the crest and the coxal and sacral tubers of the ilium, the 

 transverse and spinous processes of the first and second sacral vertebrae, 

 and the inner surface of the ilium between these points. It is attached 

 to the spinous, transverse, articular and mammillary processes of the 

 lumbar vertebrae, the transverse processes of the thoracic vertebrae, the 



^ It has been suggested that this muscle should be called longissimus costarum. 



