MYOLOGY 51 



processes of the first four or five thoracic vertebrae. In- 

 sertion is by tendon and muscle fibers onto the transverse 

 processes of the last five cervicals. 



Towards its thoracic end the main portion, constituting 

 pars thoracis of the longissimus, has origin chiefly from 

 the transverse processes of the thoracic vertebrae. The 

 lumbodorsal fascia passes deep within the anterior portion, 

 chiefly in the vicinity of the ninth rib, and gradually 

 ceases. Insertion is by tendinous fibers upon the trans- 

 verse processes of the fourth and fifth cervical vertebrae. 



The postcostal or lumbar portion, forming pars lumborum 

 of the longissimus, is divisible in a sagittal direction as 

 follows: (a) lateralis is entered by the deeper lumbo- 

 dorsal fascia, the fibers originating from both the ectal 

 and ental surfaces; origin of the extreme caudal fibers is, 

 however, from the cranial and medial portions of the 

 anterior ilium; insertion is onto the di- and anapophyses 

 of the lumbar vertebrae; (5) medialis has origin from 

 the spinous processes and metapophyses of the lumbar 

 vertebrae. Some of the fibers are bipennated onto an 

 aponeurotic tendon, and insertion is onto the diapophyses 

 of the vertebrae. The muscle is continuous caudad with 

 the extensor caudae laterahs. 



M. spinalis dorsi (figs. 6, 10, 11) occurs between the 

 thoracic spines and the longissimus. The more super- 

 ficial fibers arise directly from the lumbodorsal fascia at 

 about the ninth rib. The deep part gradually becomes 

 differentiated from the medial portion of the longissimus, 

 though not with any degree of clarity until the seventh 

 rib is reached. The medial fibers interconnect the spinous 

 processes of the vertebrae craniad to include the spine of 

 the axis, passing caudad from the spines craniad to the 

 ninth thoracic, and craniad from those caudad of the 

 eleventh thoracic. 



Various branches, not followed, of the costal nerves 

 serve this long system. 



