126 THE MUSCLES. 



(a) l\IiLscles of the Lumbar and Thoracic Regions. — The 

 muscles of the lumbar and thoracic regions are mostly covered 

 by a strong fascia, known as the lumbodorsal fascia (Fig. 

 68, y). This consists of two sheets, the superficial sheet being 

 applied directly to the outer surface of the inner sheet, or sepa- 

 rated from it by a mass of fat. 



The superficial sheet (Fig. 68, j/) overlies the lumbar region 

 and the caudal half of the thoracic region. On the medial side 

 it is attached to the spinous processes of the vertebrae and is 

 united closely to the deeper layer. Laterally this fascia is 

 continuous with the latissimus dorsi (?«) and obliquus abdominis 

 externus (/). Caudad it is attached to the spine of the ilium 

 and becomes continuous with the fascia covering the gluteus 

 muscles. 



The deeper sheet (Fig. 70, c) is of a tendinous character, 

 forming the external tendinous layer of the longissimus dorsi, 

 many of whose fibres take origin from its under surface. It is 

 described more fully in the account of this muscle. 



M. longissimus dorsi (Fig. 69, /, /', /"; Fig. 70, a 

 and b). — A very large muscle, filling most of the region 

 between the spinous processes and transverse processes of the 

 lumbar and thoracic vertebrae, and extending into the cervical 

 region. 



The muscle is largest in the lumbar region (Fig. 69, f\ 

 Fig. 70), where it is divided into a narrow medial (Fig. 70, a) 

 and a thick lateral portion (Fig. 70, b), the latter being again 

 partially subdivided by the fascia. The two parts unite farther 

 craniad. 



The medial division (Fig. 70, a) consists of muscular 

 bundles connecting the spinous processes of the vertebrae with 

 the accessory and mammillary processes of other vertebrae ; it 

 is continuous caudad with the extensor caudal lateralis (Fig. 

 70, f). The muscle-fibres take origin in the sacral region by 

 small round tendons from the spinous processes of the last two 

 sacral and the caudal vertebrae. They curve cranioventrad, 

 forming a large belly, and are inserted into the accessory and 

 mammillary processes of the lumbar vertebrae. In the thoracic 

 region this division unites with that portion of the lateral divi- 



