126 THE MUSCLES. 
(a) Muscles 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, v). 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, v) 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 vertebre and is 
united closely to the deeper layer. Laterally this fascia is 
continuous with the latissimus dorsi (7) and obliquus abdominis 
externus (f). Caudad it is attached to the spine of the ilium 
and becomes continuous with the fascia covering the gluteus 
muscles. a 
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. 60, 7, 7’, 7”; Fig. 70, a 
and 4).—A very large muscle, filling most of the region 
between the spinous processes and transverse processes of the 
lumbar and thoracic vertebrx, and extending into the cervical 
region. - 
The muscle is largest in the lumbar region (Fig. 69, /; 
Fig. 70), where it is divided into a narrow medial (Fig. 70, a) 
and a thick lateral portion (Fig. 70, 4), the latter being again 
partially subdivided by the fascia. The two parts unite farther 
craniad. 
The medial division (Fig. 70, @) consists of muscular 
bundles connecting the spinous processes of the vertebrae with 
the accessory and mammillary processes of other vertebre; it 
is continuous caudad with the extensor caude 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 vertebrze. In the thoracic 
region this division unites with that portion of the lateral divi- 
