THE FASCIiE AND MUSCLES OF THE TAIL 279 



its course further forward the muscle receives fasciculi from the lumbar and thoracic 

 spines, but diminishes somewhat in volume. About the twelfth thoracic vertebra 

 it divides into two parts. The dorsal division (m. spinalis et semispinalis), rein- 

 forced by bundles from the first four thoracic spines, passes forward under the com- 

 plexus to be inserted into the spines of the last four cervical vertebrae. The ventral 

 division passes forward and downward underneath the serratus ventralis to be in- 

 serted 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 medial; (2) transverse, attached to the transverse and articular processes, 

 which are medial and deep; (3) costal, which are lateral. 



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

 latissimus dorsi, serrati dorsales, serratus ventralis, 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, deep cervical, intercostal, and lumbar arteries. 



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



8. Multifidus dorsi. — This is a long segmental muscle which lies along the sides 

 of the spinous processes of the vertebrae from the sacrum to the neck. 



Origin. — (1) The lateral part of the sacrum; (2) the articular processes of 

 the lumbar vertebrae; (3) the transverse processes of the thoracic vertebrae. 



Insertion. — The spinous processes of the first two sacral, the lumbar, thoracic, 

 and last cervical vertebrae. 



Action. — Acting with its fellow, it is an extensor of the spine; acting singlj-, 

 it fiexes it laterally. 



Structure. — It is composed of a series of bundles which are directed obliquely 

 forward and upward. Each fasciculus passes over several vertebrae to its insertion. 

 In the posterior part of the series the bundles cross two or three vertebrae and are 

 inserted into the summits of the spines. Anteriorljr the bundles have a more 

 horizontal direction, cross three to five vertebrae, and are inserted into the sides 

 of the spines considerably below their summits. A further complication consists 

 in the fusion of several bundles into a common insertion. 



Relations. — Superficially, the longissimus dorsi; deeply, the vertebral spines. 



Blood-supply. — Intercostal and lumbar arteries. 



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



9. Intertransversales lumborum.. — These are very thin muscular and tendin- 

 ous strata, which occupy the spaces between the transverse processes of the lumbar 

 vertebrae except the fifth and sixth. 



Action. — To assist in flexing the loins laterally or in rendering the region rigid. 

 Relations. — Superficially, the longissimus dorsi; deeply, the quadratus lum- 

 borum. 



Blood-supply. — Lumbar arteries. 

 Nerve-supply. — Lumbar nerves. 



The Fascia and Muscles of the Tail 

 (fascia et musculi caud^) 

 The muscles of the tail are inclosed in the strong coccygeal fascia, which is 

 continuous in front with the gluteal fascia and blends with the lateral sacro-iUac 

 ligament. At the root of the tail it is loosely attached to the subjacent muscles, 

 but further back it is intimately adherent to them. From its deep face are de- 

 tached septa which pass between the muscles to become attached to the vertebra. 

 1. Coccygeus.i — ^his is a flat, triangular muscle which lies chiefly between the 

 sacro-sciatic ligament and the rectum. 



1 Also termed the ischio-coccygeus or compressor coccygis. 



