THE TRUNK MUSCLES. 51. i 



the splenius capitis upon the head will be similar ; the simultaneous action of the two 

 muscles of opposite sides will bend the head backward, each muscle neutralizing the 

 rotatory effect of the other. 



(d) THE TRANSVERSO-SPINAL TRACT. 



1. Spinalis. 6. Intertransversales. 



2. Semispinalis. 7. Rectus capitis posticus major. 



3. Multilidus. 8. Rectus capitis posticus minor. 



4. Rotatores. 9. Obliquus capitis superior. 



5. Interspinales. 10. Obliquus capitis inferior. 



I. Spinalis (Fig. 520). 



Attachments. — The spinalis in its lower portion is the continuation of the 

 deeper and innermost fibres of the sacro-spinalis, and, like the longissimus, with 

 which it is partly associated, it is regarded as consisting of a thoracic, a cervical, 

 and a cranial portion. The spinalis dorsi arises from the spinous processes of the 

 upper two lumbar and the lower two or three thoracic vertebrae by tendons common 

 to it and the longissimus dorsi. It forms a thin, flat muscle which passes upward, 

 inserting as it goes into the spinous processes of the thoracic vertebrae from the 

 second to the eighth or ninth, but one vertebra intervening between its uppermost 

 tendon of origin and its lowermost tendon of insertion. The spinalis cervicis arises 

 from the spinous processes of the upper two or four thoracic and the lower two 

 cervical vertebrae, and ascends alongside the spinous processes of the cervical ver- 

 tebrae to be inserted into those of the second, third, and fourth vertebrae. The 

 spinalis capitis consists of bundles arising from the spinous processes of the upper 

 thoracic and last cervical vertebrae, and passes upward to be inserted with the semi- 

 spinalis capitis. 



Nerve-Supply. — From the posterior divisions of the spinal nerves from the 

 third cervical to the last thoracic. 



Action. — To extend the spinal column. 



2. Semispinalis (Fig. 520). 



Attachments. — The semispinalis forms the superficial layer of the muscles 

 lying in the groove between the spinous and transverse processes of the vertebrae. 

 Three portions may be recognized in it. The semispinalis dorsi arises from the trans- 

 verse processes of the lower six or seven thoracic \'ertebrae ; its fibres are directed 

 obliquely upward and medially and are inse?ied into the spinous processes of the five or 

 six upper thoracic and last two cervical vertebrae. The semispinalis cervicis arises 

 from the transverse processes of the five or six upper thoracic vertebrae and is inserted 

 into the spinous processes of the second, third, fourth, fifth, and sometimes the sixth 

 cervical vertebrae. This portion of the muscle is almost concealed beneath the upper- 

 most portion, the semispinalis capitis, which arises from the transverse processes of the 

 upper six thoracic \^ertebrae and the articular and transverse processes of the lower 

 three or four cervical vertebrae. The fibres are directed almost vertically upward, 

 and are joined by the spinalis capitis to form a broad muscle-sheet which is inserted 

 into the under surface of the squamous portion of the occipital bone between the 

 superior and inferior nuchal lines. 



An intermediate tendinous intersection usually divides the semispinalis capitis 

 into an upper and a lower portion, and is much more distinct in the more medial 

 bundles than in the lateral ones. Frequently these more medial bundles are sep-- 

 arated somewhat from the others, and they have been considered a distinct muscle 

 and termed the biventer, the lateral portion of the muscle being named the complexus. 



Nerve-Supply. — From the posterior divisions of the spinal nerves from tne 

 second cervical to the last thoracic. 



Action. — The semispinalis dorsi and cervicis extend the vertebral column and 

 rotate it somewhat towards the opposite side. The semispinalis capitis draws the 

 head backward and also rotates it slightly towards the opposite side. 



