THE BACK. 407 



Describe tlic semispinalis dorsi. 



The idea in the name is founded on the facts : (i) That one of the muscle is 

 inserted into the spines, and (2) that the muscle is situated in the thoracic or dorsal 

 region of the spine. (Fig. 285.) The muscles composing the series are small and 

 tendinous. They extend obliquely inward and upward, from their origin, on the 

 back of the transverse processes of the thoracic vertebrae, from the sixth to the 

 tenth, to their insertion into the spines of the last two cervical and the first, 

 second, third, and fourth thoracic. The nerve-supply is from the posterior 

 primary divisions of the thoracic nerves. Remove this muscle and you expose 

 the multifidus spinae below. The muscle is covered by the spinalis dorsi 

 and latissimus dorsi the inner and middle parts of the erector spins, you will 

 remember. 



Describe tJie semispinalis colli. 



The verbal idea is the same in this as in the preceding muscle. The muscle 

 is located in the cervical region ; it is inserted into the spines of the cervical 

 vertebrae, from the second to the fifth inclusive. (Fig. 285.) The muscle originates 

 from the transverse processes of the five or six upper thoracic vertebrae. The 

 muscle is covered by the complexus ; under it is the multifidus spinae. Between 

 the complexus and semispinalis colli you will find : (i) Branches of the posterior 

 cervical nerves ; (2) an anastomosis between the arteria princeps cervicis, a branch 

 of the occipital, and the arteria profunda cervicis, a branch of the superior inter- 

 costal. In ligatfon of the common carotid and subclavian arteries blood may 

 reach both the hand and brain by this channel. 



Describe the nmltifidus spines. 



The multifidus spinae is found in every region of the spine. In the sacral and 

 lumbar regions it is thick and fleshy ; in the thoracic and cervical regions it is 

 thin and aponeurotic. This muscle has the following origins, which must be 

 carefully learned before this muscle can be dissected understandingly : 



(i) From the deep surface of the erector spinae. This fact of origin makes the 

 dissection of the muscle a difficult task, except in cases of zinc or formaline pre- 

 pared bodies where the cadavers have had a year to become very hard ; (2) 

 from the groove between the sacral spines and rudimentary articular processes 

 of the sacrum ; (3) from the mammillary processes of the lumbar vertebrae ; (4) 

 the transverse processes of all the thoracic vertebrae ; (5) the articular processes 

 of the cervical vertebrae from the fourth to the sixth, and from the transverse 

 process of the seventh cervical. The segments making up the collective multi- 

 fidus spinae from these diverse regions are inserted as follows : into the lower 

 borders of the vertebral spines, from the fifth lumbar to the second cervical. 

 The nerve-supply of the multifidus spinae is from the posterior primary divisions 

 of the spinal nerves, from the second cervical to the third sacral nerve. This 

 muscle covers the rotatores spinae. 



Describe the rotatores spiiue. 



These are in the region of the thorax. They derive their name from the 

 rotatory action they exert. Of these muscles there are eleven pairs. They 

 originate from the back and upper part of the transverse processes, and are 

 inserted into the lower border of the lamina of the next vertebra above. They 

 are in relation above with the multifidus spinae. Nerve-supply, the posterior 

 primary divisions of the spinals. 



Name and describe the seventh layer of muscles of the back. 



(i) The interspinales arise from the upper surface of the spine of a lower, and 

 are inserted into the lower surface of the spine of the vertebra immediately 

 above. These muscles are very small. Nerve-supply, the posterior primary 

 divisions of the spinals. (2) The intertransversales arise from the transverse 

 process below and are inserted into the one above. They are small muscles. 



