ERECTOR SPIX.E MUSCLE. 235 



highest ribs, and inserted into the transverse processes of three cervical 

 vertebrae, usually the fourth, fifth, and sixth. Its insertions are intimately 

 connected with those of the transversalis cervicis. 



d. The longissimus dorsi, the main muscle of the inner column of the 

 erector spinse, is both larger and longer than the ilio-costalis, its original 

 fibres passing as high as the first dorsal vertebra. Internally it is closely 

 connected on the surface with the spinalis dorsi, from the lower part of 

 which it generally receives one or more tendinous slips. When those slips 

 and the tendons of origin from the lumbar spines are cut through, the inner 

 surface of the muscle can be brought into view. The longissimus dorsi 

 presents two series of insertions. The inner row of insertions is a series of 

 rounded tendons attached to the inferior tubercles of all the transverse pro- 

 cesses of the dorsal, and the accessory processes of the lumbar vertebras. 

 The outer insertions form in the dorsal region a series of thin processes 

 attached to the nine or ten lowest ribs, sometimes more or fewer, and in 

 the lumbar region are attached by fleshy slips to the whole length of the 

 transverse processes, and beyond these to the lumbar fascia arising from 

 them. 



e. The transversalis cervicis prolongs upwards the column of fibres of the 

 longissimus dorsi. It arises from the internal tubercles of the transverse 

 processes of the four or five highest dorsal vertebrae, and occasionally the 

 last cervical, and is inserted into the posterior tubercles of the transverse 

 processes of five cervical vertebrae from the second to the sixth inclusive. 

 It always receives a slip of the original fibres of the longissimus dorsi. 



/. The trachelo-mastoid muscle, which may be regarded as the con- 

 tinuation of the longissimus dorsi to the head, arises in close connection 

 with the trans versalis cervicis from the upper dorsal transverse processes, 

 and also from behind the articular processes of the three or four lowest 

 cervical vertebrae, and, forming a tbiu flat muscle, passes to be inserted into 

 the posterior margin of the mastoid process, under cover of the splenius and 

 sterno-mastoid muscles. It is the only muscle which lies between the 

 splenius and complexus, and the only portion of the erector spinse concealed 

 by the former. 



g. The spinalia dorsi is a long narrow muscle placed at the inner side of 

 the longissimus dorsi, and closely connected with it. It arises by several 

 tendons from the spines of the first two lumbar, and the two lowest dorsal 

 vertebrae, and is inserted into from four to eight of the higher dorsal spines. 

 It adheres closely to the semispinalis muscle upon which it lies. 



ACTIONS. The gplenii and trachelo-mastoid muscles of both sides acting together 

 draw backwards the head and upper cervical vertebrae : when the muscles of one side 

 only act the extension is accompanied by lateral flexion and rotation, especially of the 

 head on the axis. 



The greater part of the erector muscles of the spine, when those of both sides are in 

 action, bends backwards the vertebral column and trunk ; and these muscles co- 

 operate powerfully in almost every great muscular effort of the body or limbs. It is 

 the power of these muscles to straighten the back from the bent condition, of which a 

 measure is obtained by the muscular dynamometer; the amount of this varies in 

 adults of medium strength from SOOlbs. to 400lbs. By the action of the erector 

 muscles of one side a certain amount of lateral flexion and of rotation, which is 

 greatest in the chest, accompanies the extension. These muscles also, by their costal 

 attachments, if the spine be fixed, depress the ribs, and thus assist in expiration ; but 

 it is likewise conceivable that the cervicalis ascendens and musculus accessorius, in 

 acting from fixed points above, may have the effect of elevating the ribs. 



