THE DISSECTION OF THE BACK 67 



capitis. It belongs to the third layer of muscles, of which 

 the greater number have been dissected already. The lower 

 two segments will be dissected subsequently, but it is convenient 

 to examine the semispinalis capitis at once. It is a thick 

 fleshy mass which springs by tendinous slips from the 

 transverse processes of the upper six thoracic vertebrae and 

 the articular processes of the fourth, fifth, and sixth cervical 

 vertebrae. Its massive upper extremity is inserted into a 

 somewhat oval area on the occipital bone, between the 

 superior and inferior nuchal lines close to the external occipital 

 crest. It is separated from its fellow muscle of the opposite 

 side by the ligamentum nuchae; and its most medial part, 

 which is to a certain extent distinct from the general mass, 

 is divided into two bellies by an intermediate tendon, and 

 is spoken of as the biventer cerviris. Occasionally the remainder 

 of the muscle is also intersected by a tendinous septum. 



The semispinalis capitis bends the head backwards. It 

 is supplied by the posterior rami of the upper cervical nerves. 



Dissection. The semispinalis capitis must now be reflected 

 by detaching it from the occiput and throwing it laterally. Care 

 must be exercised, not only on account of the nerves which have 

 been seen to perforate the muscle to reach the surface, but also 

 on account of the structures which it covers. In its upper part 

 it lies over the sub-occipital triangle and the muscles bounding 

 it, whilst, below, it covers the semispinalis cervicis. A thick, 

 dense fascia is placed over the subjacent parts, and in the fascia 

 lie certain of the cervical nerves and the anastomosis between 

 the descending branch of the occipital artery and the arteria 

 profunda cervicis. The dissector must specially look for a small 

 twig from the posterior ramus of the sub-occipital nerve which 

 enters the deep surface of the upper part of the semispinalis 

 capitis, and for a larger branch to the same muscle from the 

 greater occipital nerve. 



As soon as the twig to the semispinalis capitis is secured the 

 dissector should cut out a small piece of the muscle to which the 

 nerve goes, and leave it attached to the nerve to serve as a guide 

 to the other branches of the sub-occipital nerve when the bound- 

 aries and contents of the sub-occipital triangle are being dissected 

 (see p. 75). 



Ligamentum Nuchse. When the semispinalis capitis has 

 been turned aside the corresponding surface of the ligamentum 

 nuchae will be exposed (Fig 19). The ligament is a strong and 

 fibrous partition, placed in the median plane, between the 

 muscles on each side of the back of the neck. It represents a 

 powerful elastic structure in quadrupeds, which helps to sustain 

 the weight of the dependent head. In man, however, there is 

 in 5 a 



