THE DISSECTION OF THE BACK 171 



the artery crosses the apex of the posterior triangle, and 

 disappears under the trapezius, which it finally pierces near 

 the external occipital protuberance, to reach the scalp. Two 

 muscles constitute its deep relations viz., the insertions of 

 the superior oblique and the semispinalis capitis (O.T. 

 complexus). 



The following branches may be traced from this portion of 

 the occipital artery : (i) descending (O.T. arteria princeps 

 cervicis) ; (2) meningeal ; (3) muscular. 



The ramus descendens (O.T. arteria princeps cervicis) is a 

 twig of some size, which passes medially to the lateral border 

 of the semispinalis capitis (O.T. complexus) ; there it divides 

 into a superficial and a deep branch. The former ramifies on 

 the surface of the semispinalis capitis, whilst the latter sinks 

 under that muscle, where it will be followed to its anastomosis 

 with the deep cervical artery at a later stage in the dissection. 



The small meningeal branch enters the posterior cranial fossa 

 through the mastoid foramen, and supplies the dura mater 

 and cranial wall in this region. 



The muscular twigs go to the neighbouring muscles. 



The veins corresponding to the occipital artery are two, 

 or perhaps three, in number. They drain the blood from the 

 occipital portion of the scalp, and open into the sub-occipital 

 plexus, which is drained by the vertebral and deep cervical 

 veins. The most lateral of the occipital veins frequently 

 communicates with the transverse sinus (O.T. lateral) through 

 the mastoid foramen. 



Dissection. The semispinalis capitis, which has been exposed by the 

 reflection of the splenius and the turning aside of the longissimus cervicis 

 and longissimus capitis, must now be cleaned, and whilst this is being done 

 and its attachments are being defined, care must be taken of the medial 

 divisions of the posterior branches of the second, third, fourth, and fifth 

 cervical nerves. The first of these or, in other words, the great occipital 

 from its great size, runs little risk of injury, but the others are liable to 

 be overlooked. They all emerge from the substance of the muscle close to 

 the median plane. 



Semispinalis Capitis (O.T. Complexus). The semispinalis capitis is the 

 uppermost part of a muscular column consisting of three segments, which 

 are spoken of collectively as the semispinalis, and individually as the semi- 

 spinalis dorsi, the semispinalis cervicis, and the semispinalis capitis. It 

 belongs to the third layer of muscles, of which the greater number have 

 been dissected already. The two lower segments will be dissected subse- 

 quently, 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 



