66 HEAD AND NECK 



under cover of the posterior border of that muscle, the artery 

 crosses the apex of the posterior triangle, and disappears 

 under the trapezius, which it pierces afterwards, 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) (Fig. 20). 



The following branches may be traced from the second 

 portion of the occipital artery: (i) ramus descendens (O.T. 

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



The descending branch (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 the mastoid 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 sinus) 

 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 that is being done and the attachments of 

 the muscle are being defined, care must be taken of the medial 

 divisions of the posterior rami of the second, third, fourth, and 

 fifth cervical nerves. The first of the three or, in other words, 

 the greater 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. 



Musculus 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 



