158 THE AJfATOMT OF THE HORSE. 



processes of the last six cervical vertebrae. The upper division of the 

 muscle is inserted, by a flat tendon common to the splenius, into the 

 mastoid crest ; the lower division terminates in a riband-like tendon, 

 common to the splenius and mastoido-humeralis, and inserted into the 

 wing of the atlas. 



Action. — Acting singly, to bend the neck laterally; acting with the 

 opposite muscle, to extend the occipito-atlantal articulation (elevate 

 the head). 



The Complexus. This is one of the most powerful muscles of the 

 neck. It covers the lamellar portion of the ligameutum nucha?, which 

 separates the right and left muscles. It arises from the 2nd, 3rd, and 

 4th dorsal spines, in common with the splenius ; from the transverse 

 processes of the first six or seven dorsal vertebra? ; and from the articular 

 processes of the cervical vertebra?, with the exception of the first. From 

 these different points of origin the fibres converge towards the poll, 

 where they terminate in a tendon inserted into the occipital bone. 



Action. — Both muscles will extend the occipito-atlantal joint (elevate 

 the head) ; the muscle of one side will, while elevating the head, turn it 

 slightly to the same side. 



Nerves. As already stated, each cervical nerve resolves itself into a 

 superior and an inferior primary branch. The superior primary branches 

 of the last six may now be found distributing nerves to the splenius, 

 trachelo-mastoideus, complexus, and semispinalis colli muscles ; and if 

 the complexus be raised from the ligamentum nucha? other branches 

 will be found ascending between the muscle and the ligament to be 

 distributed to the integument near the middle line. 



The muscles and other structures which lie above the ' cervical 

 vertebra? receive their chief blood supply from three vessels, viz., the 

 occipital, dorsal, and superior cervical arteries. The first of these gives 

 branches to the neighbourhood of the poll, but it is not to be sought at 

 present. 



The Dorsal Artery will be found distributing branches to the upper 

 part of the neck, in front of the withers. The artery has its origin 

 within the thorax, w T here, on the left side, it is a branch of the axillary 

 artery, and, on the right side, of the arteria innominata. It leaves the 

 chest by the upper part of the second intercostal space ; and, inclining 

 backwards and upwards, it divides on the longissimus dorsi into a 

 number of branches distributed to the withers, and the neck in front of 

 that region. These will be found ascending between the splenius and 

 complexus muscles. 



The Superior Cervical Artery, like the preceding, arises within the 

 chest, and from the same source. It leaves the thorax by the upper 

 part of the first intercostal space ; and, placing itself on the inner 

 surface of the complexus, it ascends between that muscle and the liga- 



