156 THE ANATOMY OF THE HORSE. 



tendon of origin from the mastoid process and crest, which is not to be 

 exposed at present, is thin and aponeurotic ; that from the wing of the 

 atlas is common to the splenius and trachelo-mastoideus muscles ; while 

 the succeeding slips of origin are fleshj'. The muscle passes over the 

 siioulder joint, and becomes inserted into the outer lip of the musculo- 

 spiral groove. As already seen, the lower edge of the muscle forms the 

 upper boundary of the jugular channel, and at the lower part of the 

 neck it is closely united to the sternal band of the panniculus. 



Action. — It is an extensor and inward-rotator of the shoulder-joint. 

 When the limb is fixed, it bends the neck laterally. 



This muscle represents the greater part of the sterno-mastoid of man 

 (the rest being represented by the sterno-maxillaris), combined with the 

 clavicular part of the deltoid, this fusion resulting from the absence of 

 a clavicle. 



Birections. — If the mastoido-humeralis has not already been cut, it 

 should be divided in front of the shoulder, and turned upwards (Plate 

 28) to show the presoapular glands and a branch of the inferior 

 cervical artery. The stellate groups of cutaneous nerves may thereafter 

 be traced through the mastoido-humeralis to their source. 



The Presoapular Lymphatic Glands. These are arranged in the 

 form of a chain between the mastoido-humeralis and subscapulo-hyoid 

 muscles at the lower part of the neck. 



The Inferior Cervical Artery is a branch of the axillary, arising 

 at the first rib. It divides into a descending (Plate 1) and an 

 ascending branch, the latter being here seen between the mastoido- 

 humeralis and subscapulo-hyoid muscles, to which and the above- 

 mentioned glands it is distributed. 



Cervical Spinal Nerves. There are eight pairs of these, the 1st 

 issuing from the spinal canal by the antero-internal foramen of the 

 atlas, the 2nd by the foramen (converted notch) at the anterior edge of 

 the arch of the axis, and the others in succession by the intervertebral 

 foramina. They have all a common disposition in that each divides at 

 its point of exit into a superior and an inferior primary branch. Only the 

 inferior primary branches present themselves now for consideration, and 

 of these the 1st is more conveniently taken at a later stage. The 

 remaining six behave as follows : — 



The 2nd, 3rd, 4th, 5th, and 6th conmiunicate, each with the preced- 

 ing and succeeding branches of the series, and divide into three sets of 

 branches, viz.: (l)oommunicating branches to the middle cervical ganglion 

 (see vertebral nerve, page 161) ; (2) muscular branches to the mastoido- 

 humeralis, longus colli, scalenus, and rectus capitis anticus major 

 muscles, and to the diaphragm (see phrenic nerve, page 6) ; (3) cutaneous 

 branches which pierce the mastoido-humeralis, and are distributed as 

 the stellate groups already seen. Besides these, the 6th nerve sends 



