152 THE ANATOMY OF THE HORSE. 



This is a long and powerful muscle, extending between the head and the 

 shoulder, on the side of the spinal column. It takes origin from the 

 mastoid process and crest, from the wing of the atlas, and from the 

 transverse processes of the 2nd, 3rd, and 4th cervical vertebrae. The 

 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 fleshy. The muscle passes over the 

 shoulder-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 pannioulus. 



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 stei^no-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. 



Directions. — If the mastoido-humeralis has not alreadj^ been cut, it 

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

 28) to show the prescapular 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 Pebscapulab 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 Infeeioe Ceevical Aetbey 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. 



Ceevical 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 superior and inferior primary branches. 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 communicate, each with the preced- 

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

 branches, viz., (1) communicating branches to the middle cervical ganglion 

 (see vertebral nerve, page 157); (2) muscular branches to the mastoido- 



