146 THK ANATOMY OF THE HORSK. 



Action. — To depress (flex) the head or give it a hxteral incliuatiou, 

 according as the right and left muscles act singly or in concert. 



The Sterno-thyro-hyoideus (Plates 27 and 28). This is a long and 

 slender nniscle, extending along the lower face of the trachea, and closely 

 applied along the middle line to its fellow of the opposite side. It takes 

 origm from the carinifurm cartilage of the sternum. About the middle 

 of the neck its muscular substance is interrupted by a short tendinous 

 portion, rendering the muscle digastric. Above this central tendon the 

 muscle divides into two portions. The outer or thyroid band passes 

 obliquely outwards and forwards between the trachea and the sub- 

 scapulo-hyoideus muscle, and becomes inserted into the edge of the 

 thyroid cartilage of the larynx. The inner or hyoid band is continued 

 directly forwards in company with the corresponding branch of the 

 opposite imiscle, and becomes inserted into the body of the hyoid bone. 



Action — To depress the hyoid bone and larynx. 



The SuBSCAPULO-HYOiDEUS {Omo-hyoid of man) (Plates 27 and 28). 

 This is a thin, ribbon-shaped muscle having a breadth of three or four 

 inches. It takes origin at the inner side of the scapula, from the fascia 

 covering the subscapularis muscle. It then passes downwards and for- 

 wards between the scalenus and rectus capitus anticus major muscles 

 inwardly; and the supraspinatus, anterior deep pectoral, and mastoido- 

 humeralis muscles outwardly. Appearing at the lower edge of the 

 last-named muscle, to which it adheres closely, it passes between the 

 jugular vein and carotid artery ; and crossing over the upper part of the 

 trachea in a dii'ectiou obliquely forwards and downwards, it applies itself 

 at the outer edge of the hyoid band of the sterno-thyi-o-hyoideus, and 

 becomes inserted along with that muscle into the body of the hyoid 

 bone. In the lower part of the neck the ascending branch of the 

 inferior cervical artery and the prescapular group of lymphatic glands, 

 are included between this muscle and the mastoido-humcralis. 



Action. — To depress the hyoid bone. 



Nerves. At the upper part of the neck a branch from the spinal 

 accessory nerve enters the sterno-maxillaris, awd branches from the 1st 

 spinal nerve enter the sterno-thyro-hyoid and subscapulo-hyoid muscles. 

 These, however, will be better dissected at a later stage. 



Directions. — The jugular vein should now be ligatured at tlie upper 

 and lower ends of the jugiilar furrow, and the intermediate portion of 

 the vessel should be cut away. The excised portion of the vein should 

 be laid open to expose its valves. The part of the subscapulo-hyoideus 

 which passes over the trachea may be cut out after the manner of 

 Plate 28, and the sterno-maxillaris may be similarly treated. This will 

 expose for examination the trachea, the oesophagus, the carotid artery, 

 the pneumogastric and sympathetic nerves, and the recui-rent nerve. 



Valves of Veins. Three or four valves are placed in the jugular vein. 



