150 • ■ THE ANATOMY OF THE HORSE. 



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 muscle, 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 subscapulars muscle. It then passes downwards and for- 

 wards between the scalenus and rectus capitis 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 direction obliquely forwards and downwards, it applies itself 

 at the outer edge of the hyoid band of the sterno-thyro-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-humeralis. 



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, and 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 r be ligatured at the upper 

 and lower ends of the jugular 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 reourrent nerve. 



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

 Each valve is composed of two or three semilunar folds of the inner coat 

 of the vein, the folds having a close resemblance to the semilunar seg- 

 ments of the aortic valve (Fig. 13, page 132). Each flap with the adjacent 

 part of the wall of the vein forms a small pouch with its mouth directed 

 towards the heart. When the blood tends to regurgitate it distends 

 these pouches until the segments meet across the vessel and thus arrest 

 the backward current. In most veins throughout the body similar 

 valves are found ; but they are most numerous in the veins of the limbs. 

 In the small veins each valve may be composed of only a single flap. 

 The following veins have few or no valves : the pulmonary system of 



