152 THE ANATOMY OF THE HORSE. 



The Common Carotid Artery (Plate 28). This is the vessel that 

 conveys the blood to the head and upper part of the neck. It begins 

 on the under aspect of the trachea, at the entrance to the thorax, where 

 it results from the bifurcation of a short vessel termed the cephalic 

 trunk — a branch of the arteria inuominata. It ascends in the neck in 

 company with the trachea, and terminates above the cricoid cartilage of 

 the larynx by dividing into the external carotid, the internal carotid, 

 and the occipital artery. It thus crosses the trachea very obliquely, 

 being at first on its under surface, then on its lateral aspect, and finally 

 above it. It is in contact with the scalenus, longus colli, rectus 

 capitis anticus major, and subscapulo-hyoideus muscles, the last- 

 mentioned intervening between the artery and the jugular vein in the 

 upper half of the neck. In the lower half of the neck the artery and 

 vein are in direct contact, the carotid being above and slightly internal 

 to the jugular. The common cord of the pneumogastric and sympathetic 

 nerves is on the upper side of the artery, and the inferior laryngeal 

 (recurrent) nerve is below it. At the entrance to the thorax the pre- 

 pectoral group of lymphatic glands is in contact with the artery. The 

 left carotid differs from the right in being related for a considerable 

 part of its course to the oesophagus, which separates it from the trachea. 

 The collateral branches of the carotid are as follows : — 



1. Innominate and slender muscular, ossophar/eal, and tracheal branches. 



2. The Thyroid artery, which arises a few inches behind the thyroid 

 body, and passes obliquely forwards to enter the gland on its posterior 

 aspect. Sometimes this artery is distributed mainly or entirely to the 

 neighbouring muscles. 



3. The Thyro-laryngeal artery. — This is the largest of its collateral 

 branches. It arises a little in front of the preceding vessel, and, passing 

 to the inner side of the thyroid body, it divides in front of it into 

 thyroid and laryngeal branches. The former turn back to enter the 

 gland in front, while the latter pass to the larynx and pharynx. 



The terminal branches of the carotid are not to be followed at 

 present. 



The Pneumogastric and Sympathetic Nerves in the neck (Plate 28.). 



The pneumogastric, vagus, or 10th cranial nerve has its origin from 

 the medulla oblongata. It leaves the cranium by the posterior part oL 

 the foramen lacerum, and, inclining downwards and backwards on the 

 guttural pouch, it meets the cervical cord of the sympathetic, with 

 which it becomes in nearly all cases closely united. The common cord 

 resulting from the fusion of the two nerves descends in company with 

 the carotid artery, lying on the upper side of that vessel. At the lower 

 part of the neck the two nerves, in passing into the thorax, again 

 become separate. 



The cervical cord of the sympathetic begins at the superior cervical 



