196 THE ANATOMY OF THE HORSE. 



internal carotid, and occipital arteries. The first of these continues the 

 direction of the parent trunk, and is much larger than either of the 

 others, which are of nearly equal size. 



The Occipital Artery (Plate 32). The root of this vessel is external 

 to, and slightly in advance of, the root of the internal carotid. It passes 

 upwards and slightly forwards over the anterior straight muscles of the 

 head, and enters the antero-external foramen of the atlas. In the 

 groove which connects this and the antero-internal foramen the artery 

 divides into occipito-muscular and cerebro-spinal branches. Before its 

 passage through the foramen the vessel detaches three collateral 

 branches, viz., prevertebral, mastoid, and retrograde or anastomotic 

 arteries. 



1. The Prevertebral Artery. — This is the first and most slender of the 

 three branches. Passing upwards and forwards, it supplies muscular 

 twigs to the anterior straight muscles of the head, and meningeal twigs 

 that pass into the cranium by the foramen lacerum basis cranii and 

 the condyloid foramen. 



2. The Mastoid Artery, a considerable vessel, is detached about one- 

 third of an inch above the preceding. It crosses over the edge of the 

 rectus capitis lateralis, and ascends behind the styloid process of the 

 occipital bone, where it has already been exposed (page 174). 



3. The Retrograde or Anastomotic Branch varies considerably in 

 volume. Arising between the obliquus capitis superior and the rectus 

 capitis lateralis, beneath the wing of the atlas, it passes backwards 

 through the posterior alar foramen, and anastomoses with the termina- 

 tion of the vertebral artery. 



The Internal Carotid Artery (Plate 32) is a long vessel which is 

 the main source of supply to the brain. It passes obliquely upwards 

 and forwards, supported by the membrane of the guttural pouch, and 

 enters the cranium by the foramen lacerum basis cranii. Its mode of 

 entrance and its distribution to the brain will be examined at a later 

 stage. 



The External Carotid Artery (Plates 31 and 32). This vessel may, 

 for the purposes of description, be divided into two portions. The first 

 portion, comprising two-thirds of the artery, is included between the 

 guttural pouch inwardly, and the stylo-m axillaris, digastric, and stylo- 

 hyoid muscles outwardly. It emerges from beneath the last-named 

 muscle, and joins the second portion by passing between the muscle 

 and the posterior edge of the great cornu of the hyoid bone. It is 

 crossed inwardly by the 9th, and outwardly by the 12th nerve. The 

 second portion passes obliquely upwards and forwards on the great 

 cornu, at the upper part of which, and under cover of the parotid 

 gland, it divides into the superficial temporal and internal maxillary 

 arteries. The collateral branches of the vessel are : the submaxillary, 



