234 THE DOG-FISH 



the first efferent trunk. It runs forwards and 

 inwards across the hyo-mandibular cartilage, and 

 across the ventral surface of the skuU opposite 

 the hinder border of the orbit ; and divides, about 

 one-third of an inch from the middle line, into 

 external and internal carotid arteries. Of these 

 the former runs forwards and outwards across 

 the floor of the orbit, and supplies the jaws and 

 snout ; while the latter, continuing its course in 

 a groove in the ventral surface of the skull, unites 

 with its fellow to form a median vessel, which 

 passes through a foramen in the skull into the 

 cranial cavity, and then divides into right and left 

 arteries supplying the brain. 



b. The hyoidean artery arises from the anterior limb 

 of the first loop near its ventral end, and runs 

 forwards and outwards to the spiracle, where it 

 gives branches to the pseudobranch. It then 

 continues forwards and inwards across the floor 

 of the orbit, crossing the external carotid artery, 

 and, entering the skull by a small hole |-ineh in 

 front of the inter-orbital foramen, divides into 

 branches supplying the brain. 



3. The dorsal aorta and its branches. 



The aorta is a median vessel, formed by the union 

 of the efferent branchial vessels of the two sides. The 

 anterior pair of these vessels unite about the level of 

 the inner openings of the fourth gill-clefts, and the 

 succeeding ones open in pairs into the aorta so formed. 



The dorsal aorta so formed runs back the whole 

 length of the body, lying in the trunk just below the 

 vertebral column, and in the tail, where it becomes 

 the caudal artery, in the haemal arches of the ver- 

 tebrae. It gives off branches supplying the whole of 

 the body behind the head. 



The anterior part of the aorta should he dissected between 

 the kidneys ; the posterior part, which is deeply placed between 



