72 F. W. THYNG 



Arteries 



The arteries of the right side are shown in plate 1 from the 

 left side. The arteries of the left side of the head and neck are 

 shown in plate 2. 



Systemic derivatives of the aortic arches. The truncus aorticus 

 {Tr. aor., plate 1) leaves the left ventricle of the heart, extends 

 in the dorsal mesocardium across the pericardial cavity, and 

 im.mediately divides into two ventral aortae. The left ventral 

 aorta gives off the short fourth left aortic arch which becomes 

 part of the arcus aortae, and continues cephalad as the !eft 

 common carotid. It is shown in this plate as a stump, but is 

 displayed fully in plate 2. 



The left common carotid extends cephalad between the thymic 

 and thyreoid anlagen to the region of the larynx, where it divides 

 into a dorsal and a ventral branch, the internal and the external 

 carotid arteries, respectively. 



The external carotid {A. car. ex.) represents the continuation 

 of the ventral aorta. It is a short stem which terminates by 

 dividing; into five branches, viz.: superior thyreoid, occipital, 

 lingual, external maxillary and posterior auricular. The superior 

 thyreoid {A.thyr.s. and d., plates 1 and 6) runs medially skirting the 

 cephalic extremity of the lateral lobe of the thyreoid gland, and 

 at once breaks up into small branches. The occipital (A.occ, 

 plates 2 and 6) takes a cephaFc and dorsal direction around the 

 lateral side of the internal jugular vein, and gives off the sterno- 

 mastoid branch. The lingual (A.ling., plates 2 and 6) runs 

 medially to the tongue, giving off laterally a branch to the sub- 

 maxillary gland. The external maxillary (A.rnax.ex., plates 2 

 and 6) extends ventrally along the mandibular arch. The pos- 

 terior auricular (A.aur.p., plates 2 and 6) is a large vessel which, 

 having given off the stylo-mastoid branch curves laterally behind 

 the primary external acoustic meatus and passes dorsally. The 

 ascending pharyngeal and temporal arteries have not been iden- 

 tified. The internal maxillary, as such, is still wanting in this 

 embryo; it is foreshadowed by the stapedial branch of the in- 

 ternal carotid to be described later. 



