222 



VASCULAE SYSTEM 



sweeps backwards, gives branches to the brain, afterwards concentrated chiefly 

 in the posterior cerebral, and joins the cerebral portion of the vertebral (see below) 

 to form the circle of Willis. 



The ventral aortce, now the external carotids, extend forwards into the mandibular 

 arches. In its course each gives off as secondary branches the superior" thyroid, 

 lingual, and facial, and then, turning backwards to reach the proximal end^of 

 Meckel's cartilage, it turns inwards and forwards on its lateral aspect to be joined 

 to the mandibular branch of the stapedial artery by an anastomosing branch. 



The stapedial artery, though only an embryonic vessel in man and some other 

 mammals, persists in others, such for instance as the rat. It arises in that 



XI 



XII 



t.a. t.p. 



FIG. 278. DIAGRAM REPRESENTING THE NERVES AND ARTERIES OF THE HEAD OP AN EMBRYO or THE 



, FIFTH WEEK, FOUNDED ON RECONSTRUCTIONS BY TANDLER, MALL, AND SlREETER. (T. H. Bryce.) 



I to XII, cerebral nerves ; 1, 2, 3, three branches of fifth nerve ; the sixth nerve is not labelled it 

 is seen passing forwards below 3. 



The truncus arteriosus, T.A., is continued forward as the common carotid, which divides into external 

 and internal carotids. The latter arching forward between the vagus (X) and glossopharyngeal (IX) nerves 

 passes mesial to the nerve-roots to the brain, and gives off two terminal branches an anterior which 

 supplies the posterior middle and anterior cerebral arteries, and a posterior which joins the vertebral, 

 V.A., to form the primitive circle of Willis. Below the auditory vesicle av., the stapedial artery is seen 

 passing through the annulus stapedialis and supplying supra-orbital, infra-orbital, and mandibular 

 branches accompanying the three divisions of the fifth nerve. 



animal (Tandler) from the dorsal persisting portion of the second arch, and is 

 continued forwards as a longitudinal anastomosis between the second and first 

 arches. In man its origin and course is the same (figs. 277, 278), but the share 

 taken by the first arch is not certainly known (Tandler). At its origin it passes 

 through the rudiment of the stapes, and later between the limbs of the ossicle. 

 The artery gives off two trunks a superior and an inferior. The superior passes 

 forward on the mesial aspect of the Gasserian ganglion to the roof of the orbit 

 (supra-orbital) ; the inferior divides below the Gasserian ganglion into an infra- 

 orbital branch which runs forwards mesial to the mandibular root of the ganglion, 

 and a mandibular which accompanies the mandibular nerve (fig. 278). The 



