634 



THE BLOOD-VASCULAR SYSTEM 



off the pulmonary arteries. The true fifth arches are probably not always developed, but when 

 they occur they are later in development, imperfect, and very transitory. The dorsal aortae, 

 originally paired, are now united to form a single vessel as far forward as a place slightly caudal 

 to the sixth arches. 



During the separation of the heart into right and left halves (p 526.), the primitive ventral 

 aorta is^divided by the aortic septum into two vessels, the main pulmonary artery and the as- 

 cending aorta of the adult. The pulmonary trunk becomes connected with the sixth pair of 

 arches only; the other arches then communicate, by means of the aorta, with the left ventricle. 

 The further changes which occur in the arches to bring about the conditions found in the adult 

 are shown diagrammatically in fig. 506. The right and left ■pulmonary arteries arise from the 

 corresponding sixth arches. The portion of the sixth arch dorsal to the pulmonary artery dis- 

 appears on the right, on the left it persists until birth as the ductus arteriosus (lig. arteriosum of 

 the adult). The fourth arch, including the short ventral stem between the fourth and sixth 

 arch, becomes the permanent aortic arch on the left side, and the innominate and proximal por- 

 tion of the subclavian upon the right. The dorsal longitudinal stem disappears on both sides 

 between the third and fourth arches, and on the right side from the sixth arch back to the un- 

 paired dorsal aorta. A trace of the latter portion of the right dorsal stem frequently persists in 

 the adult as a small vessel (a. aherrans) connecting the dorsal aorta, directly or indirectly, with 

 the right subclavian artery (p. 590). The ventral stems between the fourth and third arches 

 form the common carotids; those between the third and first become the external carotids. The 

 internal carotids are formed by the third arches and the dorsal stems between the third and first 

 arches. The first and second arches disappear early, contributing somewhat to the formation 

 of the branches of the internal and external carotids. 



Fig. 506. — Diagrams showing the Method of Normal Development of the Aortic 

 (Arches, and Indicating the Mechanism of Some Variations.) 



The primitive aortic arches (1-6), and some of the cervical dorsal segmentals (V-VIII) are 

 shown in allithe diagrams but numbered in Y only. X., abnormal: the aortic arch is on the 

 right; the left subclavian takes the dorsal course; the right vertebral arises direct from the 

 aortic arch. Y., normal; Z., abnormal: the right subclavian arises from the sixth cervical 

 dorsal segmental; the left from the sixth and seventh. A, ascending aorta; AA, aortic 

 arch; AD, dorsal jaorta; CC, common carotid; CE, external carotid; CI, internal carotid; 

 D, ductus arteriosus; IN, innominate; S, subclavian; T, costo-cervical; V, vertebral. 



X 



In early development the segmental arteries are caudally placed with regard to the aortic 

 arch vessels. As the latter, however, become shifted following the migration of the heart from 

 the neck into the thorax, the persistent seventh dorsal cervical segmental (subclavian) reaches 

 the neighbourhood of the sixth aortic arch. 



Little is known of the share taken by the first and second aortic arches in the formation of the 

 branches of the internal and external carotid arteries. It has been shown by Tandler that the 

 internal maxillary is primarily a branch of tlie internal carotid, (the first and second arches tak- 

 ing a share in its formation). The primitive vessel is known as the stapedial since it passes be- 

 tween the crura of the developing stapes. It gives off supraorbital, infraorbital, and mandibular 

 branches; the latter two arising from the main artery by a common trunk. The common trunk 

 is later joined^by a branch from the external carotid and, together with the supraorbital, becomes 

 the middle meningeal. An anastomosis Ijetween tlic supraorbital and the ophthalmic persists 

 so that in the adult the anterior branch of the meningeal frequently takes a considerable share in 

 the blood-supply of the orbit. The stapedial trunk undergoes retrogre.ssion and is represented 

 in the adult by the carotico-tympanic of the internal carotid and by the superior tympanic of the 

 middle meningeal, 'i'he infraorbital l)ranch of the stapedial becomes the second and third parts 

 of the internal maxillary and gives off branches accordingly. The mandibular branch becomes 

 the inferior alveolar of the adult. 



(2) The segmental system (fig. 507) consists of arteries primarily arising from the aorta in 

 three longitudinal '.scries, donsal, lateral, and ventrnl on either side. The segmental arrange- 

 ment is nmch less perfect in the ventral and lateral groups than in the dorsal. So much so, in 

 fact, that the term segmental is used for the ventral and lateral groups rather as a matter 



