Fig. 364. Diagram of the arrangement of the arterial arches in an embryo of one of the Amniota. (From Gegenbaur; after Rathke.)
a. ventral aorta; a´´. dorsal aorta; 1, 2, 3, 4, 5. arterial arches; c. carotid artery.
In the Amniota the metamorphosis of the arteries is in all cases very similar. Five arches, viz. the mandibular, hyoid, and three branchial arches are always developed ([fig. 364]), but, owing to the absence of branchiæ, never function as branchial arteries. Of these the main parts of the first two, connecting the truncus arteriosus with the collecting trunk into which the arterial arches fall, always disappear, usually before the complete development of the arteries in the posterior arches.
The anterior part of the collecting trunk into which these vessels fall is not obliterated when they disappear, but is on the contrary continued forwards as a vessel supplying the brain, homologous with that found in Fishes. It constitutes the internal carotid. Similarly the anterior part of the trunk from which the mandibular and hyoid arteries sprang is continued forwards as a small vessel[230], which at first passes to the oral region and constitutes in Reptiles the lingual artery, homologous with the lingual artery of the Amphibia; but in Birds and Mammals becomes more important, and is then known as the external carotid ([fig. 125]). By these changes the roots of the external and internal carotids spring respectively from the ventral and dorsal ends of the primitive third artery, i.e. the artery of the first branchial arch ([fig. 365], c and c´); and thus this arterial arch persists in all types as the common carotid, and the basal part of the internal carotid. The trunk connecting the third arterial arch with the system of the dorsal aorta persists in some Reptiles (Lacertilia, [fig. 366] A) as a ductus Botalli, but is lost in the remaining Reptiles and in Birds and Mammals ([fig. 366] B, C, D). It disappears earliest in Mammals ([fig. 365] C), later in Birds ([fig. 365] B), and still later in the majority of Reptiles.
The fourth arch always continues to give rise, as in the Anura, to the system of the dorsal aorta.
Fig. 365. Development of the great arterial trunks in the embryos of A. a Lizard; B. the common Fowl; C. the Pig. (From Gegenbaur; after Rathke.)
The first two arches have disappeared in all three. In A and B the last three are still complete, but in C the last two are alone complete.
p. pulmonary artery springing from the fifth arch, but still connected with the system of the dorsal aorta by a ductus Botalli; c. external carotid; c´. internal carotid; ad. dorsal aorta; a. auricle; v. ventricle; n. nasal pit; m. rudiment of fore-limb.
In all Reptiles it persists on both sides ([fig. 366] A and B), but with the division of the truncus arteriosus into three vessels one of these, i.e. that opening furthest to the left side of the ventricle (e and d), is continuous with the right fourth arch, and also with the common carotid arteries (c); while a second springing from the right side of the ventricle is continuous with the left fourth arch (h and f). The right and left divisions of the fourth arch meet however on the dorsal side of the œsophagus to give origin to the dorsal aorta (g).
In Birds ([fig. 366] C) the left fourth arch (h) loses its connection with the dorsal aorta, though the ventral part remains as the root of the left subclavian. The truncus arteriosus is moreover only divided into two parts, one of which is continuous with all the systemic arteries. Thus it comes about that in Birds the right fourth arch (e) alone gives rise to the dorsal aorta.
In Mammals ([fig. 366] D) the truncus arteriosus is only divided into two, but the left fourth arch (e), instead of the right, is that continuous with the dorsal aorta, and the right fourth arch (i) is only continued into the right vertebral and right subclavian arteries.