38 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



dorsal aortae in which they end, unite together, as they do in 

 the human embryo, to form the descending thoracic aorta. 

 Probably a communicating arterial twig which runs in the human 

 body from the origin of the intercostal artery of the 3rd or 4th 

 space to join the superior intercostal of the right side represents 

 the right dorsal aorta between the 4th arch and the descending 

 aorta (see Fig. 28). 



Oases are found in which the permanent aorta is very much 

 constricted at or near the point of entrance of the ductus arteriosus 

 (5th arch) (see Fig. 29). It will be noticed that the correspond- 

 ing part of the right dorsal aorta is obliterated. Such a con- 

 striction on the left side is to be regarded as corresponding to 

 that on the right side and indicates a partial attempt to produce 

 a right aortic arch ; it may give rise to clinical symptoms. 



Dorsal Aortae. — It will be noticed that the parts of the dorsal 

 aortae between the 3rd and 4th arches disappear (Fig. 28). The 

 ventral aortae persist as the innominate, the common carotid and 

 external carotid arteries. It will be observed that, while the 1st, 

 2nd and 3rd vascular arches have almost retained their foetal 

 position, the 4th and 5th arches have been pulled downwards by 

 the descent of the heart. The 4th, which should lie opposite the 

 upper part of the thyroid cartilage, comes to lie on the 1st rib on 

 the right side and within the thorax on the left, while the 5th 

 dragging the nerve of its arch in front of it (the recurrent 

 laryngeal) is pulled right within the thorax from its foetal position 

 at the thyroid cartilage. With the descent of the heart the ventral 

 aortae between the 3rd and 4th arches are drawn out to form the 

 innominate and common carotid arteries on the right side, and 

 the left common carotid on the left. 



Muscles of the Visceral Arches. — All the muscles supplied 

 by the facial nerve — the platysma, muscles of expression, the 

 stapedius, etc. — are derived from the muscle plate of the 2nd 

 or hyoid arch. The muscles of mastication, with the tensors of 

 the palate and tympanum, are derived from the muscle segment 

 of the mandibular arch. The stylo-pharyngeus is derived from 

 the 3rd arch. 



The Platysma and Muscles of the Face and Scalp.— The 

 platysma myoides, the muscles of the face, scalp and external ear, 

 are derived from the muscle plate of the second or hyoid arch. 



