76 F, W. THYNG 



The two small pulmonary arteries {Aa.puL, plate 1) extend 

 caudad from the main pulmonary trunk, one to each developing 

 lung. The right vessel is reconstructed more completely than 

 the left, and represents not only the right pulmonary artery, 

 but the proximal portion of the right pulmonary arch, as has been 

 shown by Bremer ('02). The left pulmonary artery is shown 

 only as a stump. It represents the left pulmonary artery proper. 

 The segment of the pulmonary arch {D.a., plate 2) between the 

 left pulmonary artery and the left dorsal aorta is the ductus 

 arteriosus. 



Derivatives of the occipital and cervical segmental arteries. The 

 right hypoglossus (intersegmental) artery (see p. 75) and the 

 proximal parts of the cephalic six (five on the right side) pairs of 

 cervical intersegmental arteries have disappeared. The distal 

 parts of these intersegmentals have now become branches of 

 the large vertebral arteries {A. vert.), fonned from the series of 

 post-costal anastomoses between them. The seventh inter- 

 segmental artery (sixth on the right side) has retained its connec- 

 tion with the dorso-lateral aorta of its own side to form a portion 

 of the stibclavian and the proximal part of the corresponding 

 vertebral artery. The vertebral arteries extend cephalad medial 

 to the cervical nerves, with the exception of the first pair which 

 they accompany towards the spinal cord. The sudden change in 

 direction of the vertebral arteries opposite the suboccipital pair 

 of nerves arises from the fact that the portion of these arteries 

 between the atlas and occiput develops from the first cervical 

 intersegmental spinal rami. The vertebral arteries pass ventral 

 to the myelencephalon and medial to the hypoglossal and vagus 

 nerves. Opposite the glossopharyngeal the arteries of the two 

 sides unite to form an enormous median vessel, the basilar {A .has.) . 

 The basilar artery extends cephalad in the median line between the 

 notochord and the rhombencephalon. Between the trochlear 

 nerves (N.troch.) it divides into two lateral branches, the posterior 

 cerebral arteries. Each posterior cerebral artery (A.cer.p.) 

 communicates ventrally with a branch (posterior communicating) 

 of the internal carotid artery (A.car.i.) to participate in the for- 

 mation of the circulus arteriosus. It also dispenses several 



