ANATOMY OF A 17.8 MM. HUMAN EMBRYO 77 



branches to the lateral and dorsal surfaces of the di- and 

 mesencephalon {Dien. and Mesen.), a considerable branch to the 

 mesencephalon passing through the rootlets of the oculomotor 

 nerve {N.oc, plate 2). The left superior cerebellar artery 

 (A.cereh.s.) arising from the basilar is seen in plate 2, just 

 caudad of the trochlear nerve. Its branches overlie the isthmus 

 and the metencephalon. The other branches of the basilar 

 are seen in part in plate 1, those on the left side having been cut 

 away. 



The left subclavian artery (A.scL, plate 2) is a branch of the 

 corresponding seventh cervical intersegmental artery. Beyond 

 the origin of the vertebral artery {A. vert.), the thyreo-cervical 

 {Tr.thyr.-cerv.) arises from the cephalic aspect of the subclavian; 

 the costo-cervical (Trxost.-cerv.) from the dorsal; and the internal 

 mammary {A.mam.i.) from the ventral aspect. The costo- 

 cervical divides, as usual, into the deep cervical and superior 

 intercostal; the others can be identified by their direction. The 

 remaining portion of the subclavian trunk extends ventro-later- 

 ally into the upper limb. 



On the right side (plate 1) there is a common anomaly, the 

 sixth (not the seventh) cervical intersegmental being connected 

 with the right dorso-lateral aorta. By means of the right dorso- 

 lateral aorta and a short precostal anastomosis the sixth inter- 

 segmental is connected with the seventh. 



Consequently the right subclavian comprises three elements 

 (exclusive of the parts derived from the fourth aortic arch and 

 dorso-lateral aorta). These are: (1) the root of the sixth cer- 

 vical intersegmental artery; (2) a longitudinal precostal anasto- 

 mosis connecting the sixth and seventh intersegmentals; and (3) 

 a portion of the seventh intersegmental. From the first element 

 arises the right vertebral artery {A.vert.d.), the root of which 

 represents a part of the sixth cervical segmental. From the 

 third element the right superior intercostal arises on the caudal 

 aspect; the thyreo-cervical on the cephalic; the deep cervical on 

 the dorsal, and the internal mammary on the medial aspect. 

 The deep cervical is derived from the dorsal ramus of the seventh 

 intersegmental artery. The internal mammary represents the 



