266 



THE DEVELOPMENT OP THE VASCULAR SYSTEM 



median ventral vessel, the basilar artery, which thus is connected cranially (by 

 way of the circulus arteriosus) with the internal carotids, caudad with the verte- 

 bral arteries. 



Fig. 276. — Arterial system of a human embryo of 10 mm. (His). X 18. Ic, Internal carotid artery; 

 P, pulmonary artery; Ve, vertebral artery; III-VI, persistent aortic arches. 



The internal carotids (Fig. 271), after giving off the ophthalmic arteries, give rise cra- 

 nially to the anterior cerebral artery, from which arise later the middle cerebral artery and the 



anterior chorioidal artery, aU of which supply the 

 brain. Caudalward many small branches to the 

 brain wall are given off and quite late in develop- 

 ment (48 mm. C R fetuses) these form a true pos- 

 terior cerebral artery (Mali). 



The ventral rami of the dorsal interseg- 

 mental arteries become prominent in the 

 thoracic and lumbar regions and persist as 

 the intercostal and lumbar arteries, segmentall}- 

 arranged in the adult. Longitudinal precos- 

 tal anastomoses (Fig. 274) constitute the 

 costo-cervical and thyreo-cervical trunks (Fig. 

 275). The subclavian and a portion of the 

 internal mammary artery are derived from the 

 ventral ramus of the seventh cervical seg- 

 mental artery. The remainder of the internal 

 mammary and the superior and inferior epi- 

 gastric arteries are formed by longitudinal 

 ventral anastomoses (Fig. 274) between the 

 extremities of the ventral rami from the thoracic and lumbar intersegmental 

 arteries, beginning with the second or third thoracic (Fig. 277). 



Fig. 277. — The development of the 

 internal mammary and deep epigastric 

 arteries in a human embryo of 13 mm. 

 (Mall in McMurrich). 



