anterior cerebral artery_ 

 anterior circle of Willis 

 middle cerebroL 



posterior communicating 



ophthalmic artery 



nternol carotid 



superior cerebellar 



palatine arteries 



bosilor artery, 

 ertebral artery 



anterior inferior cerebellar 



posterior inferior cerebellar- 



— middle cerebral 



~ ophthalmic artery 



moxillory ortery, 

 internal carotid 



lingual. 



vertebral artery 



external carotid 



thyroid 



common carotid stem 

 brachiocephalic 



pulmonary artery, 

 subclavian artery 



radix oortoe 



internal mommory 



ductus arteriosus 



dorsal aorta 



Figure 1 1-26. Three stages of anterior arterial development in man. 

 (After Patten, 1946) 



the circle of Willis; these join the internal carotids. The an- 

 terior cerebrals now appear, and there is a small communi- 

 cating branch between them which completes the anterior 

 half of the circle of Willis. The anterior cerebrals extend 

 forward and upward on the inner surface of the two cere- 

 bral lobes. 



The spinal artery arises as a posterior midline develop- 

 ment from the basilar artery, and it extends back inside of 

 the vertebral canal. The ophthalmic artery arises as a branch 

 of the internal carotid just before it is joined by the poste- 

 rior communicating division of the basilar artery. 



Meanwhile the external carotid, the anterior extension of 

 each of the paired ventral aortae, has given rise to the ex- 

 ternal maxillary, the lingual, and the thyroid arteries. 



RepHles In the reptile (Figure 11-27) the external carotid 

 is represented by the lingual (or mental) branch of the com- 

 mon carotid. The carotid stem branches to form the inter- 

 nal carotid and the stapedial artery. The internal carotid 

 passes ventromedially into the parabasal canal. Here it gives 

 rise to a palatine artery, which emerges above the palatine 

 bone, then enters the sella to supply the middle and ante- 

 rior cerebral arteries. In Sphenodon a parabasal canal is 



lacking; the palatine branch arises before the internal ca- 

 rotid enters the basis cranii. An ophthalmic artery, from the 

 internal carotid, accompanies the optic nerve. 



The stapedial artery passes upward and outward in front 

 of the stapes or columella and gives rise to two branches. 

 One is a mandibular branch, which extends down inside 

 the quadrate to the mandible and enters the bone by way 

 of the mandibular fossa and foramen. The other, the inter- 

 nal maxillary stem, passes forward through the cranio- 

 quadrate passage and gives rise to a superior orbital artery, 

 then continues down and forward to serve the upper jaw 

 and snout area. 



The above description applies to Sphenodon or to Iguana. 

 In the turtle the picture is much the same, one difference 

 being that a second stem passes outward from the internal 

 carotid to join the lingual (the external carotid). This new 

 stem now replaces the old, more proximal to the heart, stem 

 of the lingual artery. In the crocodile a similar situation oc- 

 curs; the internal carotid is connected with the mandibular 

 and lingual arteries by way of a vessel similar to that of the 

 turtle. There is no common carotid vessel. 



In the bird the external and internal carotids arise as in 

 the other groups, but, as a result of the elongation of the 

 neck, the external becomes a branch of the internal. The 

 external carotid stem may be preserved as the superficial 

 cervical, as in the crocodile. 



The pattern observed in the reptiles is quite comparable 

 to that of the mammals. In mammals the external carotid 

 has become a larger stem and has taken over the distribu- 

 tion of blood to much of the surface of the head including 

 the usual maxillary and mandibular branches. This ascend- 

 ancy of the external carotid is a distinctive mammalian fea- 

 ture, even though it is not found in all mammals. In the case 

 o( Erinaceus, a fairly reptilian distribution is observed, with the 

 stapedial and internal carotids serving the entire head and 

 the external carotid serving only the lingual area. 



Amphibians In the amphibian we see further evidence of 

 the evolution of the head arteries. In the case of the sala- 

 mander, Nedurus. the carotid stem has internal and external 

 divisions which serve the head (Figure 1 1-28). The external 

 carotid goes to the musculature of the throat as a lingual 

 artery. 



The internal carotid stem passes forward below the stapes 

 and upward in front of that structure where it divides. 

 There is a posteriorly extending temporal division passing 

 above the stapes, a mandibular (and hyoid) division extend- 

 ing ventrolaterally behind the quadrate, and an internal 

 division which enters the parabasal canal. The parabasal 

 division gives rise to the palatine artery, then enters the 

 cranial cavity to branch as the middle and anterior cere- 

 bral vessels and the ophthalmic vessel. The last division of 

 the internal carotid stem passes forward through the cranio- 

 quadrate fissure, supplies the muscles in the region behind 

 the eye, and passes down laterally to become the internal and 



356 • THE CIRCULATORY SYSTEM 



