anterior cerebral 

 ethmonasal 



hyoid efferent 



1st bronchial arch 



external carotid artery / ^^ ^^ =y-^ 



hyoid afferent thyroid artery 



A PETROAAYZON 



internal carotid 



ventral aorta 



B EPTATRETUS 



Figure 1 1-23. Semidiogrommatic lateral views of aortic arches of 

 lamprey, A, and Eptotrefus, B. (B, after Muller, 1839, and Goodrich, 

 1909) 



duced the right systemic arch to a subclavian stem. The 

 mammal pattern may have been derived from the amphib- 

 ian but not from the pattern observed in living reptiles. 

 A discussion of the aortic arches of fishes is best con- 

 sidered as a part of a more general discussion of the arteries 

 of the head. 



CIRCULATION IN THE HEAD 

 Arteries 



Mammals The main arteries of the head can be described 

 in terms of the human (Figure 1 1-24). The common carotid 

 branches in the neck to form internal and external carotids. 

 The external carotid sends branches to the thyroid area, a 

 lingual artery to the tongue, and external maxillary artery 

 up over the outside of the jaw, an occipital artery to the 

 back of the skull, a posterior auricular to the region behind 

 the ear, an internal maxillary to the inside of the jaws, and 

 a superficial temporal artery up over the side of the head 

 between the ear and eye. From the internal maxillary come 

 two deep temporals serving the jaw musculature. The ex- 

 ternal carotid also sends a pharyngeal artery up along the 

 internal carotid. 



The internal carotid passes just behind the external and 

 enters the skull through the carotid canal. It turns forward 

 to either side of the sella turcica, where it expands as a cav- 

 ernous portion (enclosed in a venous sinus), gives off the 



ophthalmic artery to the orbit, and divides to form the 

 middle and anterior cerebrals. Where it divides, a connec- 

 tive from the vertebral and spinal arteries joins the internal 

 carotid. 



Examination of other mammals indicates a rather similar 

 pattern. The usual main trunks are an external carotid, an 

 internal maxillary, and an internal carotid. In the rat, there 

 is also a stapedial artery. This enters the tympanic bulla 

 through its own foramen, perforates the stapes, passes through 

 a small canal in the periotic bone, and then exits from the 

 bulla. From here it passes forward through the alar 

 canal to the orbit, where it becomes the internal maxillary 

 stem. The internal carotid is also developed in the rat. The 

 external carotid differs in not supplying the internal maxil- 

 lary division. 



The cat differs in that the internal carotid stem is present 

 in the young but eliminated in the adult: the pharyngeal 

 artery replaces the carotid stem. The pharyngeal artery en- 

 ters the skull at the anterior carotid foramen to join the 

 circle of Willis at the base of the brain. The external ca- 

 rotid bears the internal maxillary as a branch, which has 

 much the same division as in the human. 



Of particular interest is the carotid rete which is observed 

 in most mammals. In the human the carotid stem expands 

 as the carotid sinus. This is enclosed by a cavernous venous 

 sinus to either edge of the sella turcica. In the cat the in- 

 ternal maxillary stem forms a rete, in the orbit just outside 

 the skull. This is enclosed by a venous rete from the ophthal- 

 mic veins. From the arterial rete several channels lead into 

 the carotid portion of the circle of Willis, and a small inter- 

 nal rete is associated with these connections. A second con- 

 nective, the middle meningeal, also forms a rete where it 

 joins the circle of Willis. Most animals have some sort of 

 close arterial and venous relationship, with the vein enclos- 

 ing the artery, in the orbit or inside the skull at the side of 

 the sella turcica. 



EMBRYOLOGICAL DEVELOPMENT In the earliest Stage of de- 

 velopment, there is a network of small vessels permeating 

 the tissues of the head into which the arteries feed and from 

 which the veins arise (Figure 1 1-25). With the appearance 

 of the aortic arches, there is an anterior extension from the 

 ventral aorta, the external carotid, and another from the dor- 

 sal aorta, the internal carotid. The internal carotid has 

 branches to the region of the eye and to the brain, the mid- 

 dle cerebral (Figure 11-26). 



Behind the level of the aortic arches, a number of seg- 

 mental arteries extend up toward the nerve tube, and from 

 these a vessel develops forward through the capillary net. 

 This vessel is the vertebral artery. It passes below and me- 

 dial to the ear region giving off branches to the cerebellar 

 region of the brain. The vessels of either side now join and 

 form the basilar artery, which gives off additional branches 

 to the cerebellar region of the brain. The anterior end of 

 this vessel forks as the posterior communicating branches of 



354 



THE CIRCULATORY SYSTEM 



