THE SUBCLAVIAN ARTERY. 753 



number, {a) The inferior frontal is distributed to the inferior frontal convolutions, (b) the 

 ascending frontal passes to tlie lower portion of the ascending frontal convolution, (c) the 

 parietal supplies the whole of the ascending parietal convolution and the neighboring portions 

 of the inferior parietal, and (cf) the parieto-temporal passes to all the convolutions around the 

 posterior limb of the fissure of Sylvius. 



7. The Anterior Cerebral Artery. — The anterior cerebral artery (a. cerebri 

 anterior) (Fig. 700) is the smaller of the terminal branches of the internal carotid. 

 It passes -forward above the optic chiasma to the anterior end of the great longitudi- 

 nal fissure, and, bending upu'ard around the rostrum of the corpus callosum, is con- 

 tinued backward along the medial surface of the cerebral hemisphere to the posterior 

 portion of the parietal lobe. At its entrance into the great longitudinal fissure it is 

 connected with its fellow of the opposite side by a short transverse vessel termed 

 the anterior communicating artery (Fig. 702). 



Branches.— Immediately after it has crossed the optic chiasma the anterior cerebral artery 

 gives off a number of small central branches {antero-mesial ganglionic) , which penetrate the 

 base of the brain and are distributed to the lamina cinerea, the rostrum of the corpus callosum, 

 the septum lucidum, and the tip of the caudate nucleus. Throughout its course in the great 

 longitudinal fissure it gives branches to the corpus callosum and also cortical branches to the 

 medial and lateral surfaces of the cerebral hemisphere. These branches are (a) the orbital, 

 which vary in number and are distributed to the orbital surface of the frontal lobe, also sup- 

 plying the olfactory bulb ; (b) the anterior internal frontal, which supplies the anterior and 

 lower part of the marginal convolution and sends branches to the lateral surface of the hemis- 

 phere supplying the superior and middle frontal convolutions ; {c) the middle internal frontal, 

 which is distributed to the middle and posterior parts of the marginal convolution and to the 

 adjacent portions of the superior and ascending frontal and ascending parietal convolutions ; 

 and {d) the posterior internal frontal or quadrate, which, in addition to sending branches to the 

 torpus callosum, supplies the quadrate lobe and the upper part of the superior parietal convo- 

 lution. These branches anastomose upon the inferior and lateral surfaces of the hemisphere 

 with the branches of the middle cerebral artery, the main stem of the artery anastomosing 

 posteriorly with branches of the posterior cerebral. 



Anastomoses of the Carotid System. — Although the majority of the 

 anastomoses of the branches of the carotid arteries are with one another, yet there is 

 a sufhcient amount of communication with other vessels to allow of the establishment 

 of a collateral circulation after ligation of the common carotid of one side. The con- 

 nections which are available for the circulation in such a case are as follows. ( i ) 

 There is abundant communication between the branches of the right and left external 

 carotids across the median line ; (2) the anterior communicating artery forms an 

 important communication between the internal carotids of opposite sides ; (3) 

 anastomoses exist between the ascending cervical branch of the inferior thyroid, the 

 superficial cervical branch of the transversalis colli, and the deep cervical branch of 

 the superior intercostal, on the one hand, all of these being branches of the sub- 

 clavian artery, and the a. princeps cervicis, a branch of the occipital artery ; (4) 

 abundant communications exist between the terminal branches of the inferior thyroid 

 from the subclavian and the superior thyroid from the external carotid ; and, finally, 

 (5) by means of the posterior communicating artery the internal carotid may 

 receive blood from the posterior cerebral artery, which, through the basilar and 

 vertebral arteries, belongs to the subclavian system. 



THE SUBCLAVIAN ARTERY. . 



In the primary arrangement of the branchial blood-vessels, while there are two 

 aortic arches (Fig. 678), the two subclavian arteries arise symmetrically from these 

 arches as lateral segmental branches corresponding to the seventh cervical segment. 

 With the disappearance of the lower portion of the right arch, however, an apparent 

 lack of symmetry in their origin supervenes, the vessel of the right side arising from 

 the innominate stem, while that of the left side springs directly from the persist- 

 ing aortic arch. As a matter of fact, however, the proximal portion of the right aortic 

 arch is represented by the innominate stem, together v/ith a small portion of the 

 proximal end of the right subclavian artery, so that the original morphological sym- 



48 



