508 SANGUIFEROUS SYSTEM. 



few other quadrupeds, whose living habits necessitate this 

 protection, and whose cerebral blood is chiefly derived 

 from the internal carotid. In the erect bodies of man and 

 the higher quadrumana it is not developed, nor is it required 

 in the small internal carotids of the rodentia, where these 

 vessels are often less than the vertebral arteries, nor in the 

 plantigrade carnivora. On traversing the carotid canal of 

 the temporal bone, and the cavernous sinus of the dura 

 mater, the internal carotid artery, on each side, sends for- 

 wards an ophthalmic branch to pass, with the optic nerve, 

 through the foramen opticum, to the organ of vision and 

 the surrounding parts. It sends backwards a posterior com- 

 municating branch to unite with the posterior cerebral branch 

 of the basilar, and, advancing forwards, it gives off the 

 anterior cerebral which anastomoses with its opposite, by a 

 very short anterior communicating branch, before turning 

 upwards to ramify on the cerebral convolutions above the 

 corpus callosum. The internal carotid sends outwards like- 

 wise a middle cerebral artery, along the fissure of Sylvius, to 

 be distributed chiefly on the anterior and middle parts of the 

 brain. The numerous windings, anastomoses, and subdi- 

 visions of the internal carotid or cerebral arteries on the pia 

 mater of mammalia, and the more complicated retia mira- 

 bilia, serve to prepare these nutritious vessels, for pene- 

 trating in safety the delicate texture of the brain, as the 

 nutritious arteries of bone subdivide on its perios- 

 tium. 



The subclavian artery on each side (142. A. b. k.} is chiefly 

 appropriated to the atlantal extremities, and varies in its size 

 and distribution according to the development and form of 

 these members; it sends also branches to the head and 

 anterior parts of the trunk, as the vertebral, the superior 

 intercostal, the internal mammary, several scapular and 

 cervical arteries, and, in man, the inferior thyroid which 

 in other mammalia comes from the external carotid. The 

 vertebral arteries ascend to the foramen occipitale, through 

 the foramina in the transverse processes of the cervical ver- 

 tebrae, giving off branches to the spinal chord and the dura 

 mater ; and, after winding round the articular processes of 

 the atlas, and entering the occipital foramen, they unite 

 below the medulla oblongata, to compose the trunk of the 



