SANGUIFEROUS SYSTEM. 509 



basilar artery, which is distributed chiefly in the cerebellum 

 and posterior parts of the brain. In the bradypus tridactylus 

 the vertebral artery enters the foramen of the eighth 

 vertebra from the occiput, in most mammalia it enters 

 that of the seventh, and in man and many other species 

 it enters that of the sixth or the fifth cervical vertebrae. 

 Instead of a rete mirabile, it assumes a tortuous course, like 

 the internal carotid, before entering the cranium. The ver- 

 tebral artery is often as large as the internal carotid, as in 

 the guinea-pig and the agouti, where the latter artery is only 

 a small branch of the internal maxillary, and where the circle 

 of Willis is formed principally by the branches of the large 

 basilar artery. In some other species, as the marmot and 

 the porcupine, the vertebral arteries exceed in magnitude the 

 internal carotids. In the hybernating cheiroptera, which 

 generally hang suspended by the feet with their head down- 

 wards, and where there is no protecting rete mirabile on the 

 internal carotid, the cerebral blood is chiefly conveyed through 

 the large vertebral arteries. In the ruminantia, as in birds, 

 the vertebral arteries are distributed principally on the ex- 

 terior, and not the internal, parts of the head, these vessels 

 anastomosing more or less extensively with the occipital 

 branches of the external carotids, and not passing inwards 

 to form the basilar artery : so that these quadrupeds, with 

 their small internal carotids, have generally the exterior 

 parts of their head greatly developed, at the expense of the 

 more important intellectual organs. The great size of the 

 cerebellum of rodentia, and the imperfect development of 

 their smooth bird-like cerebral hemispheres, appear also to 

 be connected with the magnitude of their vertebral arteries 

 and the minuteness of their internal carotid or cerebral 

 arteries. 



The superior intercostals supply the anterior intercostal 

 spaces ; the internal mammaries send branches to the 

 diaphragm, the mediastinum, the pericardium, and to the 

 mammary glands when pectoral, and also branches to anas- 

 tomose with the epigastrics from the external iliacs ; the 

 scapular and cervical arteries chiefly supply the muscles of 

 the shoulder and neck. The great trunk of the subclavian 

 continues its course as axillary and brachial artery, sending 

 off numerous thoracic and circumflex branches to the mus- 



