THE NERVES. 155 



the seventh by numerous filaments ; being increased by an 

 accessory nerve from the spine {Plate V. Fig 1 .8,8); and 

 are of a very compound description ; consisting of three 

 separate nerves : the glosso pharyngeus going to the tongue 

 and pharynx; the pneumogastric distributed to the chest 

 and abdomen ; and the spinal accessory, bestowing its chief 

 action upon the neck before and behind. The ninth pair are 

 like distinct numerous white strings ; which ultimately unite 

 to form one nerve, and run to the tongue for motion {Plate 

 V. Fig 1 . 9, 9). These are all the nerves giving off within 

 the cranium. That which is mentioned as the tenth pair, 

 being in fact the first spinal nerves. 



The medulla oblongata {Plate V. Fig 1 . ^) is formed by 

 the continuation of the substances of the brain, and by two 

 pillars from each side ; each pillar being composed of three 

 bands : proceeding backward to the edge of the foramen 

 magnum of the occipital bone, it takes the name of spinal 

 marrow, and runs onward ; as will be shown presently. 



The arterial trunks enter the skull in a very con- 

 voluted manner; and anastomose freely with each other 

 within the substance of the brain. The blood is returned 

 by small veins ; which pour it into the sinuses, and these 

 empty their contents into the jugulars. 



The structure of the brain is fibrous. It is not in 

 itself an organ of great sensibihty ; on the contrary, some 

 portions of it, as of the cerebrum, are nearly insensible : 

 nor is the destruction of a small quantity of it neces- 

 sarily fatal. It can bear gradual pressure even to the 

 absorption of half its bulk ; as is seen in hydatids on 

 the brains of sheep ; but a slight sudden pressure, as of 

 apoplexy, destroys its life. Nothing is known of its 

 functions ; excepting that the cerebrum confers volition; that 

 the cerebelkira bestows command over muscular motion ; 

 and that the corpora quadrigemina are the roots of the 

 optic nerves. 



THE NERVES. 



The nerves are white fibrous cords, sent off either from 

 the cranium or spinal canal ; the ramifications of which nerves 

 are distributed to all parts of the body ; being enveloped in a 

 resistant membranous sheath called neurilema, to give them 



