INTRODUCTION. 25 



the globe of the eye. The fifth pair are very extensive nerves : they 

 divide into three separate branches, which ramify into numerous fila- 

 ments or twigs, and are distributed over the whole of the face, and in 

 some degree give the various muscles the power of motion, but more 

 particularly confer on the parts to which they go the faculty of feel- 

 ing. The sixth pair go to the muscles of the eye. One division of 

 the seventh pair is distributed over the internal parts of the ear, and 

 on it depends the sense of hearing : the other portion is that from 

 which the muscles of the face mainly derive their power of motion. 

 The eighth pair are principally distributed over the organs contained 

 in the chest and belly: they give the power of motion, but motion 

 altogether independent of the will ; and they have nothing to do with 

 sensation. The ninth pair go to the tongue, and give it the faculty 

 of taste. 



These nine pairs of nerves proceeding in regular succession from 

 the brain, may be readily seen by gradually raising that organ from 

 the fore part of the cavity of the skull. 



The Spinal Marrow. — When the brain passes out through the large 

 opening at ihe back part of the skull into the canal rf the spine, it is 

 called the spinal cord or marrow. It extends through the whole length 

 of the spine, and gives origin to numerous nerves, which pass through 

 notches formed between the junction of each of the bones, and are 

 distributed over the whole of the exterior and to some of the internal 

 organs of the body. They convey to the whole machine the power 

 of feeling and of moving. 



The Organs of Circulation. — Every part of the bofly is supplied 

 with blood by means of the heart and the vessels arising from it; and 

 the regular course in which it flows from the heart and back to it 

 again is denominated the circulation of the blood. 



The Heart is situated about the ndddle of the chesty rather inclining 

 to the left side, and rests upon the breast-bone. 



It may be considered as double ; and it consists of two cavities on 

 either side. The upper one, on the right side, the auricle, so called 

 from its supposed resemblance to a dog's ear, receives the blood 

 which has circulated through the frame, and pours it into the lower 

 one, tlie ventricle. As soon as that is filled, it contracts upon its 

 contents; and, as it closes, a membrane or valve rises, which pre- 

 vents the return of the blood into the auricle, and forces it into vessels 

 that carry it into the lungs, where it undergoes that purification which 

 is necessary to sustain the life of the animal. Having been thus 

 purified, it is returned to the heart, and enters the left auricle ; thence 

 it is poured into the left ventricle, and, that contracting, and a similar 

 membrane or valve rising, to prevent its flowing back into the auricle, 

 it is sent into the main trunk of the arteries, and thus distributed 

 over the whole of the frame. 



The blood flows through the arteries by the force impressed upon 

 it by the heart. This is felt in the pulsations of the arteries, which 

 correspond with the contractions of the heart, and indicate not only 

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