134 THE HUMAN BODY. 



termed a plexus; but this union is effected solely by the 

 neurilemma. One nerve-fibre properly speaking never is 

 confounded with another. It runs without interruption, and 

 always distinct, through the most intricate net-work, from the 

 nervous centre to the organ which it serves. From the 

 analogy with the union of the blood-vessels, we speak of the 

 anastomosis of the nerves, and we shall soon see that if the 

 union of the vessels with each other is an essential condition 

 of the circulation of the blood, the distinct and absolute in- 

 dependence of the nerves, even to their minutest ramifica- 

 tions, is no less necessary to the integrity of the nervous 

 functions. We may therefore compare the union of the 

 nerves by juxtaposition during their course to a bundle of 

 electric wires which, though united, are always distinct, 

 because of their isolating covering. The isolating covering 

 of the nerves is the neurilemma. 



M. Sappey has recently described, under the name of the 

 nervi nervornm, or nerves of the nerves, the filaments which 

 run to the neurilemma, and which stand in precisely the 

 same relation to the nerves that the nerves themselves do 

 to the entire organism. 



There are two orders of nerves: the one, under the influ- 

 ence of the will, causes motion in the organs ; these are the 

 nerves of animal life: the other presides over the functions of 

 the viscera without our consciousness, and without any effort 

 of will; these are the nerves of organic life. The first are 

 cranial or spinal nerves, and spring directly from the nervous 

 centres, they are white and generally of a resistant texture; 

 the second, ganglionic or visceral nerves, although connected 

 with the nervous centre, form a system apart, which is called 

 the great sympathetic. These nerves are for the most part 

 soft, and of a grayish colour. 



Cranial and spinal nerves. These nerves are all disposed 

 in twos, and form a series of pairs to the number of forty, 

 of which nine pairs are cranial or cerebral, and thirty-one 

 pairs are spinal. 



The cranial nerves are classed as follows : 



ist pair. Olfactory nerves, which are ramified in the 

 organ of smell. 



