NERYOUS SYSTEM AND GENERAL SENSATION. 29 



and mammals, the nervous system is composed of two prin- 

 cipal masses, the spinal cord (fig. 19), which runs along the 

 back, and the brain (fig. 20), contained within the skull.* 

 The volume of the brain is proportionally larger, as the animal 

 occupies a more elevated rank in the scale of life. Man, 

 who stands at the head of creation, is in this respect also the 

 most highly endowed being. 



79. With the brain and spinal cord the nerves are con- 

 nected, which are distributed, in the form of branching threads, 

 through every part of the body. The branches which unite 

 with the brain are nine pairs, called the cerebral nerves, and 

 are destined chiefly for the organs of sense located in the head. 

 Those which join the spinal cord are also in pairs, one pair 

 for each vertebra or joint of the back. The number of pairs 

 varies, therefore, in different classes and families, according to 

 the number of vertebrae. Each spinal nerve is double, being 

 composed of two threads, which at their junction with the 

 cord are separate, and afterwards accompany each other 

 throughout their whole course. The anterior thread transmits 

 the commands of the will, which induce motion ; the pos- 

 terior receives and conveys impressions to the brain, to pro- 

 duce sensation. 



STRUCTURE OF THE PRIMARY FIBRES OF NERVES. 



[ 80. Whoever would acquire a knowledge of the minute 

 anatomy of the nervous system, had better begin by examining 

 one of the peripheral nerves. Let a piece of one of the trunks 

 or branches of a nerve, that can easily be dissected out, be 

 chosen, and laid upon a glass plate : here let the nervous 

 bundles be separated or teazed out by the aid of a needle in 

 either hand, until free spaces of the glass plate appear ; let 

 the preparation now have a drop of serum or of albumen added 

 to it, and then be covered with a piece of thin glass. Under 

 a magnifying power of from three to four hundred diameters, 

 numbers of transparent cylindrical, straight, or slightly 

 sinuous filaments will- be perceived as the chief structure, 



* The brain is composed of several distinct parts, which vary greatly, in 

 their relative proportions, in different animals, as will appear hereafter. 

 They are : 1. The medulla oblongata; 2. Cerebellum; 3. Optic lobes; 

 4. Cerebral hemispheres ; 5. Olfactory lobes ; 6. The Pituitary body ; 

 7. The Pineal body. See figures 19, 20. The spinal cord is composed of 

 four nervous columns. 



