84 THE NERVOUS SYSTEM OF THE FROG 



well distended : the terminal branches must be dissected from the 

 ventral surface. 



IV. The Cranial Portion of the Sympathetic Nervous 

 System. 



The main sympathetic trunk of each side extends forwards 

 in front of the first ganglion, and enters the skull at the 

 foramen in the exoccipital bone through which the glosso- 

 pharyngeal and pneumogastric nerves pass out : it is connected 

 with the pneumogastric nerve, and then runs forwards within 

 the skull to the Gasserian ganglion of the trigeminal nerve, in 

 which it ends. 



0. Histology of Nerves. 



Nervous matter consists histologically of elements of two 

 kinds, nerve cells and nerve fibres. The nerve cells are 

 branching nucleated cells, connected by their processes with one 

 another and with the nerve fibres. The nerve cells are the 

 centres whence impulses originate, while the nerve fibres serve 

 to convey these impulses from one part to another. A local 

 accumulation of nerve cells is called a ganglion. 



1. Nerve Fibres are of two kinds, medullated and non- 



medullated. 



a. Medullated nerve fibres form almost the whole of the 

 cranial and spinal nerves, and a large part of the brain 

 and spinal cord. 



Take a small piece of the sciatic or some other nerve from a 

 freshly killed frog : spread it out and tease it in a drop of normal 

 salt solution : examine with low and high powers : note 



i. The nerve fibres : unbranched. 

 ii. The perineurium, or connective tissue binding the 

 nerve fibres into bundles, or " nerves." 



In each nerve fibre note 



iii. The primitive sheath, or sheath of Schwann : a 

 very delicate external investment, seen with 

 difficulty, and only in certain places. 



iv. The medullary sheath : a thick fatty layer within 

 the primitive sheath ; it is highly refractive, and 

 gives the nerve fibre its double contour. 



