318 ANATOMY OF VERTEBRATES. 



the limb A, in ' making ' the current, or completing the circuit ; 

 whilst contractions occur in the limb a in ' breaking ' the current, 

 as by removing one of the wires : the limb in which the current 

 is direct contracts on making the current ; the limb in which the 

 current is inverse contracts on breaking the current. It needs 

 only to leave one wire in the water, and to remove or introduce 

 the other, in order to i break ' or ' make ' the current. If the two 

 vessels be further connected by a conductor of copper wire, as at 

 O, fig. 209, contractions of both limbs take place on both making 

 and breaking the connection. 



Neuricity l is not electricity, any more than is myonicity ; 

 both are peculiar modes of polar force. Any point of the surface 

 of a nerve is positive in relation to any point of the transverse 

 section of the same nerve, just as any point of the surface of 

 a muscle is positive in relation to any point of the transverse 

 section of the same muscle. 2 Ligature of a nerve arrests the 

 nervous current, not the electric current ; a divided nerve con- 

 nected by an electric conductor transmits the electric current ; 

 but the nervous current excited by stimulus above the section 

 is arrested by the electric conductor. Neuricity is convertible 

 into myonicity and into other forms of polar force, just as 

 myonicity or the muscular force may be disposed of by conver- 

 sion into heat, 3 electricity, 4 and chemicity, the latter shown by the 

 evolution of carbonic acid. 5 Molecular change, in nervous and in 

 muscular fibre, attends the exercise of their respective forces. 



§ 57. Sympathetic system. — This consists of one or more ganglia, 

 usually a series of such arranged on each side of the vertebral 

 centres from near the occiput to the opposite end of the abdominal 

 cavity, or to the anterior caudal vertebra?. Where the ganglia are 

 numerous they are connected in each lateral series by a band of 

 nervous fibres, and resemble a pair of gangliated cords. These 

 communicate with the contiguous spinal nerves, and with the 

 cranial nerves, usually through small ganglia in different parts of 

 the head, fig. 206. At the caudal end the two sympathetic cords 

 usually unite with a single ganglion in the under or fore part of 

 the body of the anterior caudal vertebra. 



A sympathetic ganglion is a body connected with bundles of 

 nerve-fibres, the chief proceeding to or from it in the direction of 

 its axis, the smaller nerves diverging more or less transversely. 



1 'Vis nervosa,' 'Nervous force,' ' Nervous fluid;' it is in relation to the latter name, 

 expressive of an exploded idea, that the term 'current' is still used in reference to the 

 course of the polar force, whether nervous, magnetic, or electric. 



2 ccxi. 3 ccix. * lb. s lb. 



