274 FROGS, TOADS, AND NEWTS [CH. VIII 



ventral surface. The glands consist of numerous tubes 

 whose epithelium secretes a milky calcareous fluid : they 

 are highly vascular and appear in the tadpole prior to the 

 development of the limbs. Their function is unknown. 



A spinal nerve then, after the union of the roots, is 

 a mixed motor and sensory nerve. These ten pairs of 

 nerves are distributed to nearly all parts of the body not 

 supplied by the cranial nerves. The remainder is in- 

 nervated by what is known as the Sympathetic System. 

 This consists of a row of nerve ganglia, connected by 

 nerve fibres and lying on either side of the vertebral 

 column but continued forward inside the cranial cavity. 

 One sympathetic ganglion is usually associated with each 

 spinal nerve and united to it by a " ramus communicans " ; 

 with the tenth spinal nerve more than one ganglion may 

 be connected. The right and left cords are united to one 

 another by numerous fine nerve strands which surround 

 the dorsal aorta and supply fibres to the neighbouring 

 vessels and organs. Sympathetic fibres run with the 

 spinal nerves to all parts of the body, and there are special 

 bundles of them passing to the heart, stomach, liver, and 

 other abdominal viscera. They are especially concerned 

 with regulating the calibre of blood vessels and the degree 

 of contraction of the muscular walls of hollow viscera in 

 general. The heart possesses the power of continuing to 

 beat after all its nerve connexions have been severed, and 

 indeed after it has been removed from the body : its rate 

 of beating is however under nervous control by means of 

 the various nerve fibres reaching it by the vagus and 

 sympathetic branches. 



