AMPHIBIA. 215 



coccygeal nerve proceeds from the foramen in the urostyle, and 

 supplies the posterior viscera. It is also connected with the 

 sciatic plexus. 



(3) The sympathetic system is made up of a slender cord on 

 each side, lying just beneath the backbone. It is dilated at 

 intervals into ten sympathetic ganglia, each connected by one 

 or more rami communicantes with the corresponding spinal nerve. 

 In front it passes through the vagus-foramen, and is united with 

 the Gasserian ganglion, while in the region of the dorsal aorta 

 the two cords are closely bound to the dorsal side of that vessel. 

 The internal organs, including the vascular system, are supplied 

 by nerves coming off from the sympathetic ganglia, and breaking 

 up into plexuses. 



Histology (Fig. 61). The ganglion cells of the cerebro-spinal 

 axis are mostly multipolar. They make up the greater part of 

 the grey matter, which forms a central core to the spinal cord, 

 medulla oblongata, optic lobes, and optic thalami, and an external 

 crust to the cerebral hemispheres. The corpora striata are also 

 composed of grey matter. Bipolar ganglion-cells of peculiar 

 type, in which one process is twisted spirally round the other, 

 have been found in the sympathetic ganglia, and unipolar cells 

 in the spinal ganglia. The processes of the ganglion cells either 

 become continuous with the axis cylinders of nerve-fibres or 

 branch, and form networks. 



The nerve-fibres are either non-medullated or medullated. The 

 non-medullated (3) are made up of axis-cylinder and primitive 

 sheath with nerve-corpuscles. They are the only kind found in 

 the sympathetic system and olfactory nerves, besides which they 

 are common in the other parts of the nervous system. The 

 medullated fibres possess a layer of fatty matter, the medullary 

 sheath, between the axis-cylinder and primitive sheath. It is 

 broken up into short segments at the nodes of Schmidt, and at less 

 frequent intervals nodes of Ranvier occur where the primitive 

 sheath is constricted and touches the axis-cylinder. Each inter- 

 nodal segment, bounded at each end by one of these last, generally 

 possesses a single nerve-corpuscle. Medullated fibres make up 

 most of the white matter of the cerebro-spinal axis, which forms 

 the external part of the spinal cord, medulla oblongata, optic 

 lobes, and optic thalami, and the internal part of the cerebral 

 hemispheres. The crura cerebri are also of white matter. 



