98 VERTEBRATES WITHOUT JAWS iv. 9 



extends far back along the intestine. There is little contribution of 

 fibres from the spinal nerves to the alimentary canal, since this has no 

 mesentery, being attaached only at its cranial and caudal ends. There 

 are, however, numerous fibres from the spinal nerves to the rectum, 

 ureters, and cloacal region, and numerous postganglionic neurons 

 are found here. Nerve-cells are also found in the intestinal plexuses. 



The sympathetic system consists of isolated fibres running in both 

 dorsal and ventral roots. Many of these run directly to their endings, 

 for instance in the arteries, without interpolation of neurons. A few 

 postganglionic cells are present, however, but they are seldom collec- 

 ted into ganglia. The system is therefore even more scattered than 

 in elasmobranchs (p. 173). The 'adrenal' system is also diffuse. There 

 are scattered masses of interrenal (cortical) tissue and large groups 

 of suprarenal (medullary) cells, especially in the walls of the veins and 

 the heart. The suprarenal tissue receives 'preganglionic' fibres from 

 the spinal nerves. Its cells sometimes seem to be connected with each 

 other by fibres like those of neurons and they may operate a form of 

 control intermediate between nervous and hormonal (Johnels, 1956). 



The nerve-fibres in the nervous system of cyclostomes are not pro- 

 vided with myelin sheaths ; in this they resemble the nerves of amphi- 

 oxus. Conduction is slow in such non-medullated fibres, the only case 

 actually investigated in cyclostomes being the lateral line nerve of 

 Bdellostoma, found by Carlson to conduct at the low rate of 5 metres 

 a second (frog about 50 m/sec, mammals up to 100 m/sec). 



The spinal cord is of a uniform transparent grey colour and is 

 flattened dorso-ventrally, apparently to allow access of oxygen, and 

 metabolites, no blood-vessels being present within the cord. How- 

 ever, vessels are present in Myxine in which the cord is also flat. The 

 nerve-cell bodies lie, as in higher vertebrates, towards the centre, but 

 the synaptic contacts are not made in this 'grey' matter but at the 

 periphery, in what would correspond to the white matter of higher 

 forms. The outer part of the cord is thus made up of a neuropil or 

 nerve feltwork, formed of the terminations of the incoming sensory 

 fibres and the dendrites of the motor-cells. These cells (Fig. 63) lie 

 in the ventral part of the cord, their axons running out to make the 

 large fibres of the ventral roots and their dendrites passing to all parts 

 of the peripheral regions of both the same and the opposite sides of the 

 cord. They are thus presumably able to be stimulated directly by 

 impulses in the processes of the afferent fibres that end in these 

 regions. 



Direct control of the spinal cord from the brain is obtained through 



