390 COMPARATIVE ZOOLOGY OF CHORDATES 



No autonomic system is known in Amphioxus. In Petro- 

 myzon, neurons are found along the gut, connected with the 

 vagus and probably with " pelvic " nerves. The parasympa- 

 thetic system is therefore present. The sympathetic system 

 is, on the other hand, not well developed, and imperfectly 

 differentiated from the supra-renal elements. Groups of these 

 cells are found near the spinal nerves and the blood-vessels, but 

 they are not joined together by sympathetic chains. Parallel 

 with this poor development of the sympathetic component of 

 the autonomic system in Petromyzon, it may be mentioned 

 that that animal has no oviduct or bladder, and no smooth 

 muscle under the skin. In the head the eyes are degenerate, 

 and there are no salivary glands, and this is parallel with the 

 absence of differentiated cranial autonomic ganglia. In 

 Selachians, the sympathetic ganglia are joined together by 

 the longitudinal lateral chains, and the ciliary ganglion is 

 present in the head. With the land-vertebrates the full 

 development of the autonomic system appears. 



It is not easy to see why the exciter neurons for smooth 

 muscles and glands should migrate out of the central nervous 

 system as they do, and take up positions outside it. It 

 is also very remarkable that some of them should connect 

 with the central nervous system through dorsal nerve-roots 

 (hindbrain outflow of parasympathetic), while others should 

 connect through ventral nerve-roots (midbrain and sacral 

 outflow of parasympathetic and the entire sympathetic). In 

 this connexion it may be noted that in Amphioxus the smooth 

 muscles of the body are innervated through the dorsal nerve- 

 roots, while the ventral roots contain only fibres belonging to 

 the somatic system. The primitive course for fibres innervat- 

 ing smooth muscle, therefore, appears to be through the dorsal 

 nerve-roots, and this primitive feature is retained in the case 

 of the hindbrain (facial, glossopharyngeal and vagus) outflow 

 of the parasympathetic system, but lost in all the rest. 



Literature 



luntary Nervoi 

 Lanoley, J. N. Autonomic Nervous System. Heffer, London, 1921. 



Gaskell, W. H. The Involuntary Nervous System. Longmans, Green, 

 London, 1920. 



