THE CEANIAL AKD SPINAL NEEVES. 385 



glion of the first spinal nerve, with which it is closely connected 

 (of. Pig. 115). The roots of origin of the pneumogastric nerve 

 converge to form a single large trunk, which lies immediately 

 behind the trunk of the glosso-pharyngeal nerve ; it expands to 

 form a large oval ganglion, beyond which it divides into a small 

 branch to the second branchial arch, and a much larger visceral 

 branch which runs backwards to the heart, lungs, and stomach. 



XI. The spinal accessory or eleventh cranial nerve. The 

 development of this nerve has not yet been determined in the 

 rabbit. 



XII. The hypoglossal or twelfth cranial nerve. Exactly 

 opposite the roots of the pneumogastric nerve in rabbit embryos 

 of the twelfth day, but arising from the medulla oblongata at a 

 more ventral level, and nearer the median plane, is a second 

 set of nerve roots. These are quite as numerous as the more 

 dorsally placed pneumogastric roots, but are more slender ; they 

 converge, and unite to form the hypoglossal nerve. Anatomically, 

 the roots of the pneumogastric and hypoglossal nerves, at this 

 stage, have relations closely comparable to those between the 

 dorsal and ventral roots of a spinal nerve ; but it is not yet 

 clear whether this comparison has any morphological value. The 

 roots of the hypoglossal nerve are probably correctly regarded 

 as belonging to the same category as the ventral spinal roots, 

 but their relations to the roots of the pneumogastric must be 

 considered at present as much more doubtful. 



The Spinal Nerves. 



The earliest stages in the development of the spinal nerves 

 have not yet been described in the rabbit. The dorsal roots 

 and the ganglia are clearly established by the end of the ninth 

 day ; the ventral roots develop later, apparently during the 

 tenth day. 



The ganglia of the dorsal roots are at first of very consider- 

 able width ; almost as wide, in fact, as the mesoblastic somites ; 

 so that on the twelfth day there are hardly any intervals between 

 the spinal nerves, the successive ganglia being practically in 

 contact with one another along the greater part of the length of 

 the spinal cord. 



Beyond the ganglia the nerves narrow rapidly, and have the 

 normal proportions. The main divisions of the spinal nerves are 



C C 



