THE CRANIAL AND SPINAL NERVES. 271 



ridge, common to it and the glossopharyngeal nerve. At first the 

 pneumogastric is, if anything, the smaller of the two nerves, 

 but it soon becomes distinctly the larger. Like the glosso- 

 pharyngeal nerve, it early acquires multiple roots, the most 

 anterior of which is directly continuous with the hindmost of 

 the roots of the glossopharyngeal nerve, without entering the 

 brain. 



Beyond the roots of origin, the main stem of the pneumo- 

 gastric nerve runs downwards and backwards, parallel to the 

 glosso-pharyngeal nerve; it expands into a large fusiform gan- 

 glion, from which branches are given off to the second and third 

 branchial arches, as well as large branches to the heart, lungs, 

 and intestines. 



From the hindmost root of origin of the pneumogastric nerve 

 from the brain, a long commissural branch (Fig. 115, x") runs 

 backwards along the side of the medulla oblongata, and is con- 

 tinuous posteriorly with the ganglion of the first spinal nerve. 

 This commissural branch is derived from the part of the neural 

 ridge between the pneumogastric and first spinal nerves. 



The mode of development of the spinal accessory or eleventh 

 cranial nerve, and of the hypoglossal or twelfth cranial nerve, 

 has not been satisfactorily determined in the chick. The hypo- 

 glossal nerve has, from the first, the relations characteristic of 

 the ventral roots of the spinal nerves ; though whether it corre- 

 sponds to one, or to more than one, of such roots is not determined 

 with certainty. 



It is interesting to note that the definite relations of the 

 fifth, seventh, ninth, and tenth cranial nerves to the visceral 

 arches are as characteristically shown in an embryo chick of the 

 fifth day (Fig. 1 1 5) as they are throughout life in a typical water- 

 breathing Vertebrate such as a dogfish. 



c. The Spinal Nerves. 



I The dorsal roots of the spinal nerves develop, as already 

 ticed, in a manner practically identical with the typical 

 mial nerves. Their first appearance is almost simultaneous 

 th that of the cranial nerves ; they may be recognised in 

 ibryos in which the first two or three pairs of mesoblastic 



