ix.] THE NERVOUS SYSTEM. 399 



medulla, and giving off a branch to border the opposite sides 

 of the successive branchial apertures, and then continuing on 

 to the alimentary canal. It also, in Fishes, gives off a special 

 nerve, called the nervus lateralis (Fig. 349, /, and Fig. 350, o\ 

 to the outer skin of the lateral body-wall, immediately be- 

 neath what is called the " lateral line." By very rare ex- 

 ception the two pneumo-gastric nerves may unite and run 

 along the intestine to its extreme end. 



The third part of the eighth nerve (named the spinal 

 accessory) supplies the trapezius and sterno-mastoid muscles. 

 It may be altogether absent, as is the case in Serpents, and 

 in the whole of the Ichthyopsida. 



26. The NINTH NERVE is called the hypoglossal. It 

 arises by several roots from the medulla oblongata, and then 

 passes out through the anterior condyloid foramen, and is 

 distributed to the muscles of the tongue, the os hyoides, and 

 the larynx. In Fishes this nerve is usually called the "first 

 spinal ;" and in fact the essential nature of the hypoglossal 

 nerve is rather spinal than cerebral, being often closely con- 

 nected with what is commonly reckoned the first spinal, 

 even in Vertebrates above Fishes. 



Thus we see that the so-called cranial nerves belong to two 

 categories, (i) those of special sense, and (2) those which 

 border the primitive visceral clefts and send off filaments to 

 other structures. 



The fifth, seventh, and eighth nerves belong to the second 

 category ; the third, fourth, and sixth nerves may be regarded 

 as derived from the fifth. 



The eighth nerve is a most complex one, the glosso- 

 pharyngeal portion alone being made up of an undetermined 

 number of cranial nerves related to those visceral clefts and 

 arches which become obliterated in all Vertebrates above the 

 Ichthyopsida. 



27. 'The SPINAL NERVES arise in pairs from opposite sides 

 of the spinal marrow. They are related in number to the 

 divisions of the axial skeleton, or vertebrae, and (as has been 

 said in describing that skeleton) they pass out of the neural 

 canal either in the intervals between the neural arches, or by 

 direct perforations of those arches. 



Each nerve arises by two roots one anterior (or ventral), 

 the other posterior (or dorsal). The posterior, or dorsal, 

 root has a ganglionic enlargement (Fig. 339, B, Gii). 



After leaving the neural canal each nerve divides into two 

 conspicuous branches one dorsal, the other ventral ; but 



