NERVOUS SYSTEM. 47 



striped and voluntary. The mesenchymatous muscles for the 

 most part consist of unstriped fibres, but some of them are 

 cross-striped, and even voluntary. The muscles of the heart 

 and oesophagus are examples of cross-striped mesenchymatous 

 muscles in the trunk ; they are not under the control of the will. 

 In the head many of the mesenchymatous muscles are cross- 

 striped and voluntary ; e.g. the facial muscles, the mandibular 

 and the branchial muscles. The eye-muscles are myotome 

 muscles, and supplied, as stated above, by ventral roots ; but 

 they differ from the muscles of the great lateral sheet of myotome 

 trunk muscles in the fact that their fibres are directed dorso- 

 ventrally, and not longitudinally, as in the latter. 



The dorsal nerve-cord extends in front of the notochord, and 

 is enlarged in front to form the brain, which is constructed on 

 the same fundamental plan in all classes. The posterior part 

 constitutes the spinal cord. The skeletal investment of the 

 brain is unsegmented, and constitutes the skull, while the spinal 

 cord lies in a tube of the vertebral column, which always shows 

 some sign of segmentation and is usually completely segmented. 

 The spinal nerves, which are segmentally arranged, possess 

 two roots, a dorsal and a ventral, which join. The dorsal of 

 these roots carries a ganglion and contains afferent nerve fibres ; 

 the ventral contains efferent fibres only. The brain possesses 

 ten pairs of nerves, which are very similarly arranged in all 

 Vertebrata. They differ from the spinal nerves in the fact 

 that except in the case of three of them, they have the dorsal 

 roots only. The third and sixth nerves may be regarded as 

 the ventral roots of the fifth and seventh nerves respectively, 

 and the fourth nerve must also be regarded as a ventral root, 

 though it arises from the dorsal surface of the brain. The ninth 

 and tenth nerves appear to be altogether without ventral roots. 

 In the higher Vertebrata there are two additional pairs of cranial 

 nerves, the eleventh and twelfth. Of the cranial nerves, the 

 fifth, seventh, eighth, ninth and tenth are usually regarded 

 as being serially homologous with the posterior roots of spinal 

 nerves, and are supposed to be related to a vanished segmenta- 

 tion of this part of the body.* They resemble these in having 



* A short account of the modern views on the nature of cranial nerves 

 and of nerves in general, and of cranial segmentation is given in the 

 chapter on Pisces. 



