192 NERVOUS SYSTEM OF VERTEBRATES. 



histogenesis of motor nerves (cf. p. 63). In higher vertebrates 

 when the myotomes lose their simple segmental arrangement 

 and become divided into special muscles, the relations of the 

 nerves to the muscle segments become more obscure. 



The greatest modifications affecting the ventral nerves occur 

 in connection with the innervation of the Hmbs. The limbs are 

 innervated by rami going out from plexuses formed by the union 

 of the ventral rami of several spinal nerves. Two such plexuses 

 are formed, the brachial for the fore limb, the lumbar plexus for 

 the hind limb. The presence of such plexuses is explained by 

 the mode of origin and evolution of the vertebrate limbs. The 

 Hmbs first arose as folds of skin and muscle extending along the 



Fig. 103. — An early stage in the formation of the pectoral fin and brachial 

 plexus in a selachian, Spinax. The muscle buds are in dark shading, the nerves 

 in black. 



side of the body. These long limb folds are formed in the cmbrj^os 

 of lower vertebrates by outgrowths or buds from a number of 

 myotomes together with mesenchyme (Fig. 103). The myotome 

 buds form the muscles of the limb; the mesenchyme gives rise to 

 the skeleton and the connective tissue into which the blood vessels 

 and nerves grow as the development of the limb proceeds. As many 

 nerves are involved in the innervation of the limb muscles as there 

 are myotomes involved in the limb fold. In higher vertebrates (and 

 in later stages in the growth of the individual), as the limb grows 

 in length it comes to have a shorter base where it is attached to 

 the trunk. At the same time the nerves in order to reach their 

 muscles which have shifted out into the limb, must converge to 

 enter the narrow base of the Umb. This together with various 

 modifications of position due to the formation of the special 

 muscles of the limbs have caused the nerves to unite into intricate 

 plexuses. 



