'604 Dgnamic Theory. 



tion of the optic nerves, the disadvantage imposed upon them by the 



first decussation in the medulla oblongata. 



The steps through which the second decussation have been brought 

 sibout are undoubtedly repeated in the development of the embryo, but 

 in the mammals and birds it is with such celerity, and is so much short- 

 ened that it appears not to have been traced. It is reasonable to sup- 

 pose that in all cases in which there is decussation of the optic nerves 

 there is also a decussation of fibres somewhere below the optic lobes. 

 In the clupeidse ( Herring family ) there is a decussation in the anterior 

 pyramids of the medulla oblongata, followed by that of the optic nerves. 

 It must be that this decussation in the medulla is of earlier origin than 

 that of the optic nerves, and has, in fact, made the latter necessary. 

 It is difficult to see how a bilateral body could be operated without 

 nervous connections between the opposite sides, and there are such con- 

 nections amongst all, even down to the worms. But such general cross- 

 Ing of fibres as takes place in the mammals, by which the chief, if not 

 the entire, motor nerve service of one side of the body is furnished by 

 i the brain of the other side, must have originated from a state of things 

 in which the chief activity of the body was transverse. All articulates 

 possessed of jaws, work them by a lateral motion instead of a vertical 

 one. It seems probable that the great crossing of nerves, together with 

 that of muscles, originated in connection with these articulate jaws, and 

 that the arrangement persisted after the original articulate throat gan- 

 glia had been supplemented by the great additions that gradually placed 

 themselves in front of it, forming, in the vertebrates, the optic lobes, 

 cerebrum, &c. 



As to the principal cranial nerves they are generally homologous 

 throughout the mammal class, and, to a greater or less degree, through- 

 out the vertebrate sub-kingdom. The spinal nerves generally agree in 

 number with the vertebrae, and are classified as the cervical, dorsal, lum- 

 bar, sacral and caudal. Those supplying the limbs are necessarily 

 varied somewhat to suit the case, as in the Cetacea the nerves from the 

 lumbar plexus cannot supply hind limbs as in the other mammals, since 

 there are none. The anterior, or motor, part of the caudal nerves in 

 cetacea, form a trunk to supply the strong muscles of the tail. 



All the vertebrates above the Hags and Lampreys, have a sympathetic 

 nerve sj^stem. Owen observes that as this sympathetic system begins 

 in the myxinoids as an addition to an already existing cranio spinal sys- 

 tem, it is to be regarded as a specialty of the vertebrate sub-kingdom, 

 and as not having a counterpart in the nervous system of articulates. 

 The semilunar ganglions and the solar plexus, which are the great cen- 

 ters of this system, where it is fully developed, are located in the posi- 

 tions in which the first development of the system is seen in the myx- 

 inoids and in the embryo of the higher vertebrates. 



