NERVOUS SYSTEM 877 



. 



ply the viscera and circulatory system (Fig. 491). The dorsal and ven- 

 tral rami, after leaving the connection formed by dorsal and ventral roots, 

 contain mostly somatic fibers with just a few of the visceral type. The 



visceral ramus contains visceral fibers 

 alone. 



In the lower vertebrates, some of 

 the visceral motor fibers actually pass 

 through the dorsal root, so that one 

 cannot in all strictness say that the 



sh ' Fig 491 M dorsal root is exclusively sensory, but 



of cross section of spinal cord in mammals, dorsal roots are purely 



and ventral roots 



half of figure is that of a fish and right rnotnr 

 half that of the spinal cord of man. Note 



the relatively greater size of the dorsal TVi^ Vricr^ral fiK^rc '" nf ' fTi* eninol 



gray columns and dorsal funiculi in man. * ne VlSCCral hDCrS Ot tne Spinal 



This is correlated with the greater im- rtf * r , rf >~ ni - t-k-n^f i^o IKr nil ff*t-ti4- r\r 



portance in man of the assembling con- nerves are poetically all ctterent or 



nections between the cord and the frame. motor j n function. They ido'.IlOt paSS 



to their terminal organs, such as the smooth muscles, glands, etc., 

 directly, but always end in some sympathetic ganglion where there is 

 a functional connection made. From this ganglion the impulse is then 

 carried to a peripheral organ by axones from the sympathetic nerve cells. 



In the lower vertebrates, in the regions where the limbs form, there 

 are usually networks or plexuses formed. These are paired, as "cervico- 

 brachial" for the fore-limbs, and "lumbo-sacral" for the hind-limbs, 

 while in the higher vertebrates these have separated and there are defi- 

 nate cervical, brachial, lumbar and sacral plexuses. There are usually a 

 great number of nerves (up to about tweny-five) going to form a plexus, 

 in fact, twenty-five is the largest number known, and this occurs in the 

 pectoral fin of skates (Fig. 336). 



. As there is also a plexus in snakes and limbless lizards in the regions 

 where limbs usually develop, it is often thought that these animals de- 

 scended from limbed ancestors, although no trace of limbs occur during 

 their embryological development. It is from these plexuses that dorsal 

 and ventral branches of nerves pass out to the two sides of the limbs. In 

 all four-footed animals there is this simple arrangement of a single main 

 nerve trunk on the dorsal and ventral side of the limb, but in mammals 

 there are two nerve trunks on the ventral side of each limb. 



In fishes a connecting nerve, which joins some Of the anterior seg- 

 mental nerves with the limb-stem, passes to the lumbo-sacral plexus. 

 In some fishes, however, not only is there no connecting nerve, but even 

 a plexus is wanting, the spinal nerve entering directly into the limb. 

 The spinal nerves pass to and from the central nervous system, a pair 

 between each two vertebrae. Thev* receive their names from those of 



