902 



THE NERVOUS SYSTEM 



branches are similar in function to the descending brunches, save that they become associated 

 with the grey substance of segments of the cord above rather than below the level of their 

 entrance. The long branches convey impulses destined for the structures of the brain, and pass 

 upwards in the fasciculus gracilis or fasciculus cuneatus of the cord, and terminate in the 

 nuclei of these fasciculi in the medulla oblongata (lifts. li-Hi and 048). 



Aberrant spinal ganglia. In serial sections on either side of the spinal ganglion of a 

 nerve there may often be found outlying cells either scattered or in groups of sufficient, size 

 to be called small ganglia. Such are more often found in the dorsal roots of the lumbar and sacral 

 nerves. These cells are nothing more than spinal ganglion-cells displaced in the growth 

 processes, and have the same nature and function as those in the ganglion. In some 

 animals occasional cells very rarely have been found in the outer portion of the vent.-al root. 

 These probably represent afferent fibres which enter the cord by way of the ventral root. Like- 

 wise, especially in the birds and amphibia, it has been shown that occasional efferent fibres 

 may pass from the grey substance of the cord to the periphery by way of the dorsal instead of 

 the ventral root. 



Relative size of the roots. The sensory or dorsal root is larger than the ventral 

 root, indicating that the sensory area to be supplied is greater and perhaps more 

 abundantly innervated than the area requiring motor fibres. It has been shown 

 that in the entire thirty-one spinal nerves of one side of the body of man the dorsal 

 root fibres number 653,627, while all the corresponding ventral roots contain but 233,- 

 700 fibres, a ratio of 3.2 : 1. (Ingbert.) In the increase in the size of the nerves for the 



FIG. 671. DORSAL AND VENTRAL VIEWS OF SPINAL CORD SHOWING MANNER OF ATTACHMENT 



OF DORSAL AND VENTRAL ROOTS. 



AXTERO-LATERAL SULCUS {line of ventral roots) 



POSTERIOR UED1AK xrl.i'f! 

 / rriKTKKKIH r\: 



/ / TV; /M/ /:/</. i />: 

 SULCUB 



A.VTERIOR MEIIIAX FISS17RE 

 FII.A RAIllrri.AIIIA 



I'llSTERO- 



l.\ ! I i: II. 

 8ULCVS 



./' roofo] 



supply of the limbs the gain of dorsal root or sensory fibres is far greater than the gain 

 of ventral root-fibres. The first cervical or the sub-occipital nerve is always an ex- 

 ception to the rule; its dorsal root is always smaller than its ventral, and in rare cases 

 may be rudimentary or entirely absent. The spinal ganglion and, therefore, the sen- 

 sory root of the coccygeal nerve, is also quite frequently absent. 



The dorsal and ventral root-fibres of each spinal nerve proceed outwards from 

 I heir segment of attachment to the spinal cord, pierce the pia mater and arachnoid, 

 collect to form their respective roots, and pass into their respective intervertebral 

 foramina. On the immediate peripheral side of the spinal ganglion the two roots 

 blend, giving origin to the thus mixed nerve-trunk. As the trunk, the sensory 

 and motor fibres make their exit from the vertebral canal through the intervertebral 

 foramen. 



Relation to the meninges. The root filaments of each nerve receive connect ive- 

 1 issue support from the pia mater and arachnoid in passing through them. In the 

 arachnoid cavity they become assembled into their respective nerve-roots, and the 

 roots, closely approaching each other, pass into the dura mater, from which they re- 

 ceive separate sheaths at first, but at the peripheral side of the ganglion these sheaths 

 blend into one, which, with the subsequent blending of the roots, becomes the sheath 

 or epineurium of the nerve trunk. By means of the sheaths derived from the 



