THE SPINAL NERVES 965 



about the cells within the grey column of the cord, forming either associational, commissural, 

 or reflex connections, or about cells whose fibres form cerebellar connections. The ascending 

 or cephalic branches are either short, intermediate, or long. The short and intermediate 

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

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

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

 upward in the fasciculus gracilis or fasciculus cuneatus of the cord, and terminate in the nuclei 

 of these fasciculi in the meduUa oblongata (figs. 618 and 620). 



Aberrant spinal ganglia. — In serial sections on either side of the spinal gangUon 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 ganghon-ceUs displaced in the growth processes, 

 and have the same nature and function as those in the ganghon. In some animals occasional 

 cells very rarely have been found in the outer portion of the ventral root. These probably 

 represent afferent fibres which enter the cord by way of the ventral root. Likewise, 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 

 abundantty 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 

 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 exception 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 sensory root of the coccygeal nerve, is also 

 quite frequently absent. 



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

 their 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 interverte- 

 bral foramen. 



Relation to the meninges. — The root filaments of each nerve receive connec- 

 tive-tissue support from the pia mater and arachnoid in passing through them. In 

 the sub-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 receive separate sheaths at first, but at the peripheral side of the ganghon 

 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 meninges, especially the dura, the nerve-roots and the trunk are 

 attached to the periosteum of the margins of the intervertebral foramina and 

 thus are enabled to give some lateral support to the spinal cord in the upper por- 

 tion of the canal. 



The majority of the spinal gangha lie in the intervertebral foramina, closely ensheathed, 

 and thus outside the actual sac or cavity of the dura mater. The gangha of the last lumbar 

 and first four sacral nerves lie inside the vertebral canal, but since the sheath derived from the 

 dura mater closely adheres to them, they are still outside the sac of the dura mater. The gan- 

 gha of the last sacral and of the coccygeal nerves (when present) he in tubular extensions of the 

 sub-dural cavity, and thus not only within the vertebral canal, but actually withinithe sac of 

 the dura mater. The trunk of the first cervical nerve is assembled within the sac of the dura 

 mater, and, therefore, the spinal ganghon of this nerve, when present, may lie within the sac. 



Course and direction of emergence. — Invested with the connective-tissue 

 sheath derived from the meninges, each thoracic, lumbar and sacral nerve emerges 

 from the vertebral canal through the intervertebral foramen below the correspond- 

 ing vertebra, and all the nerves are in relation with the spinal rami of the arteries 

 and veins associated with the blood supply of the given localities of the spinal cord. 



The first cervical nerve does not pass outward in an intervertebral foramen proper, but 

 between the occipital bone and the posterior arch of the atlas and beneath the vertebral artery. 

 Thus the eighth or last cervical nerve emerges between the seventh cervical and the first thor- 

 acic vertebra. 



The first and second pairs of cervical nerves pass out of the vertebral canal almost at right 

 angles to the levels of their attachment to the spinal cord. During the early periods of develop- 

 ment the level of exit of each pair of spinal nerves is opposite the level of its attachment to the 



