THE NERVOUS SYSTEM. 411 



154). With respect to the interesting observations on the structure of 

 the spinal ganglia of the lower animals, and particularly of Fishes, I 

 would refer especially to the works of R. Wagner, Bidder, Robin, and 

 Stannius, cited below. 



121. Further course and termination of the Spinal Nerves. Below 

 the spinal ganglion, the sensitive and motor roots unite to form a common 

 trunk, their fibres being intermixed in diverse ways, as may be very 

 distinctly perceived in small animals. All the subsequent branches, 

 both of the anterior and posterior main divisions, as well as their fur- 

 ther continuations, are consequently of a mixed nature, formed of por- 

 tions derived from both roots ; a condition which they retain up to their 

 ultimate distribution. Here, however, an alteration takes place, the 

 motor fibres going off in by far the larger proportion into the muscular 

 branches, and the sensitive chiefly to the cutaneous. Where the gan- 

 glion-fibres which arise in the spinal ganglia are distributed, cannot be 

 ascertained anatomically.* When their physiological relations, how- 

 ever, are considered, it would appear as by far the most probable sup- 

 position, that they do not, as at first sight one would be inclined to 

 suppose, join the sympathetic in the rami communic antes, but, that ac- 

 companying the spinal nerves, they are continued chiefly into the vascular 

 branches, and consequently are distributed in the integuments, muscles, 

 bones, joints, tendons, and membranes (periosteum, pia mater, &c.), but 

 also, perhaps, to the glands and involuntary muscles of the skin. The 

 nerve-fibres in the main trunks of the spinal nerves present the same 

 diameter as in the roots, that is to say, there are finer and thicker 

 tubes, and a certain number of intermediate forms ; but, as they pro- 

 ceed, the fibres separate, the thicker going more to the muscular 

 branches, and the thinner into the cutaneous nerves. According to the 

 statements of Bidder and Volkmann, the proportion of the fine to the 

 thick fibres is, in man, as 1. 1 : 1, in the muscular nerves as 0. 1-0*33 : 1 ; 

 statements which I can but confirm, adding to them, that the nerves of 

 the bones contain, in the trunks, one-third of thick and two-thirds of 

 fine, whilst those of the articulations, tendons, and membranes, exhibit 

 a great preponderance of fine fibres. In my opinion, most of the fine 

 fibres contained in the branches of the spinal nerves must be regarded 

 as derived from the spinal cord, and as being, in their function, quite of 

 equal importance with the thick fibres, and at present, the only thing 

 that remains unascertained, is whether they all ascend to the brain, or 

 perhaps in part arise in the spinal cord ; upon which point reference 

 may be made to 112. 



The spinal nerves are composed in general of parallel tubes, for the 



* [Vid. note, 120. DaC.] 



