NERVES SUPPLYING SKELETAL MUSCLES. 639 



form a series, as they would if they arose from separate ganglia. So also with 

 the coccygeal ganglion, this sends grey rami to six coccygeal nerves ; when 

 the arrangement of nerves is posterior, the pilo-motor connections with the 

 several grey rami may be as follows : The second lumbar nerve sends a few 

 fibres to the first coccygeal ramus, and none to the rest; the third lumbar 

 nerve sends a considerable number of fibres to the first, second, and third 

 grey rami, and a few to the rest ; the fourth lumbar nerve sends fibres to all 

 the grey rami. 



A question which naturally arises from the fact that one spinal nerve is 

 connected with a number of ganglia, is whether a single nerve fibre runs to 

 one ganglion only, or whether it divides and runs to some or to all. Since a 

 given spinal nerve does not affect equally all the ganglia of the series to 

 which it runs, it is fairly certain that the constituent nerve fibres in it do not 

 divide into branches equal in number to that of the ganglia, and send one 

 branch to each ganglion. That in some cases the constituent fibres run to 

 comparatively few ganglia, is obvious from a comparison of the innervation 

 with different arrangements of the spinal nerves. In the anterior arrange- 

 ment, the third lumbar nerve causes strong erection of hairs in the whole of 

 the tail and over the sacrum, i.e. it sends fibres to the fourth lumbar and all 

 the lower ganglia, and the fourth lumbar nerve has no effect at all upon the 

 hairs. As, however, the nerves shift downward, some of the fibres of the 

 third lumbar nerve pass into the fourth ; these will naturally be the ones 

 contained in the lowermost nerve roots. Thus a certain number of fibres 

 ordinarily contained in the third lumbar nerve become separated from the 

 rest, and pass into the fourth lumbar nerve. On stimulating these, it is 

 found in some cases that they do not affect the hairs in the whole of the 

 region from the sacrum downwards, but in part of it only ; for example, only 

 in the latter two-thirds of the tail. Thus these pilo-motor fibres send no 

 branches to the ganglia from the fifth lumbar to the third sacral, but end 

 entirely in connection with the coccygeal ganglion. As the coccygeal ganglion 

 is made up of about six single segmental ganglia, the nerve fibres may 

 still divide and give a branch to each. We may conclude, then, that the 

 ganglionic connection of individual nerve fibres in a spinal nerve is usually less 

 extensive than that shown in the table (p. 634) for the several spinal nerves. 

 We shall later consider a method of determining approximately the number 

 of single ganglia with which individual nerve fibres are connected (cf. p. 681). 



It will be noticed also that a spinal nerve sending fibres to a 

 ganglion, even when this is a single segmental ganglion, does not 

 necessarily send fibres to all the kinds of nerve cells in the ganglion. 

 Thus the fourth lumbar nerve sends fibres to the sacral ganglia, but 

 these fibres are in connection with the cells which innervate the 

 external generative organs, and not in connection with the cells which 

 innervate the vessels and hair muscles of the root of the tail. 



SYMPATHETIC NERVES SUPPLYING THE SKELETAL MUSCLES. 



We have seen that the grey ramus running to a spinal nerve 

 contains fibres which accompany the cutaneous branches of the nerve, 

 and which supply various structures occurring in the skin in the region 

 to which the cutaneous branches run. We might, then, naturally expect 

 that the grey ramus would also send fibres to the other branches of the 

 spinal nerve, and thus supply vasomotor fibres to the arteries of the 

 skeletal muscles. 



We shall consider separately the evidence with regard to vaso- 

 constrictor and vaso-dilator fibres. 



