630 SYMPATHETIC AND OTHER SYSTEMS OF NERVES. 



A complication occurs, in that the ganglia may unite with one 

 another and form a compound ganglion. A compound ganglion gives 

 off a grey ramus to more than one nerve. In some places compound 

 ganglia are constant in occurrence, in others they are occasional. The 

 constant compound ganglia are the ganglion stellatum which sends grey 

 rami to the cervical nerves from about the third downwards, and to the 



first three, possibly four, 

 s y- thoracic nerves ; and 



when there are coccy- 

 geal nerves, the coccy- 

 geal ganglion. A com- 

 pound ganglion will, of 

 course, send sympathetic 

 fibres to (approximately) 

 the whole skin region 

 supplied by the spinal 

 nerves, to which it sends 

 grey rami. Thus the cells 

 of the ganglion stella- 

 tum send axons to the 

 glands and blood vessels 

 of the whole of the fore- 

 limb, as well as to the 

 hair muscles of part of 

 the neck and thorax. 



Origin and nerve 

 cell connection of pre- 

 ganglionic fibres ; in- 



Fig. 309. — The variations we have mentioned are repre- 

 sented in the diagrams A, B, and C. In A each ganglion 

 sends its nerve fibres to the corresponding spinal nerve. 

 In B the upper ganglion sends the great majority of 

 its nerve fibres to its own nerve, and a few (represented 

 by the dotted line) to the nerve below in the grey ramus 

 of that nerve. In C each ganglion sends the majority 

 of its fibres to its own nerve, but the lower sends a few 

 (represented by the dotted line) to the nerve above in 

 the white ramus of that nerve. 



dividual variations. — 

 Lastly, we have to determine the connection which exists between 

 the spinal cord and the several ganglia, or rather we have to determine 

 with which ganglia the efferent fibres in the roots of each spinal nerve 

 are connected. The fundamental line of experiment is to stimulate 

 each spinal nerve near its exit from the spinal cord, and to observe 

 the area of the skin in which sympathetic effects occur. Since we 

 know, within narrow limits, the area of the skin innervated by each 

 ganglion, we can tell within narrow limits which ganglia are in connec- 

 tion with each spinal nerve — supposing always that the effects are 

 brought about by means of the ganglia. 



The spinal nerves which, on stimulation, give rise to sympathetic 

 effects in the skin in the regions we are here considering, are, in the cat, 

 the fourth thoracic (occasionally the third) to the third or fourth lumbar 

 inclusive. The region, it will be remembered, is that of the whole body, 

 except the head and the ano-genital region. 



The distribution of the several nerves has been determined by 

 observation on the pilo-motor nerves to the neck, back, and tail in the 

 cat ; and on the secretory and vasomotor nerves to the feet and limbs. 

 Some confirmatory observations have been made in the vasomotor 

 nerves to the back and trunk. As we shall see, there are some differences 

 in the distribution of the different classes of nerve fibres, although the 

 differences are inconsiderable. 



The pilo-motor nerves present especial advantages in dealing with 

 this question, because the movement of hairs is readily observed. And 



