THE SYMPATHETIC NERVE. 621 



of the cranial nerves, and there is a free exchange of fibres between these nerves. (The function 

 and significance of these exchanges are referred to under the physiology of the cranial nerves).] 



[Dorsal and Abdominal Portion. Numerous fibres pass from these parts chiefly to the thoracic 

 and abdominal cavities, where they form large gangliated plexuses, from which functionally 

 different fibres proceed to the different organs.] 



[In the dog, the 2nd, 3rd, 4th, and 5th thoracic pass upwards into the cervical sympathetic, 

 those in the dorsal region being directed downwards from the lateral ganglia to form the 

 splanchnics (fig. 439). The grey non-medullated nerve-fibres of each grey ramus are connected 

 with the cells of its ganglion (lateral) ; the fibres do not go beyond the ganglion, but really run 

 to the corresponding spinal nerve to ramify in the sheaths of the nerves, the connective-tissue 

 on the vertebrae and the dura mater, and perhaps the other spinal membranes ; so that, accord- 

 ing to Gaskell, no non-medullated nerves leave the central nervous system by the spinal nerve- 

 roots. Thus, the white rami communicantes alone constitute the rami viscerales of the morpho- 

 logist, and all the visceral nerves passing out from the central nervous system into the sympa- 

 thetic system pass out by them alone. All the nerves in the white ramus are of small calibre 

 (1*8 /J. to 2 '7 fx) and medullated, while the true motor fibres are much larger (14 "4 /* to 19 /x). 

 The small, white fibres can be traced upwards as medullated fibres into the superior cervical 

 ganglion, and in the thorax over the lateral ganglia to form the splanchnics into the collateral 

 ganglia, beyond which they cease to be medullated. By the 2nd and 3rd sacral nerves some 

 fibres of smallest calibre issue to form the nervi erigentes, which pass over and do not com- 

 municate with the lateral ganglia, but enter the hypogastric plexus, whence they send branches 

 upwards to the inferior mesenteric plexus and downwards to the bladder, rectum, and generative 

 organs. Gaskell proposes to call them the pelvic splanchnic nerves (fig. 439).] 



[In the cervical region, there is no white ramus, and the nerve-roots contain no nerve-fibres 

 of small calibre. But in this region rises the spinal accessory nerve, between the anterior and 

 posterior roots. It contains small and large nerve-fibres; the former pass into the internal 

 division of this nerve and join the ganglion of the trunk of the vagus, while the large motor 

 fibres form its external branch and supply the sternomastoid and trapezius muscles.] 



[All the vaso-motor nerves arise in the central nervous system, and they leave the spinal 

 cord as the finest medullated fibres in the anterior roots of all the spinal nerves between the 2nd 

 thoracic and 2nd lumbar (dog) "along the corresponding ramus visceralis, enter the lateral or 

 main sympathetic chain of ganglia, where they become non-medullated, and are thence distri- 

 buted either directly or after communication with other ganglia" (Gaskell).] 



[The vaso-dilator nerves leave the central nervous system among the fine medullated fibres, 

 which help to form the cervico-cranial and sacral rami viscerales, and pass without altering their 

 character into the distal ganglia" (Gaskell).] 



["The viscero-motor nerves upon which the peristaltic contraction of the thoracic portion 

 of the oesophagus, stomach, and intestines depends, leave the central nervous system in the out- 

 flow of fine medullated nerves which occurs in the upper part of the cervical region, and pass 

 by way of the rami viscerales of the accessory and vagus nerves to the ganglion trunci vagi, 

 where they become non-medullated " (Gaskell).] 



[" The inhibitory nerves of the circular muscles of the alimentary canal and its appendages 

 leave the central nervous system in the anterior roots, and pass out among the fine medullated 

 fibres of the rami viscerales into the distal ganglia without communication with the proximal 

 ganglia" (Gaskell).] 



[Structure of a Ganglion. The structure of the sympathetic nerve-fibres and nerve-cells has 

 already been described in 321. On making a section of a sympathetic ganglion, e.g., the 

 human superior cervical, we observe groups of cells with bundles of nerve-fibres chiefly non- 

 medullated running between them, and the whole surrounded by a laminated capsule of con- 

 nective-tissue, which sends septa into the ganglion. The nerve-cells have many processes, and 

 are, therefore, multipolar, and each cell is surrounded by a capsule with nuclei on its inner 

 surface (fig. 368, II). The processes pierce the capsule, and one of them certainly and perhaps 

 all the processes are connected with a nerve-fibre. Ranvier states that each cell has a 

 fibrillated outer portion and a more granular inner part. Each of the processes becomes con- 

 tinuous with a fibre of Remak. Not unfrequently yellowish-brown pigment is found in the 

 cell-substance. Similar cells have been found in the ophthalmic, sub-maxillary, otic, and 

 spheno-palatine ganglia. The number of medullated nerve-fibres diminishes as the sympathetic 

 nerves are traced towards their distribution. Ranvier states that it is possible in the rabbit to 

 trace the conversion of a medullated fibre into a branched fibre of Remak. The blood-vessels of 

 the sympathetic ganglia in mammals are peculiar. The arteries are small, and after sub- 

 division form a capillary network, each mesh of which encloses several ganglionic cells. The 

 veins on the contrary are very large, tortuous, varicose, and often terminate in culs-de-sac, 

 into which several capillaries open. The arrangement of the veins is spoken of as the venous 

 sinuses of these ganglia, being compared by Ranvier to the sinuses of the dura mater and 

 venous plexuses of the spinal canal. ] 



Functions. The following is merely a general summary : 



