CONSTRUCTION OF THE GANGLIA. 331 



pass through the nearer ganglia without joining the nerve-cells and are continued 

 towards their peripheral distribution as fine medullated fibres, but lose their 

 medullary sheath in passing through distal ganglia of the sympathetic system. 

 It has further been shown by Gaskell that there is a functional difference 

 between these two kinds of fibres, those at least which are distributed to the 

 bloodvessels, for whereas the fibres which early lose their medullary sheath produce 

 increased contraction of the muscular coat, the others produce diminished contraction 

 or inhibition. Amongst the nerves stimulation of which produces inhibition of con- 

 traction, that which has been longest known is the cervical part of the vagus which 

 sends branches to the cardiac plexus, and to the heart. This nerve is found to 

 contain a large number of fine medullated fibres, which for the most part reach it 

 from the upper roots of the spinal accessory. 



The proportion of fine medullated nerve-fibres which the roots of the cerebro- 

 spinal nerves contain may be taken as a direct indication of the extent to which 

 they feed the sympathetic system. This is at least true for the region of the thoracic 

 outflow of vascular and visceral nerves. All the anterior roots in this region con- 

 tain a large number of fine medullated fibres which leave the root in a bundle and 

 pass directly to the sympathetic chain forming the white ramus communicans (fig. 385) 

 between the spinal root and the sympathetic (Gaskell has shown that the so-called 

 " grey ramus communicans " is in all cases merely a branch from the sympathetic to 

 the bloodvessels of the spinal cord and of the nerve-roots). A section therefore through 

 one of these anterior roots taken before the white ramus leaves it shows a large number 

 of fine medullated fibres intermingled with the ordinary large fibres of the root, 

 and contrasts forcibly with a section across one of the anterior roots of other regions 

 where there is no sympathetic outflow, and, therefore, no white ramus communi- 

 cans, and few, if any, fine medullated fibres. (Compare fig. 384, C and D.) 



The bearing of these facts upon the morphology of the cerebro-spinal nerves and 

 of the sympathetic will be dealt with in the part of this work which treats of the 

 anatomy of the nervous system (Neurology). 



CONSTRUCTION OF THE &ANOLIA. 



Situation. Ganglia are found in the following situations viz. : 1. On the 

 posterior root of each of the spinal nerves ; on the corresponding root of the fifth 

 cranial nerve ; on the facial, auditory, glosso-pharyngeal and pneumogastric nerves. 

 2. In a series along each side of the vertebral column, connected by nervous cords, 

 and constituting what is known as the trunk of the sympathetic. 3. On branches of 

 nerves, especially of the sympathetic ; occurring numerously in the abdomen, thorax, 

 neck, and head ; generally in the midst of plexuses, or at the point of union of two or 

 more branches. Those which are found in several of the fossae of the cranium and 

 face are for the most part placed at the junction of fine branches of the sympathetic 

 with branches, usually larger, of the cerebro-spinal nerves. 



The roots of certain of the cranial nerves, which do not exhibit true ganglia, such as the 

 3rd and 6th, have been shown by Thomsen and by Gaskell to possess what appear to be the 

 remains of atrophied ganglion cells in the form of masses of granular material lying between 

 the nerve-fibres, and devoid of all cellular structure. According to Hale White some of the 

 sympathetic ganglia in man also show signs of atrophy of many of their cells in the adult. 



The ganglia differ widely from each other in figure and size : most of those which 

 have been longest known to anatomists are conspicuous objects ; but there are 

 numerous small or microscopic ganglia disposed along the branches of nerves 

 distributed to the heart, the lungs, and other viscera ; and also connected with fine 

 plexuses of nerves between the coats of the stomach and intestines. 



Structure. Ganglia are invested externally with a thin, but firm and closely 

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