SUMMARY. 245 



The disposition of the fibrils of a nerve is altered in a ganglion according to the 

 required uses ; ganglia exist as centres of origin or connection of nerves from, to, and 

 through which impressions proceed. They combine and harmonise different nerves, or 

 allow a large one, as the splanchnic, to be diffused in a wider centre, giving it a firm 

 attachment in a convenient situation for furnishing the very numerous branches to the 

 viscera, whose extent and varying condition require a particular distribution. They 

 may, throughout in some animals, or only in parts of others, effect greater or less 

 changes according either to the complexity or simplicity of arrangement of the fibrils of 

 the nerves entering them, by their more or less close structure, and the quantity of the 

 red intervening matter; they may therefore be very slightly varied as to their 

 constituent parts in many instances, from the nerves entering them, the disposition of 

 the fibrils being merely altered in the more open or plexiform ganglia, or they may be 

 entirely changed in the more close or solid ganglia, no vestige of the fibrils remaining. 

 In close ganglia of the sympathetic nerve, the nervous fibrils being obliterated, it is 

 presumed that the perceptiveness of the organs supplied by their branches is of a 

 peculiar kind. When the fibrils are not entirely obliterated in the ganglionic matter, 

 the perception of the organ receiving its branches approaches nearer to that allowed by 

 the cerebral and spinal nerves, and still nearer when the ganglia have almost entirely 

 the form of a plexus. So in the ganglia of the spinal nerves, the greater the quantity 

 of red matter in which the fibrils of the nerve are subdivided, the more the perceptive 

 faculty is modified. 



A plexus which is in the place of the semilunar ganglia in the turtle, snake, and 

 crocodile, is regarded as belonging to the more open or thready ganglia. The 

 arrangement to be generally considered as a plexus in various parts of animals is in 

 much wider meshes. The more intricate these are, the nearer they approach the 

 functions of ganglia. All the intermediate changes are connected with altered 

 functions for keeping up a more concentrated exciting power, or a greater or less 

 communion with other nerves : they are modifications of ganglia, to which they are, in 

 some degree, subservient, for they are frequently placed as the first arrangement of 

 branches proceeding from ganglia to their destination. Plexuses, however, need not 



