MAMMALIA. 181 



to the space occupied by the origin of the muscles. It is safely conducted along 

 arteries, as the internal carotid, the vertebral, and several more. It belongs principally 

 to the fifth, sixth, and spinal nerves, but it is also connected with others. It is not 

 combined with sentient nerves only, but motive ones also, as the sixth and the hard 

 portion of the seventh. It is not necessary that it should be joined only with nerves 

 proceeding from a ganglion, for at its very beginning it communicates with the sixth, 

 which has not one. The separation of its own ganglia, and those of the spinal nerves, 

 is not always distinct, as in the thoracic nerves of birds and turtles. The com- 

 municating branches are not in proportion to the size of each spinal nerve, but are 

 frequently large, for keeping up a more particular connexion between the internal 

 organs and the external part on which the spinal nerve terminates. Some of the 

 variations of the form and disposition of the prolongation of the sympathetic and the 

 origins of the splanchnic nerves must be attributed to the shapes and motion of the 

 vertebrae, and the convenience of conducting them to the viscera, and not to any 

 required difference in their functions generally. The closer approximation of the 

 prolongation and splanchnic nerves is connected with the form and motion of the 

 vertebrae in animals bending forward at the dorsal, as in the calf, sheep, goat, dog, 

 jaguar, and pig; but the prolongation may be sufficiently secure in man and the 

 baboon, which have a greater power of bending the spine at the lumbar vertebra, and 

 be more conveniently situated for supplying the viscera, which become so depending in 

 the erect position of the body. 



The ganglia of the sympathetic nerve in man, and some animals, consist of a 

 peculiar substance, in which the nerves entering them are entirely lost, and from which 

 others proceed ; but as this structure is not absolutely necessary in animals of the same 

 class, it is provided for modifying the functions of the sympathetic, and the organs 

 supplied by it. In whatever part of many animals corresponding portions of the 

 sympathetic nerve be examined, a difference of the structure of the ganglia in each 

 region, and of the disposition of the nerves proceeding from them, will be found, as 

 well as several variations of corresponding ones ; it is therefore most probable that 

 each form of ganglion gives its own degree of power, and determines some peculiarity 



