THE NERVOUS SYSTEM. 151 



THE SYMPATHETIC NERVE. 



The general differences between the fibres of the cerebro-spinai and 

 sympathetic nerves have been already stated (pp. 71, 72, Vol. II.), but 

 the different modes of action of the two systems cannot be referred to the 

 different structure of their fibres. It is probable, however, that the laws 

 of conduction by the fibres are in both systems the same, and that the 

 differences manifest in the modes of action of the systems are due to the 

 multiplication and separation of the nervous centres of the sympathetic: 

 ganglia, or nerve-centres, being placed in connection with the fibres of 

 the sympathetic in nearly all parts of their course. 



Distribution. 1. Fibres are distributed to all plain or unstriped 

 muscular fibres, as those of the blood-vessels (vaso-motor nerves), of the 

 muscular coats of the intestines and other hollow viscera, of gland-ducts, 

 of the iris and ciliary muscle in the eye, and elsewhere. 



The vaso-motor fibres come originally from the vaso-motor centre in 

 the medulla oblongata; and, issuing from the spinal cord, communicate 

 with the prae-vertebral chain of ganglia, and are thence, as branches 

 from these, distributed to the Blood-vessels. 2. Fibres (accelerating) are 

 distributed to the Heart. 3. Secretory fibres (in addition to vaso-motor) 

 are distributed to the salivary, and presumably to other secreting glands. 

 4. Intercentral or inter-ganglionic fibres. 5. Centripetal fibres proceed- 

 ing to the vaso-motor centre in the medulla; to the various sympathetic 

 ganglia; and probably to all cerebro-spinai nerve-centres. The periph- 

 eral distribution of these centripetal fibres is, without doubt, chiefly in 

 the parts or organs to which the centrifugal fibres of the same sys- 

 tem are mainly distributed. But they are also present in all those 

 other parts of the body which belong more especially to the Cerebro- 

 spinai system. 



Structure. The sympathetic ganglia all contain (1), nerve-fibres 

 traversing them; (2), nerve-fibres originating in them; (3), nerve or 

 ganglion corpuscles, giving origin to these fibres; and (4), other corpuscles 

 that appear free. In the sympathetic ganglia of the frog, ganglion-cells 

 of a very complicated structure have been described by Beale and sub- 

 sequently by Arnold. The cells are enclosed each in a nucleated capsule: 

 they are pyriform in shape, and from the pointed end two fibres are 

 given off, which gradually acquire the characters of nerve-fibres: one of 

 them is straight, and the other (which sometimes arises from the cell by 

 two roots) is spirally coiled around it. 



In the trunk, and thence proceeding branches of the sympathetic, 

 there appear to be always (1), fibres which arise in its own ganglia; (2), 

 fibres derived from the ganglia of the cerebral and spinal nerves; (3), 

 fibres derived from the brain and spinal cord and transmitted through the 



