178 . ANIMAL PHYSIOLOGY. 



of chains and irregular collections of ganglia, and of nerves 

 supplied principally to viscera and blood-vessels.* 



The rule that the nerve-fibres remain distinct, each from 

 every other, and never branch until within a microscopic 

 distance of their termination, holds good of both cerebro- 

 spinal and sympathetic systems; but nerves consist of 

 bundles of fibres, which may be re-arranged in other bundles, 

 forming what is called a plexus; and while in the cerebro- 

 spinal system these plexuses are comparatively simple, and 

 confined to the large trunks near their origin and a few fine 

 twigs here and there, in the sympathetic system they are 

 abundant and complex, giving a meshed and intricate appear- 

 ance to the bundles, which are seldom collected in mode- 

 rately large trunks. 



134. Nervous Action. The simplest idea of the use of 

 the nervous system is got from what is termed reflex action, 

 because that is uncomplicated with consciousness. In a 

 reflex action an irritation is applied to a part, and produces 

 in a nerve a change of condition, which is called an impres- 

 sion; this impression is termed sensory or centripetal, and 

 travels to the nerve-centre with which the nerve is connected, 

 and thence it is reflected along some other nerve-fibre, and 

 takes a centrifugal course to a muscle or other organ, which 

 it stimulates to action. If the organ so stimulated be a 

 muscle, the nervous action is excito-motor ; if it be a secreting 

 cell, the action is excito-secretory ; if it be an electric organ, 

 such as exists in various fishes, the nervous action excites it 

 to give a shock of electricity. But in every case it is one 

 description of change which, under the name of an impression, 

 passes up one nerve, through the nerve-centre, and down 



* The typical arrangement of nervous system found in segmented 

 invertebrata is a chain of ganglia running along the lower part of the 

 body, with the foremost placed in front or above the opening of the 

 mouth; and the question naturally arises, what relation has this 

 chain to the vertebrate nervous system? Now, certain researches 

 seem to point out a relationship between vertebrate and invertebrate 

 animals through the tunicate molluscs. These molluscs have only 

 one ganglion, which, I believe, may be fairly considered as homo- 

 logous with the anterior or pre-cesophageal ganglion of articulataj 

 and it appears most probable that the cerebro- spinal axis of verte- 

 brata is a highly developed structure corresponding with that one 

 ganglion, 



