312 SPECIAL PHYSIOLOGY. 



fered with ; and when the sympathetic nerves belonging to a gland 

 are divided, its secretion is arrested. The reflex action of the sympa- 

 thetic system on secreting glands is well exemplified by an experiment 

 in which, when the oesophagus was divided, a large quantity of saliva 

 was secreted, on injecting broth into the stomach. (Gairdner.) 



The effects of division of the sympathetic, in causing dilatation of 

 the vessels and congestion of any part by paralyzing the muscular 

 fibres of the coats of the small arteries, which have already been noticed 

 (p. 308), are supposed to offer an explanation of the mode in which 

 the sympathetic nerves may influence the processes of nutrition and 

 secretion, as observed in the increased flow of tears and saliva under 

 certain circumstances; but besides that indirect mode of action, it ap- 

 pears probable that the nerves may, in certain instances, exercise a 

 direct influence over the various chemical processes of nutrition and 

 secretion (see Secretion and Nutrition). The function of nutrition 

 would seem to be more intimately connected with the sympathetic 

 than with the cerebro-spinal system; for it has been found that, in 

 frogs, the nutrition of the parts to which the spinal nerves are distrib- 

 uted is much more impaired when these are divided after they have 

 passed the intervertebral ganglia than when they are divided behind 

 those ganglia. (Axmann.) In like manner, division of the fifth cra- 

 nial nerve in front of the Gasserian ganglion leads to more rapid in- 

 flammation and consequent destruction of the eye than division of the 

 same before it enters the ganglion. (Magendie and Longet.) Lastly 

 it has been noticed that paralysis of both the sensory and motor roots 

 of the spinal nerves is followed by greater disturbance of nutrition than 

 when the motor roots only are implicated. 



Bilateral or Double Action of the Nervous System. 



In describing the nervous system we have repeatedly alluded to the 

 strictly bilateral character of its anatomical construction; and in treat- 

 ing of its functions it must not be forgotten that it possesses a perfect 

 physiological duality; and this fact, coupled with the decussating 

 structures met with at certain points, and with the cross action of those 

 parts from side to side, leads to certain curious results. 



Thus, passing from below upwards in the cord, sensory impressions 

 cross over to the gray matter of the opposite side, immediately through 

 the whole length of the cord; whereas, the motor impressions pass from 

 side to side in the medulla oblongata. In the cerebellum the cross 

 effect is noticed in the rolling over or turning round of the animal on 

 the opposite side to that on which an injury is inflicted; though this 

 might depend either on stimulation or on loss of control of the muscles 

 of the opposite side, or on a loss of power of the muscles of the same 

 side; still there is a cross effect. In regard to the large ganglia at 

 the base of the brain, similar cross effects are noticeable, injuries to 

 the optic thalami or corpora quadrigemina affecting vision in the oppo- 

 site eye. The decussations at the optic commissure also lead to remark- 

 able results in reference to vision, which will be noticed in the Section 

 on Sight. Above the medulla, in the pons, and in the peduncles of 



