PHYSIOLOGY OF THE NERVOUS SYSTEM. 



722r 



interference is removed, or kept under con- 

 trol, physical actions develope themselves ; 

 being effected through the same nerves as those 

 which volition influences or which sensitive 

 impressions affect. The latter are, in such 

 instances, the exciters of the former, no doubt 

 through the vesicular matter in which they are 

 implanted. These actions become most mani- 

 fest when the connection of the brain with the 

 spinal cord has been severed ; and they occur 

 in the most marked way in those situations 

 where the cutaneous nerves are so organized 

 as readily to respond to the application of a 

 stimulus applied to the surface, or they be- 

 come universal when the cord is in a state of 

 general excitement. 



The movements in locomotion and the main- 

 tenance of the various attitudes are effected 

 through the ordinary channels of the physical 

 and volitional actions ; ar.d the posterior co- 

 lumns of the cord, by their influence on the 

 vesicular matter of the segments in which the 

 nerves are implanted, co-ordinate and har- 

 monize the complicated muscular actions of 

 the limbs and the trunk under the controul of 

 that portion of the encephalon which probably 

 is devoted to that purpose. This power of co- 

 ordination is probably mental, and intimately 

 connected with the muscular sense. 



FUNCTIONS OF THE ENCEPHALON. It 

 will be convenient first to examine the func- 

 tions of those parts of the encephalon which in 

 structure most nearly resemble the spinal cord. 



Functions of the medulla oblongata, mesuce- 

 pha/e, corpora striata, tutd optic tlialami. 

 The medulla oblongata most nearly resembles 

 the cord in form and structure, at the same 

 time that it exhibits most marked and impor- 

 tant differences from it. Its subdivisions form 

 connections superiorly with other parts of the 

 brain, namely, the mesocephale, corpora striata, 

 and optic tlialami. These connections are so 

 intimate, that, however convenient it may be to 

 the descriptive anatomist to describe these 

 parts each by itself, it is impossible, in exa- 

 mining into their functions, to separate them 

 completely. The functions of one part are so 

 readily affected by a change in any or all of 

 the others, that the effects of experiments are 

 not limited only to the part operated upon, but 

 affect or are affected by the rest. Thus, the 

 olivary columns, which form the central and 

 most essential part of the medulla oblongata, 

 extend upwards through the mesocephale to 

 the optic thalami ; and the anterior pyramids 

 form an intimate connection not only with the 

 vesicular matter of the mesocephale, but, to a 

 great extent, with that of the corpora striata. 

 All these parts taken together, with the quadri- 

 geminal tubercles, will be found to be the 

 centre of the principal mental nervous actions, 

 and of certain physical actions which are very 

 essential to the integrity of the economy. 



The office of the nerves which arise from this 

 segment of the encephalon throws light upon 

 its function. These nerves are partly destined 

 lor respiration, partly for deglutition, and partly 

 also for acts of volition and sensation. 



Destruction of the medulla oblongata is fol- 

 lowed by the immediate cessation of the pheno- 

 mena of respiration ; and this takes place whe- 

 ther it be simply divided, or completely re- 

 moved. Vi hen an animal is pithed, he falls 

 down apparently senseless, and exhibiting only 

 such convulsive movements as may be due to 

 the irritation of the medulla by the section, or 

 such reflex actions as may be excited by the ap- 

 plication of a stimulus to some part of the trunk. 



If, in an animal which breathes without a 

 diaphragm, as in a bird or reptile, the spinal 

 cord be gradually removed in successive por- 

 tions, proceeding from below, up to within a 

 short distance of the medulla oblongata, loss of 

 motor and sensitive power takes place succes- 

 sively in the segments of the body with which 

 the removed portions of the cord were connected. 

 But the animal still retains its power of per- 

 ceiving impressions made on those parts of the 

 body which preserve their nervous connection 

 with the medulla oblongata, and continues to 

 exercise voluntary control over the movements 

 of those parts. The movements of respiration 

 go on, and deglutition is performed. The 

 higher senses are unimpaired.* 



These phenomena are sometimes observed in 

 man in such cases as that alluded to in a 

 former page ; where, from injury to the spinal 

 cord in the neck, below the origin of the phrenic 

 nerve, the patient appears as a living head with 

 a dead trunk. The sensibility and motor 

 power of the head are perfect; respiration goes 

 on partially, and deglutition can be readily 

 performed. The senses and the intellectual 

 faculties remain for a time unimpaired. 



Irritation of any part of the medulla oblon- 

 gata excites convulsive movements in muscular 

 parts which receive nerves from it, and, through 

 the spinal cord, in the muscles of the trunk. 

 Spasm of the glottis, difficulty of deglutition, 

 irregular acts of breathing, result from irritation 

 of the medulla oblongata; and, if the excite- 

 ment be propagated to the cord, convulsions 

 will become more or less general. 



If a lesion affect one half of the medulla ob- 

 longata, does it produce convulsions or paralysis 

 on the opposite side of the body ? This ques- 

 tion may be certainly answered in the affirma- 

 tive, when tho seat of the lesion is in the conti- 

 nuations of the columns of the medulla oblon- 

 gata above the posterior margin of the pons. 

 It is not so easily solved, however, when the 

 disease is situate below the pons. The results 

 of experiment on this subject are contradictory, 

 owing probably to the extreme difficulty of 

 limiting the injury inflicted to a portion of the 

 medulla on one side; and those of Flourens 

 are of no value for the decision of this question, 

 as it appears that he injured chiefly the resti- 

 form bodies. Anatomy suggests that a lesion 

 limited to either anterior pyramid would affect 

 the opposite side of the trunk, for it is known 

 that such an effect follows disease of the conti- 

 nuation of it in the tr.esocephale or crus cerebri ; 

 and that lesion limited to the posterior half of 



* Flourens, p. 179. 



