PHYSIOLOGY OF THE NERVOUS SYSTEM. 



experiments show that the brain readily imbibes 

 fluid, and that parts in the vicinity of and 

 bathed in fluid may present a pseudo-morbid 

 softening from such imbibition. The fact, 

 thus ascertained, serves to account for the 

 more frequent occurrence of softening in the 

 fornix and septum lucidum than in other 

 parts of the brain. It is obvious that pseudo- 

 morbid softening of this kind would occur 

 only in parts within the ventricle or in the 

 cerebral substance forming their walls, or 

 on the surface of the brain itself, and that it 

 is less likely to be limited to one side than the 

 morbid softening. Now and then, however, in 

 cases of general anasarca, where the blood is 

 in a very watery condition and much fluid is 

 effused, the brain exhibits a softened state from 

 the imbibition of this fluid. 



ABNORMAL ANATOMY OF NERVES. Certain 

 nerves are sometimes absent, from a defect in 

 the developement of the organ to which they 

 are devoted ; as the optic nerve, or the olfac- 

 tory, when their respective organs are wanting. 

 The non-developement of the eye will also 

 cause a non-developement of the fourth pair 

 and the other orbital nerves which influence the 

 movements of the eyeball. 



The morbid states of nerves are few and rare. 

 Inflammation of a nerve rarely occurs idiopa- 

 thically or primarily. Occurring from what- 

 ever cause, it would be distinguished by hyper- 

 semia, enlargement, and by deposit of more or 

 less of lymph or pus. In the acute inflam- 

 mation the nerve would be softened ; but in the 

 chronic it would become indurated. Abscess 

 of a nerve is of very rare occurrence. 



Inflammatory affections of nerves occur chiefly 

 in connexion with rheumatic or gouty states of 

 the system. Sciatica is, no doubt, an in- 

 flammatory affection of the sciatic nerve of the 

 gouty kind. In lumbago probably the mus- 

 cular nerves of the lumbar muscles are similarly 

 affected. 



Atrophy is a condition into which nerves 

 may fall from disuse or from pressure. In it 

 the nerve-fibres shrink, their central axis wastes, 

 and in extreme cases disappears entirely, the 

 tubular membrane becoming plicated and as- 

 suming the characters of fibrous tissue. The 

 nerve experiences a great diminution in size, 

 and the wasting is obvious to the naked eye. 



Hypertrophy. Whether a nerve becomes 

 enlarged when more work is thrown upon it, as 

 a muscle does, is as yet quite uncertain. I 

 am disposed to think that the nerve-fibres may 

 acquire some increase of size; but it seems 

 to me impossible that they should become more 

 numerous. The number of nerve-fibres in in- 

 dividual nerves, as that of muscular fibres in 

 muscles, is probably determined at their pri- 

 mary developement,* and they undergo no 

 change but that of length and thickness subse- 

 quently. It would not be difficult, by destroy- 

 ing the office of the vagus nerve on one side, 

 to ascertain whether, after the lapse of some 



* A similar law probably prevails with other tis- 

 sues, namely, that the number of their proximate 

 elements is determined at primary developement, 

 and that in subsequent growth these elements may 

 increase in bulk but not in number. 



time, the other, upon which its function would 

 devolve, acquired any increase in the size of its 

 nerve-fibres. 



Certain gangliform tumours are formed upon 

 nerves, to which the term neuroma has been 

 applied. They consist of areolar tissue and of 

 nerve-fibres, and seem to be formed by an 

 increased developement of the areolar tissue 

 between the nerve-fibres. These tumours vary 

 considerably in size and number ; sometimes 

 they are not larger than a filbert or a gooseberry 

 sometimes as large as a walnut. In genera 

 they are few and limited to one nerve, and their 

 size is proportionate to that of the nerve with 

 which they are connected. In a few rare cases 

 tumours of this kind have been found in im- 

 mense numbers scattered over the whole cere- 

 bro-spinal system. 



(R. B. Todd.) 



NERVOUS SYSTEM, PHYSIOLOGY OF 

 THE. In inquiring into the physiology of the 

 nervous system, the first step is to determine 

 the vital endowments of nerves and of nervous 

 centres. 



When a nerve is laid bare in a living animal, 

 and a mechanical or electrical stimulus is ap- 

 plied to it, we do not find as in muscle that a 

 visible change in the nerve takes place ; on the 

 contrary, the nerve seems to be uninfluenced 

 by the applied stimulus, and the evidence we 

 have to the contrary is derived from the con- 

 traction of certain muscles, if the nerve be 

 muscular, or from indications of pain, if it be 

 a nerve of common sensation. 



We infer, then, from the contraction of the 

 muscle in the one case, or from the affection of 

 the mind in the other, that the application of 

 the stimulus has wrought a change in the 

 nerve, which, however, is of such a nature as 

 not to be discerned by any means of observation 

 within our reach. We get, however, excellent 

 proof of the excitation of the change in the 

 nerve, from the fact that when a ligature is ap- 

 plied to a nerve sufficiently tight to produce a 

 solution of continuity in the nerve fibres, the 

 propagation of the influence of the stimulus 

 beyond the ligature is checked. No kind nor 

 degree of stimulation of a muscular nerve above 

 a ligature so applied is capable of exciting 

 muscular contraction. 



The most remarkable feature which we notice 

 in the experiment of stimulating a muscular 

 nerve, is the instantaneousness with which the 

 muscular contraction takes place. Although 

 the muscles may be at a considerable distance 

 from the point of the nerve to which the stimu- 

 lus is applied, there seems no appreciable 

 interval of time between the application of the 

 stimulus and the contraction of the muscle. 

 And the cessation of the muscular contraction, 

 instantly upon the removal of the stimulus, is 

 equally conspicuous. 



It would appear, then, that the change in 

 the nerve is produced and is propagated along 

 the nerve to distant parts, as it were at one and 

 the same moment. This rapidity of the pro- 

 duction, and the instantaneousness of propaga- 

 tion of the change in the nerve, denote that the 

 nerve fibres must be the seat of a molecular 



