216 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



371. It has been ascertained by the researches of MM. Becquerel and 

 Breschet, that the temperature of a muscle rises, when it is thrown into 

 energetic contraction. The increase is ordinarily but about 1 Fahr. ; 

 but it may amount to twice as much, if the muscle be kept in action for 

 some time, as in the exercise of sawing. Two causes may be assigned 

 for this increase. It may depend upon the chemical changes which 

 take place in the Muscle, as a necessary condition of the production 

 of its force ( 361) ; or it may be the result of the friction taking 

 place between different parts, during the constant interchange of their 

 actions ( 356). Perhaps both these causes concur in producing the 

 effect. 



372. The Nervous System, taken as a whole, is the instrument of all 

 those operations, which peculiarly distinguish the Animal from the Plant ; 

 and it serves many additional purposes, connected with the Organic or 

 Vegetative functions, which the peculiar arrangements of the Animal 

 body involve. Wherever a distinct Nervous System can be made out 

 (which has not yet been found possible in the lowest Animals), it con- 

 sists of two very different forms of structure ; the presence of both of 

 which, therefore, is essential to our idea of it as a whole. We observe, 

 in the first place, that it is formed of trunks, which are distributed to 

 the different parts of the body, especially to the muscles -and to the sen- 

 sory surfaces ; and of the ganglia, which sometimes appear merely as 

 knots or enlargements on these trunks, but which in other cases, have 

 rather the character of central masses, from which the trunks proceed. 

 Now it is easily established by experiment, that the active powers of 

 the nervous system reside in the ganglia; and that the trunks serve 

 merely as conductors of the influence, which is to be propagated towards 

 or from them. For if a trunk be divided in any part of its course, all 

 the parts to which the portion thus cut off from the ganglion is distri- 

 buted, are completely paralysed ; that is, no impression made upon them 

 is felt as a sensation, and no motion can be excited in them by any act 

 of the mind. Or if the substance of the ganglion be destroyed, all the 

 parts, which are exclusively supplied by nervous trunks proceeding 

 from it, are in like manner paralysed. But if, when a trunk is divided, 

 the portion still connected with the ganglion be pinched, or otherwise irri- 

 tated, sensations are felt, which are referred to the points supplied by 

 the separated portion of the trunk ; which shows that the part remaining 

 in connexion with the ganglion is still capable of conveying impressions, 

 and that the ganglion itself receives these impressions and makes them 

 felt as sensations. On the other hand, if the separated portion of the 

 trunk be irritated, motions are excited in the muscles which it supplies ; 

 showing that it is still capable of conveying the motor influence, though 

 cut off from the usual source of that influence. 



373. When we minutely, examine the trunks of the nerves, we find 

 that they are composed, in the first place, of a Neurilemma or nerve- 

 sheath, consisting of areolar tissue ; the office of which is evidently that 

 of protecting the nerve-tubes, and of isolating them from the surround- 

 ing structures, at the same time that it allows blood-vessels to pass into 

 the interior of the Jrunk. From the interior of the neurilemma, thin 

 layers of areolar tissue pass into the midst of the enclosed bundle of 



