FUNCTIONS OF THE SPINAL CORD. 



649 



frogs for 24 hours or more. Thus, two frogs, A and B, had weights fastened 

 to the hind legs until they were unable to raise them ; the weighting 

 amounted in each case to 925 grains. Immediately after division of the 

 Cord they were able to raise 



We shall hereafter see how much the influence of the Will in producing 

 the active contraction of a muscle, is dependent upon sensations received 

 from it ; and it seems highly probable that the impression of the state of the 

 muscle, conveyed by the afferent fibres proceeding from it to the Spinal 

 Cord, is sufficient to excite this state of moderate tension through the motor 

 nerves arising from the latter. Such a view derives probability from the 

 fact, which must have fallen under the observation of almost every one, that 

 most reflex actions become increased in energy, if resistance be made to 

 them. Of this we have familiar examples in the action of the expulsor 

 muscles which operate in defecation, urination, and parturition, if, when 

 they are strongly excited, their efforts be opposed by spasmodic contraction 

 of the sphincters, or by mechanical means. Many forms of convulsive 

 movement exhibit the same tendency, their violence being proportional to 

 the mechanical force used to restrain them. Here it is evident that the 

 impression of resistance conveyed to the Spinal Cord is the source of the 

 increased energy of its motor influence ; from which we may fairly infer that 

 the moderate resistance occasioned by the natural antagonism of the mus- 

 cles, is the source of their continued and moderate tension, whilst they are 

 under the influence of the Spinal Cord. This constant though gentle action 

 serves to keep up the nutrition of the muscles which are paralyzed to the 

 will ; and this is still more completely maintained, if the portion of the 

 nervous centres with which they remain connected be so unduly irritable, 

 that the muscles are called into contraction upon the slightest excitation, 

 and are thus continually exhibiting twitchings, starlings, or more powerful 

 convulsive movements. It is upon the continuance of the nutrition of the 

 muscles, that the persistence of their contractility depends ; and hence the 

 Spinal Cord has an indirect influence upon the peculiar property, which is 

 more likely to be retained when the muscle is still subject to the influence of 

 the Spinal Cord, though cut off from that of the Brain, than when it is com- 

 pletely paralyzed by the entire severance of its connection with the nervous 

 centres. 



3. Of the Sensory Ganglia and their Functions. Consensual Movements.. 



513. At the base of the Brain in Man, concealed by the Cerebral Hemi- 

 spheres, but still readily distinguishable from them, we find the series of 

 ganglionic masses which have been already mentioned as being in direct 

 connection with the nerves of Sensation, and which appear to have func- 

 tions quite independent of those of the other components of the Encephalon. 

 These are the Olfactory and Optic ganglia (Corpora Quadrigemina), with 

 the Auditory and Gustatory centres contained in the Medulla Oblougata. 

 The structure of the Corpora Quadrigemiua has been shown by Mr. Lock- 

 hart Clarke 1 to consist of nerve-cells, together with oblique, transverse, and 



1 Proceedings of the Royal Society, June 20th, 1861. 



42 



