I 



H. II.] EXTERNAL DIRECT NERVE-ACTIONS. 239 



ot constituted of longitudinal fibrils like those of muscles 

 (161), although supplied with nerves ; as the substance of the 

 liver, the osseous medulla, the glands, and the membranes not 

 made up of sensitive muscular tissue, as the mucous. When 

 such a tissue is irritated, no immediate movement results; 

 nevertheless, an external impression is transmitted along the 

 nerves, and felt or reflected in its course upwards, and can 

 produce indirect nerve-actions and sentient actions of external 

 sensations in the same or other mechanical machines (424, iii). 

 Note. — This peculiar capability of muscular fibre for direct 

 nerve-actions, as compared with other machines, has probably- 

 been the principal source of the erroneous doctrine, that the 

 animal motor force of an external impression, or, in other 

 words, their irritability, is a property peculiar to the tissue, 

 and independent of the nerves. It is probable, however, that 

 it is not the only seat of direct nerve-actions, as wiU be shown 

 subsequently (463). 



446. When many muscular fibres are united together, so as 

 to form bundles, the motion excited in one readily extends to 

 others, and puts the whole bundle into action ; or, if a viscus 

 be made up wholly of such bundles, the whole machine may be 

 thus excited to action, as is the case with the heart, stomach, 

 intestines, &c. This compound and communicated action is as 

 much a direct nerve-action, as if only one fibril had been 

 excited to contract; consequently, when the motions of the 

 heart, or of portions of intestine, are renewed after their 

 removal from the body, by pricking with a needle, direct 

 nerve-actions are produced. {Vide Haller's 'Elem. Physiol.,^ 

 tom. iv, p. 467.) 



447. Nevertheless, it is not advisable thus to consider them. 

 For, firstly, the movements excited in the fibrils connected with 

 those primarily irritated, is only a mechanical result. Secondly, 

 it may be considered as a nerve-action, the direct result of an 

 internal, or the indirect nerve-action of an external impression, 

 and this may take place as follows. The nerve entering a 

 muscle is distributed to every fibril of it, otherwise every 

 portion would not be sensitive (35). Consequently, there must be 

 numerous points of division of the nerve in the substance of 

 the muscle, at which an external impression on its way upwai'ds 

 can be reflected, and changed into a non-conceptional impression 



