CH. III.] THE NERVES OF MUSCLES. 87 



an external impression transmitted to the brain, or one which 

 does not reach the brain (3i, 47), or a cerebral impression from 

 conceptions, or not, or an internal impression in the medulla of 

 the nerve itself in its course (121). An irritation of the nerve 

 of a muscle (an external impression) produces movement in the 

 muscle it supplies ; it does this also when the animal is de- 

 capitated, or with its nerves tied and rendered insensible, nay, 

 even in the separated muscle itself (Haller's ' Physiol.,' § § 404, 

 409, 575.) If the medulla of a nerve be irritated, the muscle 

 or muscles to which it is distributed, are convulsively con- 

 tracted; and this so long as the animal lives, or even after 

 death, so long as it is moist. It is not at all necessary that 

 the nerve be entire, since if it be divided and then irritated, 

 contractions are equally excited. If a nerve be compressed, or 

 tied, the muscles to which it is distributed are paralysed, and 

 although the will endeavours to act on them, they remain 

 motionless. But if the ligature, or compression, be relaxed, 

 the muscles regain the power of movement, provided the 

 nerve be uninjured. If the crura cerebri be irritated deeply 

 in their substance, the most violent convulsive movements are 

 excited in the whole body; the same occurs when the spinal 

 cord is irritated. If the brain be compressed, that portion 

 of the body is deprived of motion, the nerves of which arise 

 from the compressed portion : in injuries of the spinal cord 

 it is more distinctly manifest, that convulsions or paralyses are 

 induced in that part of the body supplied with nerves from 

 the injured portion. When the spinal cord is injured in the 

 neck, death follows immediately, because the nerves of the 

 heart arise from this portion (Haller's ^ Physiol.,^ §§ 367, 368). 

 An impression on a motor nerve, of whatever kind it may be, 

 developes the movements proper to the muscle with which it is 

 incorporated in a way quite incomprehensible. It is a movement 

 caused by an animal force (7), and in so far as it is produced 

 by the impressions of conceptions in the brain, it is a sentient 

 action (97) . No other movement of a muscle is such, whether 

 it be from mechanical or physical forces, or even from im- 

 pressions (except those of conceptions), although in the latter 

 case they are animal (6, 7) . On the other hand, the volitional 

 movements must not be considered as the only sentient actions, 

 — as is carelessly taught in many elementary treatises, causing 



