II.] VIS NERVOSA OF EXTERNAL IMPRESSIONS. 225 



mimals endowed with brain, it is requisite that the nerves be 

 supplied with these, as the medium for the transmission of 

 impressions, the brain must be considered as being necessary 

 at least to the continued production of nerve-actions; unless 

 the animal be so constituted, that the vital spirits are secreted 

 in the medulla of the nerves themselves, or in their ganglia, 

 as is the case in avertebrate animals (362). Compare also 

 § 673. This is one of the reasons why in animals endowed 

 with brain, the vis nervosa is abolished so soon after de- 

 capitation ; for the vital spirits gradually flow out of the nerve- 

 medulla, and are not re-produced. So long, however, as a 

 sufficiency of vital spirits remain in the nerves, their vis nervosa 

 continues to act with vigour, thus establishing its independence 

 of the brain. In many animals, the vis nervosa is retained after 

 decapitation for days and weeks, and in turtles for half a year. 

 The destruction or removal of the brain, which destroys con- 

 sciousness, hinders therefore the continuance of the vis nervosa, 

 but only so far as it prevents the influence of the blood in the 

 muscles (161) ; or, in other words, in proportion as the nerves 

 thereby become gradually more feeble and dead, but not because 

 the co-operation of the brain is a part of the vis nervosa, 



417. Those portions of animals which are supplied with 

 nerves highly susceptible of impressions (160), are endowed 

 with more acute external feeling, and a stronger vis nervosa 

 from external impressions (403) than others ; as for example, 

 the heart, stomach, and intestines. Structures with few nerves, 

 or nerves little irritable, are endowed with a feebler vis nervosa 

 from external impressions ; and those to which no nerves are 

 distributed, have neither external impressions, nor vis nervosa 

 (390). Thus bones, tendons, cartilages, and ligaments, however 

 they may be irritated, display no traces of movement. A part, 

 to possess the vis nervosa of external impressions must have 

 nerves that can receive an impression, fully incorporated with 

 it ; the more numerous such nerves, the more varied the im- 

 pressions, and the more susceptible they are to these impressions, 

 the more vivid its external feeling, and vice versa (44, 47) . 



418. When an external impression in a nerve distributed to 

 a mechanical machine, excites a nerve-action in the latter at 

 the point where the impression is received, it is termed, 

 whether the impression be transmitted onwards or not (415, i), . 



15 



