CH. I.] NERVE-ACTIONS IN GENERAL. 195 



by their means in the mechanical without the co-operation of 

 the cerebral force, and in fact may take place without any 

 brain whatever. Consequently, we cannot infer, 



i. That those actions cannot originate from a sentient brain 

 which are not animal actions or nerve-actions ; or if the sen- 

 tient brain be separated from the body, that no animal actions 

 or nerve-actions can take place in the latter. 



ii. That those impressions which are not felt, and do not 

 reach the brain^ cannot develop animal or nerve-actions. 



iii. That those impressions which are not produced by con- 

 ceptions, and do not depend upon material ideas, cannot cause 

 animal actions, or nerve- actions. 



There is an important distinction to be noted in all. Nerve- 

 actions require the presence and free action of the vital spirits 

 in the animal machines (357, 359). The cortical substance 

 of the brain secretes the vital spirits from the blood, and dis- 

 tributes them to the nervous system. To this extent, the vital 

 spirits and the brain can be considered as being necessary to 

 the two kinds of vis nervosa. But again, the brain is not the 

 secreting organ of the vital spirits in all animals, since there 

 are some, in the first place, that have no brain or head distinct 

 from the trunk, yet are nevertheless endowed with vis nervosa, 

 and in which in all probability the vital spirits are secreted in 

 every part of the system, — in every nerve, and probably in 

 every ganglion — for their limbs often retain animal life, and 

 have an independent existence, when separated from the body. 

 Secondly, in those that have a distinct brain secreting nervous 

 fluid, it is not the medullary substance, the seat of the cerebral 

 forces (11) that is necessary to the vis nervosa, but the con- 

 nected cortical substance. Lastly, even in these the cortical 

 substance is only necessary, because it prepares and supplies 

 for the vis nervosa its animal nourishment, namely, the vital 

 spirits, and is, consequently, unnecessary so long as the nerves 

 retain a sufficient supply of the latter (1 59) ; just as animals 



I will live for some time without food, or plants survive after being 

 separated from the stem. The reader will better understand 

 what follows by keeping this view in mind. 

 363. Although the two kinds of vis nervosa be thus inde- 

 pendent of the brain and of the cerebral forces, they can 



