CH. n.] EXTERNAL DIRECT NERVE-ACTIONS. 251 



e often not felt^ although usually the natural functions of the 

 bladder are volitional, and therefore sentient actions. 



479. The spleen, being composed of vessels, is in the same 

 ^^ategory with the liver. 

 ^B 480. The organs of the senses (considered as mechanical 

 ^fciachines, and not as organs of sensation), being supplied with 

 ^fcauscles, are, to that extent, capable of nerve- actions : the spas- 

 ^Biodic affections of the tongue and of the muscles of the orbit, 

 horripilation, and, in lower animals, twitchings of the ears, are 

 manifest examples of these. Are not the actions of the iris in 

 contraction and dilatation, and the tension of the muscles of 

 the ossicula auditus, although usually sentient actions, caused by 

 external sensation, at the same time also nerve-actions caused 

 by the same external impressions ? It is difficult to decide the 

 question. (Compare Haller's ' Physiology,^ §§ 494, 496, 513.) 

 481. The functions of the sexual organs in mammalia, and 

 in animals of a similar organisation in this respect, are usually 

 sentient actions ; but, nevertheless, there are some reasons for 

 thinking, that the cerebral forces only co-operate, and that they 

 may be induced by the vis nervosa alone, as direct nerve-actions 

 of external impressions. Erection of the penis will take place 

 in sleep, or in disease, without any external sensation, and is, 

 therefore, a direct nerve-action. The uterus is endowed with 

 as much irritability as the intestinal canal : and, lastly, 

 Bibiena^s experiments, lately instituted, show that butterflies 

 and silkworm-moths copulate, and deposit their eggs, after de- 

 capitation ; and thus a natural instinct is fulfilled by nerve- 

 actions alone. (Compare § 269.) 



