io NERVOUS SYSTEMS PART m. 



out any exertion of reason or will, in such, a manner as to 

 produce actions as directly and obviously adapted to the 

 well-being of the individual as the reflex system. For 

 example, a grain of dust irritates the nostrils, and in- 

 voluntarily excites the complicated muscular movements 

 concerned in the act of sneezing. This class of actions, 

 which is called sensori-motor, or consensual, includes 

 most of the purely instinctive motions of the lower 

 animals, which, being prompted by sensations, cannot 

 be assigned to the reflex group. 



Purely emotional movements are nearly allied to the 

 preceding. Sensation excites a mental feeling, or im- 

 pulse, which reacts upon the muscular system without 

 calling either the will or the instinct into exercise. 

 These emotional movements are often performed in op- 

 position to the strongest efforts of the will, as when a 

 sense of something ridiculous may excite irresistible 

 laughter at an improper time. It is probable that the 

 strong emotions exhibited by many of the lower ani- 

 mals, which have been ascribed to instinct, are referable 

 to this group. 1 



The movements of such animals as have no nerves 

 are merely owing to the vital contractility of muscular 

 fibre. 



In the highest province of animal life, which includes 

 the mammalia, birds, reptiles, and fishes, the general 

 structure of the nervous system consists of a double 

 lobed brain, from whence a spinal cord proceeds, pro- 

 tected by articulated bones, which extend along the 

 back of the animals, and from thence nerve-fibres extend 

 to every part of the body. But in order to suit a great 

 variety of forms, this system undergoes many modifica- 

 tions. In all the lower grades of life that have nerves, 

 the system chiefly consists of small globular masses, or 



1 The nervous system is ably explained in Dr. Carpenter's ' Manual of 

 Physiology.' 



