There are numerous actions, especially among the lower animals, as little under the influence of the will or intelligence as the reflex nerves, which nevertheless depend upon sensation for their excitement. The sensation may call the muscular apparatus into action without 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 involuntarily 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 impulse, which reacts upon the muscular system without calling either the will or the instinct into exercise. These emotional movements are often performed in opposition 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 animals, 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, protected 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 modifications. In all the lower grades of life that have nerves, the system chiefly consists of small globular masses, or nuclei, of nervous matter, technically called ganglia, which are centres of nervous energy, each of which is endowed with its own peculiar properties; the nervous cords and filaments proceeding from them are merely organs of transmission. The arrangement of these centres of nerve-force is symmetrical, or unsymmetrical, according to the form of the animal.