114 THE MECHANISM OF LIFE 



arise as outgrowtlis from the lateral fore-brain vesicles that 

 become, later on, the cerebral hemispheres. Therefore the 

 connection of the nerve terminations in the mucous membranes 

 of the nose are with the cortex cerebri direct, and this may be 

 the reason why the sensation of smell is said to be more 

 " reminiscent " than those of vision or hearing: it sets up more 

 immediate " associations," because of its place of entrance into 

 the higher brain. Less, however, is known about the internal 

 tracts along which the olfactory impulses travel in man than 

 in the lower animals, because of the degeneracy of the human 

 organ of smell in comparison with that of most other vertebrate 

 animals. In some fishes, for instance, the great development 

 of the olfactory organs and their nervous tracts suggests that 

 the sensation in question plays a very important part in the 

 general behaviour of these animals. 



The Auditory Organs. — Two entirely different sense organs 

 are contained in the " internal ear." " Vibrations of sound " — 

 that is, very rapid, periodically repeated alternations of com- 

 pression and rarefaction of the air — are made to impinge on the 

 tympanic membranes, or " drums of the ear," and then, by 

 means of the chain of little bones, called the auditory ossicles, 

 these vibratory movements of the drums are transmitted to 

 the fluid contained in peculiarly shaped cavities in bones on each 

 side of the head. Two organs are contained in this cavity, or 

 " bony labyrinth " — the organ of equilibrium, and that of true 

 hearing. The former consists of a little membranous sac from 

 which proceed three semicircular canals, which are arranged in 

 the three planes to which we refer all positions in abstract space. 

 That is, one canal is vertical and runs forward and backward, 

 another is vertical and runs from side to side at right angles to 

 the first one, while the third is horizontal and is at right angles 

 to the first and second. The dendrites of one branch of the 

 eighth or auditory nerve project into the fluid contained in the 

 bases of the semicircular canals. Operative interference in 

 many animals and diseased conditions in man show that destruc- 

 tion of one or other of the canals produces well-marked abnor- 

 malities in locomotion, apparent giddiness, and lack of co- 

 ordination over the muscles of the limbs. The " vestibular " 

 part of the ear is therefore an organ of equilibrium, and its nerve 

 terminates in a nucleus in the medulla, which is connected in 

 ways that are not well known with the mid- brain on the one 



