73 2 A MANUAL OF PHYSIOLOGY 



equilibrium. W r hen the soles of the feet are anaesthetized by chlon> 

 form or by cold, and the person is directed to close his eyes, he 

 staggers and sways from side to side. The disturbance of equili- 

 brium in locomotor ataxia must be partly attributed to the loss of 

 these tactile sensations, for numbness of the feet is a frequent 

 symptom, and the patient asserts that he does not feel the ground. 

 An interesting illustration of the importance of afferent impulses 

 from the skin in the maintenance of equilibrium is afforded by the 

 behaviour of a frog deprived of its cerebral hemispheres. Such a 

 frog will balance itself on the edge of a board like a normal animal, 

 but if the skin be removed from the hind-legs, it will fall like a log. 



(3) Afferent Impulses from the Semicircular Canals. The semi- 

 circular canals are three in number, and lie nearly in three mutually 

 rectangular planes : the external canal in the horizontal plane, the 

 superior canal in a vertical longitudinal plane, and the posterior 

 canal in a vertical transverse plane. Each canal bulges out at one 

 end into a swelling, or ampulla, which opens into the utricular 

 division of the vestibule (Figs. 263, 324). The other extremities of the 

 superior and posterior canals join together, and have a common 

 aperture into the utricle, but the undilated end of the external or 

 horizontal canal opens separately. The utricle and the semicircular 

 canals are thus connected by five distinct orifices. The greater part 

 of the internal surface of the membranous canals, utricle and saccule, 

 is lined by a single layer of flattened epithelium. But at one part of 

 each ampulla projects a transverse ridge, the crista acustica, covered 

 not with squamous, but with long columnar epithelium. Hair-like 

 processes (auditory hairs) are borne by some of the columnar cells, 

 between which lie more elongated fibre-like supporting cells. The 

 hairs project into a mucus-like mass, sometimes containing stoconia, 

 or crystals of calcium carbonate. The ampullae, like the rest of the 

 membranous labyrinth, is filled with a watery fluid called endolymph. 

 The utricle and saccule have each a somewhat similar but broader 

 elevation, the macula acustica, covered with epithelium and hair-cells 

 of the same character, and the hairs project into a similar mass in 

 which otoconia are constantly present. In some animals, as fishes, 

 the calcareous matter in the utricle and saccule forms masses of con- 

 siderable size (otoliths). Fibres of the auditory nerve end in arboriza- 

 tions around the bodies of the hair-cells of the maculae and cristae 

 acusticae. We have already seen that it is the vestibular division 

 of the nerve which is especially related to the vestibule (p. 723). 



There is very strong evidence that the semicircular canals are con- 

 cerned, not in hearing, but in equilibration. A pigeon from which 

 the membranous canals have been removed still hears perfectly well 

 so long as the cochlea is intact, but exhibits the most profound 

 disturbance of equilibrium. If the horizontal canal is destroyed 

 or divided the pigeon moves its head continually from side to 

 side around a vertical axis; if the superior canal is divided, the 

 head moves up and down around a horizontal axis. The power 

 of co-ordination of movements is diminished, but not to the same 

 extent in all kinds of animals. Thrown into the air, the pigeon is 



