THE CENTRAL NERVOUS SYSTEM 



833 



of the proprio-ceptive organs being the labyrinth, the central organ 

 of the whole proprio-ceptive mechanism is built up over the central 

 connections of the labyrinth. Thither converge connecting (inter- 

 nuncial) paths from the central endings of proprio-ceptive neurons 

 in all segments of the body (from joints, muscles, tendons, ligaments, 

 viscera, etc.). Thus a central organ is developed, which varies in 

 size and complexity in different kinds of animals according to the 

 complexity of their habitual movements. This is a convenient 

 place to consider a little more in detail the nature and peripheral 

 sources of some of the most important afferent impressions concerned 

 in equilibration. 



(i) 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. 

 348,421). The 

 other extremi- 

 ties of the su- 

 perior and pos- 

 t e r i o r canals 

 join together, 

 and have a 

 common aper- 

 ture 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 otoconia, 

 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 

 considerable size (otoliths}. Fibres of the auditory nerve end in 

 arborizations around the bodies of the hair-cells of the maculae 

 and cristae acusticae. We have already seen that it is the ventral or 



53 



FIG. 348. THE SEMICIRCULAR CANALS (DIAGRAMMATIC) 

 (AFTER EWALD). 



H, horizontal or external ; S, superior ; P, posterior. 

 The two horizontal canals lie in the same plane. The 

 plane of the superior vertical canal of one side is parallel 

 to the plane of the posterior vertical canal of the opposite 

 side. 



