400 PHYSIOLOGY 



animal itself, we are justified in assigning it to the proprioceptive system, of 

 which indeed it represents the most important receptor. Just as the pro- 

 prioceptive nerves of a limb are responsible for the tonu's of the limb muscles, 

 so, as Ewald has shown, each labyrinth is responsible to a considerable degree 

 for the tonus of the corresponding side of the body. Extirpation of one 

 labyrinth causes a lasting loss of tone in the muscles of the same side. A 

 further functional resemblance lies in the part played by the labyrinth in the 

 determination of posture. The resultant effect of the impulses arising in it 

 is to maintain a reflex posture of the head and eyes, so that the optic axes in 

 a position of rest are directed towards the horizon. Stimulation of the 

 labyrinth causes therefore movements of the eyes which may or may not be 

 associated with correlated movements of the head. 



As in the case of the other sense-organs of the anterior end of the body, 

 the reflexes excited from the labyrinth dominate over those evoked by pro- 

 prioceptive impulses from the hinder portions of the body. At the entry 

 of its nerve into the brain stem a mass of grey matter is developed, which 

 must be regarded as the head ganglion of the proprioceptive system, and 

 the chief co-ordinating organ of all the reflex systems which determine 

 posture of the limbs and of the whole animal, and therefore the maintenance 

 of equilibrium both at rest and during locomotion. This organ is the 

 cerebellum, associated with the grey matter in the upper part of the fourth 

 ventricle at the point of entry of the vestibular nerves. The cerebellum 

 commences in early foetal life as a small elevation in the dorsal wall of the 

 neural tube, where the eighth nerve enters. Simple in structure and small 

 in extent in most of the fishes and amphibia, it grows in extent with increasing 

 complexity of the animal's motor reactions, and attains its greatest develop- 

 ment in the mammalia. In this class the cerebellum, like the cerebrum, is 

 most highly developed in man and the higher apes. It is generally described 

 in man as consisting of a middle lobe, composed of the superior and inferior 

 vermis, with two lateral hemispheres, and these are subdivided by anatomists 

 according to the situation of the chief sulci. From the physiological point 

 of view the structure of the organ is relatively simple, as is shown by the 

 uniformity of its structure throughout all parts. It may be considered as 

 formed of two main structures, viz. the cortex and the central or roof ganglia. 



The surface of the cerebellum is increased by being thrown into folds or laminae, 

 so that a section of this organ has a tree-like appearance. A section through a lamina 

 shows three distinct zones : an outer molecular layer presenting a granular appearance 

 with a few nuclei ; internal to this a granule layer composed of many nuclei of nerve- 

 cells ; and most deeply a central core of white matter. Between the molecular and 

 granular layers are situated the cells of Purkinje, large flask-shaped cells each with one 

 apical dendrite, distinguished above all other dendrites of the central nervous system 

 by the richness of its branching, and with one axon, which leaves the base of the cell 

 and passes down into the central white matter, giving off collaterals in its course. 

 In preparations made by Golgi's method we are able to distinguish the various elements 

 composing these layers and their relations. The molecular layer, besides neuroglia- 

 cells and the branching dendrites of the cells of Purkinje, contains certain star-shaped 

 cells (a Fig. 201), which give off an axon running parallel with the surface in the 

 molecular layer. From this axon branches dip down towards the cells of Purkinje, 



