XII. 24-25 B R A I N 349 



of optic tract fibres that end in its walls; other sensory projections 

 also reach here. In anurans, but not in urodeles, there is a partial 

 division into separate thalamic sensory nuclei, such as are found in 

 mammals, for touch, sight, and other receptor modalities. 



The pineal organ shows evidence of a retina in a few amphibia; 

 in most it is a simple sac. 



The pituitary body is well developed and the usual partes, anterior, 

 intermedia, nervosa, and tuberalis can be recognized, though they are 

 not in the same relative position as in mammals. 



Fig. 206. Rhythmical changes of potential between electrodes placed on the surface 

 of the olfactory bulb of the frog as in Fig. 205. (After Gerard and Young.) 



The midbrain is very well developed and shows many similarities to 

 that of fishes. The cells it contains do not all lie round the ventricle, 

 many have moved out to make an elaborate system of cortical layers. 

 Electrical stimulation of various parts of the optic tectum produces 

 movements of the limb and other muscles; there can be no doubt 

 that this region plays a dominant part in behaviour. Most of the fibres 

 of the optic tract end here, and there are also other pathways from 

 the olfactory, auditory, medullary (gustatory?), and spinal regions. 

 Efferent fibres leaving the tectum pass to the midbrain base, medulla, 

 and perhaps back into the cord. This region therefore has wider con- 

 nexions than any other part of the nervous system and thus nearly 

 reaches the status of a dominant integrating organ. 



The cerebellum of amphibia, on the other hand, is very small; 

 perhaps because these are mostly animals that do not have to adjust 

 themselves freely in space during locomotion, they move mainly in a 

 single plane. There is little need for control of speed or distance of 

 movement, except of the head and tongue, which are controlled by 

 the tectum. 



25. Skin receptors 



Lateral line organs are present in the skin of all aquatic amphibian 

 larvae and in some aquatic adults, such as those of the anuran family 

 Pipidae. They are of simple form, consisting of groups of cells in an 



