26 LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



and a pigmentary layer lying in and under the latter, in this 

 " parietal organ." We owe its discovery to Graaf and Spencer. 

 In the other vertebrates we detect no relation between the epi- 

 physeal tube and the organ of special sense in the adult animal. 

 The tube has disappeared in the depths of the skull, and the 

 parietal eye, as is shown by transitional forms in amphibians 

 and reptiles, is so completely lost that no trace of it can be 

 found in birds or mammals. The blunt, knotted end of the 

 tube remains as a nodule, the " pineal gland," in front of the 

 mid-brain. One of the thalamic ganglia, the ganglion haben- 

 uhe, demonstrable in all animals, is united with its fellow of the 

 opposite side by a commissure. This commissura thalami dorsalis 

 forms a part of the roof of the inter-brain in front of the 

 epiphysis. 



In all animals the optic tract, lying on the outside of the 

 inter-brain, passes in a gradual descent from the mid-brain to the 

 base of the brain. Between it and the inter-brain proper there 

 is found, in fishes, amphibians, reptiles, birds, and mammals, 

 another ganglion which lies more or less firmly imbedded in the 

 mass of the thalamus (corpus geniculatum laterale). It is one of 

 the points of origin of the optic nerve. The main point of 

 origin of these nerves, however, is the roof of the mid-brain. 

 This roof changes less in the different classes of animals than 

 any other part of the brain. Only its relative size changes, and 

 he who has only seen the small corpora quadrigemina of the 

 human brain will be astounded when he sees the huge optic 

 lobes of a fish or a bird ; but the finer structure is always the 

 same. From the dorsal layer of the hemispherical lobe, which 

 is somewhat flattened by a sagittal furrow, the optic nerve 

 always takes its origin. From the deeper layers arises a system 

 of sensory fibres, the deep marrow. The latter forms a net- 

 work around the aqueduct of Sylvius, and the greater part of it 

 passes caudad as the lemniscus, or fillet. 



In the posterior part of the mid-brain roof there is in all 

 animals a separate nucleus, fibres from which associate them- 



