586 Dynamic Theory. 



In the birds and reptiles the optic lobes are a single pair, and so they 

 are in the monotremes amongst the mammals. But in the monotremes 

 the valvula of vieussens becomes thickened at its anterior edge by the 

 addition of transverse white fibres just back of the optic lobes. This 

 swelling is the beginning of the second pair of optic bulbs, the testes. 

 These become well developed in the marsupials, and occur in all the 

 families above these, the whole four bulbs generally going under the 



FIG. 303. 



FIG. 302. 



FIG. 302. Side view of Brain of Ornithorhynchus, 



FIG. 303. Bottom view. 



1. Anterior pyramids. 



2. Olivary bodies. 



3. Corpora trapezoidea ( Arcif orin tibres in man). 



4. Pons Varolii. 



6. Cerebellum. [large). 7. Third branch of same. 



6. rwo roots of the Tngeminal nerve (very 8. Ganglion in which it originates. 



name of the corpora quadrigemina, the original pair being the nates, 

 and the new pair the testes. 



In birds the ventricle of the hemisphere, or lateral ventricle, is al- 

 most filled by the corpus striatum, and in them there is no hippocampus. 

 In the mammals, however, the shell of the cerebrum begins to enlarge, 



FIG. 304. Brain of Dasyure, or Native Devil (a marsupial), 

 a. Olfactory lobes. c. Optic lobes. 



6. Cerebrum. d. Cerebellum. 



e. Medulla oblongata. 



and in doing this commences to form itself into 

 folds or convolutions. One of the first and most 

 constant of these is a fold pushed into the ven- 

 tricle from the inner margin of the temporal 

 lobe, from the inside wall of each hemisphere, 

 where the two abut against each other. This 

 fold of brain matter pushed into the lateral ven- 

 tricle is called the hippocampus, (hippocampus 

 major, or cornu ammonis, in Man and the Quad- 

 rumana) and the crease formed on the external 

 part of the inner face of the hemisphere, by the 

 fold, is called the hippocampal fissure. Longi- 

 tudinal fibres are developed in connection with the hippocampus, form- 

 ing a band named the taenia hippocampi (or corpus fimbriatum in man). 

 This band, starting at the pes hippocampi at the lower, or posterior, ex- 

 tremity of the ventricle, runs up to the roof of the ventricle and be- 

 comes the posterior pillar of the fornix. It then bends forward along 



FIG. 304. 



