ECONOMY OF THE BODY 121 



with, these optic lobes He in the middle line as in fishes, 

 but they are gradually shunted outwards and downwards 

 by their own growth and that of the adjacent parts. The 

 olfactory lobes in front of the cerebral hemispheres are 

 very small, in correlation with the poorly developed sense 

 of smell. A slight posterior separation of the cerebral 

 hemispheres will disclose the region of the optic thalami. 

 From this region in the embryo the eyes grow out, and it 

 also gives origin to the pineal body above and the pituitary 

 body below. The cerebellum is large, ridged transversely, 

 and divided into a median lobe and two small lateral lobes. 

 It has in part to do with the control of movements. In a 

 fish the brain lies quite flat ; in birds it has a marked curva- 

 ture, and the medulla oblongata is hidden below the cere- 

 bellum. There are, thus, five chief parts of the brain — the 

 cerebral hemispheres, the optic thalami, the optic lobes, the 

 cerebellum, and the medulla oblongata. 



Pineal Body. — On the roof of the bird's brain, between 

 the cerebral hemispheres and the cerebellum there lies a 

 small reddish body, the pineal body. It is borne on the end 

 of a stalk or epiphysis, which arises as usual as a pouch-like 

 diverticulum from the roof of the optic thalami region, and 

 afterwards becomes ampulla-like. Funkquist (191 2) notices 

 that the simple ampulla form is retained in the sparrow, 

 and that in divers and ducks tubules arise as buds from the 

 floor of the pouch, which may be subsequently constricted 

 off'. It is an epithelial organ to start with, but is modified 

 into neuroglia, and is invaded by connective tissue septa 

 with blood-vessels. In some cases, e.g. canary, the tubules 

 remain hollow ; in other cases, e.g. fowl, the lumen is more 

 or less completely obliterated. Apart from fine nerve- 

 threads accompanying the blood-vessels, there is no trace of 

 nerve-cells or nerve-fibres in the pineal body of birds. 



In some of the lower Vertebrates, e.g. lampreys, the 

 dorsal outgrowth from the roof of the optic thalami is 

 distinctly paired. There is (i) a pineal organ or epiphysis 

 proper, and there is (2) a parietal organ which arises from (1) 

 or independently in front of it. Perhaps they were originally 



