NERVOUS SYSTEM, SENSES, AND SENSE ORGANS 179 



intimately associated with the sense of smell. Starting with 

 fishes, and passing upwards through the amphibians and 

 reptiles to the birds and mammals, this region of the brain 

 becomes progressively larger, until in man it forms a very large 

 and important part. It is of interest to note that the Selachians, 

 so much more primitive in many other characters, have the 

 cerebral region of the brain much better developed than in the 

 majority of Bony Fishes. Other important parts derived from 

 the primary fore-brain are the optic vesicles, which arise from 

 the sides and later become transformed into parts of the eyes 

 and their associated nerves, and the pineal body or gland, 

 arising from the roof, which will be considered further in 

 connection with the eyes. From the floor of the fore-brain a 

 hollow outgrowth, the infundibulum, develops, to which is 

 attached the pituitary body or gland, whose secretion plays an 

 important part in the regulation of the activities of the body : 

 at the sides of this gland are two small lobes and a tiny sac. 

 The remaining structures, which arise inside the original fore- 

 brain, are of minor importance and need not be detailed here. 



The roof of the mid-brain bulges out to form a pair of optic 

 lobes (Fig. 71), which may or may not be connected with the 

 main central cavity or vesicle. They vary greatly in size in 

 different fishes, and may cover the fore-brain and press against 

 the cerebral hemispheres. In some of the Lung-fishes (Dipneusti) 

 the two are united to form a single oval body. As their name 

 implies, the optic lobes are associated with the sense of sight. 



The principal part formed from the original hind-brain is a 

 large single lobe, the cerebellum (Fig. 71), lying behind the 

 optic lobes. Below this is the medulla oblongata, the cavity 

 of which communicates with the cerebellum above and with the 

 central canal of the spinal cord behind. In the Lampreys 

 {Petromyzonidae) the cerebellum is very small, in the Hag-fishes 

 [Myxinidae) it is absent altogether; in the Selachians and Bony 

 Fishes it is very large, sometimes almost covering the optic 

 lobes. 



The brain never entirely fills the cavity of the cranium, the 

 space between it and the membrane lining the inner surface of 

 the cranial cavity being filled with a sort of gelatinous tissue. 

 In a young fish it is very much larger in proportion to the size 

 of the body than in an adult. The size of the brain also exhibits 

 considerable variation in diflferent fishes, although on the whole 

 it may be regarded as relatively small. The brain of the 

 Burbot [Lota) has been estimated to be -\j^ of the weight of 



