$60 COMPARATIVE ANATOMY 



known as the inferior cerebellar peduncle, on either side. The rest of 

 the fiber tract forms a pair of bands, called pyramids, on the ventral sur- 

 face of the medulla which extend cephalad beneath the mid-brain. These 

 extensions are called the cerebral peduncles or crura cerebri. They are 

 easily found in the lower vertebrates, but in the mammals the pons makes 

 them difficult or impossible to see. 



TELAE CHORIOIDEAE 



While the brain is supplied by blood vessels distributed over the 

 outer surface, extensions from, the outer vessels push the roof and floors 

 of most of the fore- and hind-brain behind them, into the ventricle of 

 these two regions, very much on the same principle as an outgrowth of 

 the digestive tract, such as the liver, pushes its peritoneum-covering be- 

 fore it. These foldings of the plates are called telae chorioideae or 

 choirioid plexuses, and it is through these that the nourishing blood 

 passes by osmosis into the ventricle and into the inner surfaces of the 

 brain. It is practically impossible to remove the brain and have the 

 fourth ventricle complete. Usually the chorioid plexus of this fourth 

 ventricle is torn away as it is very thin in this particular region. The 

 large open space or cavity that one sees when this has been torn away is 

 -called the fossa rhomboidalis. 



SUMMARY OF THE BRAIN 

 AMPHIOXUS 



The brain is extremely small, hardly as large in diameter as the rest 

 of the neural tube. There are but two pairs of cranial nerves, which 

 have been called olfactory and optic, but in so reduced a brain, homolo- 

 gies are uncertain. The sense organs consist of a median olfactory fun- 

 nel opening into the neurocoele, a median eye-spot (not sensitive to 

 light) on the anterior end of the brain, representing probably a rudiment 

 of paired eyes. The notochord extends the entire length of the body, 

 projecting in front of the brain. This may mean that the brain has re- 

 treated from its primitive anterior position. There is no cranium. 



CYCLOSTOMATA 



The brain is small but typically vertebrate in structure. The vagus 

 nerve is not included in the cranial region. In the myxinoids, a groove 

 runs the entire length of the dorsal surface. There are four pairs of lobes 

 (!) olfactory, (2) cerebral hemispheres, (3) mid-brain, and (4) cere- 

 bellum. The nasal capsule is enormously developed. The eyes are de- 

 generate and without muscles or nerves. There is only one semi-circular 

 canal in the inner ear. In the lampreys the cerebral hemispheres are 

 distinct and a band-like cerebellum is recognizable. Eyes are well devel- 

 oped with both muscles and nerves. There are two semi-circular canals 

 in the ear, a condition intermediate between that seen in the myxinoids 

 and that in the true fishes, where three canals are always present. 



