78 AN INTRODUCTION TO ZOOLOGY 



terminate in a small knob-like enlargement just beneath the fronto- 

 parietal bones. This is the pineal body or epiphysis cerebri, the 

 vestigial remnant of what was once apparently a pair of eye-like 

 structures ; it is generally removed in dissecting out the brain, but 

 its stalk can usually be made out readily. 



The succeeding part of the brain is the mid-brain or mesence- 

 phalon. Dorsally it takes the form of two conspicuous ovoid lobes, 

 the optic lobes or corpora bigemina, whose long axis is inclined out- 

 wards at an angle of about 45 degrees to the median line, and over 

 which, as already noticed, the pia mater is deeply pigmented. The 

 hind-brain or rhombencephalon is divided into two portions, that in 

 front, immediately behind the optic lobes, is the cerebellum or 

 metencephalon, appearing as a small transversely running fold of 

 tissue. The remaining part of the hind-brain is a good deal larger, 

 almost as long as the cerebral hemispheres, and variously known as 

 the bulb, medulla oblongata or myelencephalon. This again is 

 conspicuous in the freshly killed animal, owing to the presence of a 

 blood network, the posterior choroid plexus, which in the form of a 

 long isosceles triangle covers a large part of its dorsal surface. The 

 hinder part of the medulla is continuous with the spinal cord. 



Turning now to the ventral surface of the brain, we find in front 

 the olfactory lobes, followed by the cerebral hemispheres separated 

 by a marked groove. The thalamencephalon is a small but an 

 important part of the brain, and under it the nerves going to the 

 eyes, the optic nerves, form a very characteristic X-shaped structure 

 known as the optic chiasma. Immediately behind this is a bi-lobed 

 swelling with a median groove, the tuber cinereum or infundibulum. 

 It lies in the mid-ventral line and has attached to it the pituitary 

 body or hypophysis cerebri. This is composed of a flat median 

 cushion, immediately behind the tuber cinereum, which gives off 

 two small lateral tongue-like processes running forward. 



The ventral part of the mid-brain is formed by two large columns 

 of nervous matter, the crura cerebri, that connect up the hemispheres 

 with the medulla which forms the remaining part of the brain and 

 has running down the middle of it the ventral fissure. 



The whole of the central nervous system is hollow, and in the brain 

 the central cavity swells out to form a series of spaces known as 

 ventricles. The first are the ventricles in the cerebral hemispheres, 

 an outgrowth from each of which extends forward into the olfactory 

 lobes. The cavity of the thalamencephalon is known as the third 

 ventricle. Its roof is formed mainly by the anterior choroid plexus, 

 and in its nervous portion run two transverse bands of fibres, the 

 superior and posterior cerebral commissures, joining the cortex of 

 one hemisphere to that of the other. From its floor a pocket projects 



