3OS Elementary Biology. 



are given off many nerves, known as cranial and spinal 

 nerves, according as they arise from the brain itself or from 

 the spinal cord. 



The brain. The brain consists essentially of three 

 portions a fore-brain, a mid-brain, and a hind-brain. 



The fore-brain is composed of two portions, the pros- 

 encephalon and the thalamencephalon. The former consists 

 of two pear-shaped masses, narrowed anteriorly and known 

 as the cerebral hemispheres. From these there project two 

 smaller lobes, the olfactory lobes. The thalamencephalon 

 is much smaller and consists dorsally of a thin plate uniting 

 the two laterally placed optic thalami, from which in turn 

 on the ventral surface spring two broad bands of nerve-tissue 

 known as the optic tracts. The optic tracts cross each 

 other and form what is known as the optic chiasma. Pos- 

 teriorly from the dorsal surface of the thalamencephalon 

 there springs a small vascular prominence known as the 

 pineal gland. This structure has recently been shown to 

 be in all probability the rudiment of an eye. Ventrally also 

 a similar structure, the infundibulum, terminated by a small 

 mass of tissue, the pituitary body, arises from the posterior 

 portion of the thalamencephalon. The pituitary body is, 

 however, of totally different origin from the pineal gland, 

 and is indeed a bud from the roof of the mouth, which 

 becomes separated off and united to the base of the brain 

 in the course of development. 



The mid-brain is composed dorsally of two large 

 prominences the optic lobes, connected ventrally by a 

 broad belt of nerve-tissue. The mid-brain is also known 

 as the mesencephalon. 



The hind-brain, like the fore-brain, consists of two 

 regions the metencephalon, or cerebellum, and the 

 myelencephalon, or medulla oblongata. These two parts 

 cannot be easily distinguished from each other on the 

 ventral aspect; but dorsally the metencephalon is coincident 

 with a ridge of nerve-tissue, the cerebellum, which overlaps 



