BRAIN OF MAMMALIA. 9 



the cerebrum, the cerebellum, and the medulla oblongata, the continuation 

 of which latter is termed the medulla spinalis. 



The cerebrum constitutes, in Mammalia, by far the largest mass, and, in 

 Man, it occupies the whole of the upper and anterior portions of the cranial 

 cavity. It is primarily divided by a deep vertical fissure, running, longi- 

 tudinally, into two nearly equal parts, termed hemispheres. Each of 

 these hemispheres is made up of three lobes, an anterior, a middle, and 

 a posterior. The anterior lobe is frontal, and rests on the orbital plate of 

 the frontal bone, a plate which constitutes the roof of the orbit ; the middle 

 rests on the sphenoid and temporal bones ; the posterior covers and rests on 

 the cerebellum, in Man, but not in the lower Mammalia. Gently separating 

 the two hemispheres of the cerebrum (between which is inflected a redu- 

 plication of the dura mater), at the bottom of the fissure may be observed 

 a white transverse body, or commissure, termed the corpus callosum,* and 

 also an anterior and posterior medullary chord or commissure (commis- 

 surae cerebri anterior et posterior). Underneath the corpus callosum is 

 the delicate partition between the ventricles, termed septum lucidum, of 

 which the inferior side is vaulted, and forms the fornix. On the respec- 

 tive sides of the septum lucidum are the cavities of the cerebrum, termed 

 its lateral ventricles, which communicate with each other near the middle 

 of the fornix. In the lower and interior parts of each ventricle is a curved 

 medullary process, termed, from its form, cornu Ammonis. In the anterior 

 part of each ventricle is a pyramidal process of a cineritious colour, 

 exposing, when divided, a striated texture, whence it is termed corpus stria- 

 turn. Behind these are two eminences of a white colour, called thalami 

 nervorum opticorum, and between them is a third ventricle. After death 

 these ventricles are found to contain a serous fluid, more or less in 

 quantity. Behind the thalami is the pineal gland (which Des Cartes re- 

 garded as the seat of the soul), remarkable for containing, at least very 

 commonly, small gritty concretions of phosphate of lime, which, when the 

 gland is crushed between the fingers, communicate a rough sand-like sen- 

 sation to the touch. Beneath the pineal gland, are the optic lobes, of 

 an insignificant size compared to what they are in fishes (where they 

 are hollow, and exceed the cerebral hemispheres), and divided into four 

 parts ; hence called corpora quadrigemina. 



In each lateral ventricle is a tissue of vessels, termed the choroid plexus, 

 which is spread upon the thalami, the pineal gland, and the optic tubercles. 



The cerebellum, in Man, is covered by the posterior lobes of the cere- 

 brum, and rests on the floor of the occipital bone ; in the lower Mammalia, 

 the Orangs and Monkeys excepted, it is not covered by the cerebrum, but 

 is placed behind that portion of the brain. Its surface is beautifully 



* It will be seen, hereafter, that the corpus callosum does not exist in the marsupials. 

 VOL. I. c 



