350 THE MAINTENANCE OF THE INDIVIDUAL 



become exceptionally well protected. In all mammals this little gland 

 is lodged in a protective median depression in the sphenoid bone of the 

 skull called the "Turk's saddle," or sella turcica. The 'twixt-brain 

 also gives rise laterally to outgrowths of the lateral wall which form 

 the optic stalks that are essential to the development of the eyes. In 

 this region of the brain several problematical structures, of particular 

 interest to the comparative anatomist, such as the pineal eye, have 

 their origin. The cavity of the 'twixt-brain is called the third ventricle. 



The Mid-Brain or Mesencephalon. This portion of the brain 

 has kept many of its primitive embryonic characters, its gray matter 

 being still found largely in ganglionic masses. Anatomically, it is a 

 small region, the lumen of which, communicating anteriorly with the 

 third ventricle, is called the aqueduct. In lower forms like the fishes 

 and amphibia, the roof of this cavity is expanded dorsally into two 

 rounded protuberances, the optic lobes, or corpora bigemina. The 

 optic lobes of reptiles, birds, and mammals become further divided into 

 two pairs of centers known as the corpora quadrigemina, from which 

 are sent out bands of fibers, the anterior pair being connected with the 

 eyes, and the posterior pair with the ears. In forms below the mam- 

 mals the mid-brain functions as a co-ordinating center for impulses 

 entering through the eye, ear, and certain nerves of the body. In 

 the mammals much of this co-ordinating function has been taken over 

 by the cerebrum. Upon the latero-ventral surface of the mid-brain 

 may be seen a band of fibers, the crura cerebri, forming a highway of 

 communication between the cerebrum and the posterior parts of tlie 

 central nervous system. Two motor cranial nerves, the oculomotor 

 (III) and the trochlear (IV), which supply muscles of the eye, arise 

 here. 



The Cerebellum or Metencephalon. While the surface of the 

 cerebellum is not convoluted in the same manner as the cerebrum, 

 nevertheless its surface of gray matter is increased by being thrown 

 into numerous furrows. It is composed of two hemispheres connected 

 by a bridge, the vermis, and has consequently been likened to a butter- 

 fly with the bridge forming the body. The cerebellum lies just pos- 

 terior to the cerebrum and dorsal to the mid-brain. When cut in 

 sagittal section it is seen to be composed of radiating folds, arranged 

 in an outer layer of gray matter and an inner core of white matter. 

 Taken as a whole the white matter somewhat resembles a tree and so 

 has been called the arbor vitae, or "tree of hfe." 



Ventrally, a swollen band of fibers, called the pons varolii, is 



