856 COMPARATIVE ANATOMY 



In the higher forms this is also true, and the cerebellum (in man, 

 the pons also in a sense) are suprasegmental extensions. 



In both lower and higher forms the "eye. brain" includes the retina 

 of the eye, the optic nerve, and a part of the roof of the midbrain. In fish 

 only a few fibers from the optic nerve pass to the thalamus, but in the 

 higher forms the number of fibers to this portion are many, in fact so 

 many, that the entire thalamus, as stated, is often called the "optic thala- 

 mus." 



In the fish there are no true cerebral hemispheres, the seemingly 

 similar organs are hemispheres of the olfactory tract (with the exception 

 of the very small "somatic area" which becomes the corpus striatum and 

 cerebral cortex in the higher forms. The olfactory apparatus of the fish 

 also embraces the entire epithalamus and hypothalamus. 



It follows from all that has been said, that no nervous impulses can 

 enter the cortex without passing through the reflex centers of the brain- 

 stem first. The brain-stem therefore must have all the fibers lying within 

 it which are to carry such impulses. The suprasegmental portions are 

 therefore correlation, coordination, and readjustment centers. 



DIENCEPHALON 



The twixt-brain or inter-brain lies directly in front of the posterior 

 commissure. Still further to the front it is bounded by the velum trans- 

 versum above and the lamina terminalis below (Fig. 282, A, C). The 

 cavity in the center is a portion of the third ventricle which extends to 

 the optic chiasma. The fiber tracts running from the cerebral hem- 

 isphere backward pass into the side walls. Those lying in the dorsal 

 region go through the thalamus ( ) where there is 



a large nerve center. The ventral tracts are the cerebral peduncles al- 

 ready mentioned. Directly above and in front of the thalamus is the 

 epithalamus which also contains a nerve center known as the habenula 

 ( ). The hypothalamus, lying as its name implies 



below the thalamus, consists of the tuber cinereum in front and the mam- 

 millary bodies behind. Both the epithalamus and the hypothalamus bear 

 a relation to the sense of smell, and are, therefore, developed to a greater 

 extent in all lower animals in which this sense is highly developed. Di- 

 ^ectly behind the velum transversum is the superior or habenular com- 

 missure. 



EPIPHYSIAL STRUCTURES 



It is customary to call various parts developed in the roof plate of 

 the primitive fore-brain epiphysial structures (Fig. 202). Just where the 

 cerebral hemispheres and the twixt-brain meet there is a little fold of 

 epithelium, already mentioned, called the velum transversum hanging 

 from the roof of the cerebrum. Directly in front of this is a little chorioid 

 plexus that secretes a fluid. This is called the paraphysis. The other 



