56 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



transverse band, the corpus callosum. Traces of this occur in 

 amphibia and reptiles, but it acquires its highest development 

 in the higher mammals. The pons Varolii, passing beneath the 

 anterior pyramids of the cord, similarly connects the cerebellar 

 hemispheres in the higher vertebrates. Here, too, must be num- 

 bered the decussation, or crossing of the fibres of the anterior 

 pyramids from one side to the other. 



li 



FIG. 58. Longitudinal section of the brain of a frog, after Gaupp. The epi- 

 thelium blocked, ah, anterior part of hypophysis; ca, anterior commissure; cb, 

 cerebellum ; cc, corpus callosum ; ch, optic chiasma ; e, epiphysis ; fin, foramen of 

 Monro ; he, habenular commissure ; i, iter ; li, infundibular lobe ; Iti, lamina ter- 

 minalis, infraneural portion ; Its, lamina terminalis, supraneural portion; o, olfac- 

 tory lobe ; PC, posterior commissure ; pci, inferior and median choroid plexus ; pcp y 

 posterior choroid plexus ; q, posterior portion of mid brain ; rn, recessus neuropori ; 

 ro, recessus opticus ; v, velum medullare ant. ; 3?', 4?', third and fourth ventricles. 



In its earlier stages the brain lies in the same horizontal 

 plane with the spinal cord. Soon, by unequal growth of its 

 dorsal and ventral surfaces, bends or flexures appear. Most 

 constant of these is the cephalic flexure between fore and mid 

 brains, by which the axis of the fore brain is bent ventrally at 

 nearly right angles to the rest. Two other flexures may also 

 appear ; they are most prominent in mammals. The pontal 

 flexure, in the region of the pons Varolii, is in the opposite 

 direction ; the nuchal flexure, in the medulla, is ventral again. 

 <In the ichthyopsida these flexures largely disappear with growth ; 

 in the amniotes they persist throughout life. 



In the lower groups the five divisions of the brain are sub- 

 equal in size, but the higher vertebrates are characterized by 

 a great increase in size of the cerebellar, and especially of the 

 cerebral, regions, so that these completely cover over the twixt 



