XERVOU8 SYSTEM OF THE YERTEERATA. 



347 



in/ v 



the body, while in the older one the cerebral hemispheres (cer) have 

 grown very greatly, especially for- 

 wards and dorsalwards. They have 

 thus come to lie in front of the mid- 

 brain, and to form the end of the 

 long axis of the body, and have at 

 the same time compressed the origi- 

 nally large thalamencephalon against 

 the mid-brain. The same general 

 features may be seen in fig. 250 re- 

 presenting a longitudinal section of 

 the brain of an embryo fowl, and 

 fig. 255 representing a longitudinal 

 section of the brain of a Mammal. 



The infundibulum or perhaps 

 rather the point of origin of the 

 optic nerves is to be regarded as 

 the anterior termination of the axis 

 of the base of the brain. 



The cranial flexure is least marked 

 in. Cyclostomata (tig. 253), Teleostei, 

 Ganoidei, and Amphibia, while it is 

 very pronounced in Elasmobranchii, 

 Reptilia, Aves, and Mammalia. In 

 Teleostei, and still more in Cyclosto- 

 mata, it permanently remains slight, 

 owing to the small development of the 

 cerebral hemispheres. 



In addition to the cranial flexures, two other flexures make their 

 appearance in the base of the brain. A posterior at the junction of the 

 brain and spinal cord, and an anterior at the boundary between the 

 cerebellum and medulla oblongata, just at the point where the pons Varolii 

 is formed in Mammalia. The anterior of these is the most marked and 

 constant ; it is shewn in fig. 250. It arises considerably later than the main 

 cranial flexure, and since it is turned the opposite way it assists to a con- 

 siderable extent in causing the apparent straightening of the cranial axis. 



Histogenetic changes'. The walls of the brain are at first very 

 thin and, like those of the spinal cord, are formed of a number of 

 ranges of spindle-shaped cells. The processes of each of these cells 

 are stated to be continued through the whole thickness of the wall. 

 In the floor of the hind- and mid-brain a superficial layer of delicate 

 nerve-fibres is formed at an early period. This layer appears in 

 the first instance on the floor and sides of the hind-brain, and very 

 slightly, if at all, later on the floor and the sides of the mid-brain. 

 The cells internal to the nerve-fibres become differentiated into an 



1 It is not within the scope of this work to give an account of the histogenesis of the 

 brain ; in the statement in the text only a few points, of some morphological import- 

 ance, are touched on. 



f 



FIG. 248. LONGITUDINAL SECTION 

 THROUGH THE BRAIN OF ScYLLIUM CA- 

 NICULA AT AN ADVANCED STAGE OF DE- 

 VELOPMENT. 



cer. cerebral hemisphere ; />. pi- 

 neal gland ; op.th. optic thalamus, con- 

 nected with its fellow by a commissure 

 (the middle commissure). In front of 

 it is seen a fold of the roof of the fore- 

 brain, which is connected with the cho- 

 roid plexus of the third ventricle; op. 

 optic chiasnia ; pt. pituitary body; in. 

 infundibulum; cb. cerebellum; au.r. 

 passage leading from the auditory vesi- 

 cle to the exterior; mt'l. medulla ob- 

 longata; C.in. internal carotid artery. 



