CHORD ATA 347 



limbs. Their general arrangement will be seen from the study 

 of the frog. In the higher vertebrates the segmentation of the 

 axial muscles becomes less conspicuous especially in the head 

 region, and the appendicular muscles become relatively of 

 greater importance because of the greater use of the appendages. 

 The muscles associated with the entoderm are unstriate and not 

 under conscious control. 



364. The Nervous System. The nervous system in verte- 

 brates consists of two portions, the central and peripheral. 

 The central nervous part embraces the deep-seated organs, the 

 brain and spinal cord, and has for its most characteristic feature 

 numerous ganglion cells. From these central cells the cell-proc- 

 esses or fibres pass to the various tissues of the body, terminat- 

 ing in a manner appropriate to the special case whether it be a 

 muscle, sense organ, or gland. These nerves and their endings 

 constitute the peripheral part of the system. 



365. The Central Organs. The central nervous system origi- 

 nates from the ectoderm as a longitudinal groove-like depres- 

 sion in the mid-dorsal line of the embryo. The union of the 

 edges of this fold produces a tube and an overgrowth of the 

 ectoderm separates it from the outside world (Fig. 156). It 

 becomes surrounded by mesodermal elements (bone and con- 

 nective tissue, Fig. 158), and itself undergoes numerous and 

 complex changes.- At the anterior end of the tube occur three 

 distinct enlargements (Fig. 172, A). These are known as the 

 primary vesicles of the brain, and by the later growth and 

 differentiation of their walls they give rise to the five brain- 

 regions of the adult. The brain must be considered merely as 

 the specially modified anterior portion of the spinal cord. 



Three sets of changes occur in the development of the adult 

 vertebrate brain from this primitive condition : 



1 . The axis becomes more or less curved, the concavity being 

 ventral. 



2. Out-pocketings of the walls occur, in special regions, 

 whose cavities (ventricles) retain connection with the central 

 cavity (e. g., the hemispheres). See Fig. 172, h, pi. 



