344 ZOOLOGY; 



of the body-wall and those which move the skeleton. The 

 fibres of these muscles are cross-striped or voluntary (Fig. 

 28). It is by means of them that locomotion is effected. The 

 skeletal muscles may be classed as (i) axial, and (2) appendi- 

 cular. The axial are well shown in Amphioxus (Fig. 152) and 

 the fishes, where the whole body is made up of repeated seg- 

 ments (myotomes) of muscle fibres. The muscle segments 

 alternate with the segments of the spinal column, as one would 

 expect. The appendicular muscles are those which move the 

 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 append- 

 ages. 



356. The Nervous System. The nervous system in ver- 

 tebrates 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 fea- 

 ture numerous ganglion cells. From these central cells the 

 cell-processes or fibres pass to the various tissues of the body, 

 terminating 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. 



357. The Central Organs. The central nervous system 

 originates from the ectoderm as a longitudinal groove-like 

 depression 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. 154). 

 It becomes surrounded by mesodermal elements (bone and 

 connective tissue, Fig. 156), and itself undergoes numerous and 

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

 distinct enlargements (Fig. 170, 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- 



