THE NERVOUS SYSTEM. 467 



type, like the visceral musculature of the body, but is of the striated voluntary 

 type, like the somatic musculature. The branchial receptors are naturally 

 visceral in character and there is also in this region a series of specialized 

 visceral receptors, the end buds of the gustatory system. The development of 

 this whole specialized visceral apparatus in this region of the head has appar- 

 ently caused a corresponding reduction of the somatic musculature. 



The musculature of the mouth is also splanchnic, the mouth itself being 

 regarded by many morphologists as a modified pair of gill clefts which has re- 

 placed an older mouth lying further forward in the region of the hypophysis. 

 The existence of this series of gill clefts has naturally caused a branchiomeric 

 or splanchnic segmentation of the musculature of this region as opposed to the 

 somatic muscular segmentation seen in the trunk. Whether these two kinds 

 of segmentation correspond in this region is uncertain. (In this connection see 

 Fig. 428 and p. 496.) 



In the acustico-lateral system three parts may be distinguished : (i) a remark- 

 able series of cutaneous sense organs, extending in lines over the head and body 

 and known as the lateral line organs; (2) the vestibule, including the semicircu- 

 lar canals; (3) the cochlea (organ of hearing proper Corti's organ). In the 

 higher Vertebrates, the lateral line organs have disappeared, owing to a change 

 from a water to a land habitat; the labyrinth has remained unchanged, and 

 the cochlea has undergone a much higher development and specialization. 



Regarding the optic apparatus, it is sufficient to point out here that its motor 

 part, the eye muscles, is usually taken to represent the sole remaining somatic 

 musculature belonging to the head proper. 



The peripheral nerves of the epichordal part of the brain have fundamen- 

 tally the same arrangements as the spinal nerves, namely, the peripheral af- 

 ferent neurone bodies are separate from the nerve tube, forming ganglia, while 

 the bodies of the efferent neurones are located centrally in the morphologically 

 ventral portions of the lateral walls of the nerve tube. There are, however, 

 important differences, clearly correlated with the peripheral differentiations and 

 specializations outlined above, and affecting the afferent and efferent nerves. 



First to be considered is the afferent part of the trigeminus (Figs. 405 and 

 406). The peripheral branches of the ganglion (semilunar or Gasserian 

 ganglion) of this nerve innervate that part of the external (somatic) surfaces of 

 the head (skin and stomodaeal epithelium) which have not been encroached 

 upon by the spinal afferent nerves. This nerve is accordingly more strictly 

 comparable with the afferent spinal nerves. The central processes of the 

 semilunar ganglion cells, after entering the brain, form a separate descending 

 bundle, the spinal V. It is interesting to note that the terminal nucleus of 

 this bundle of fibers is the morphological continuation in the brain of the 

 dorsal gray column of the cord. The extensiveness of the area innervated by 



