100 ZOOLOGY 



SECT. 



a cerebral nerve as regards its final exit. It is purely motor 

 supplying certain of the muscles of the shoulder. 



The twelfth or hypoglcssal (XII.) arises from the ventral aspect 

 of the medulla oblongata, after the manner of the ventral root of 

 a spinal nerve. It is purely motor, and supplies the muscles of 

 the tongue and certain neck-muscles. In the Amphibia its place is 

 taken by the first spinal nerve, and there is no doubt that it is to 

 be looked upon as a spinal nerve which has become included in 

 the cranial region : even in some Fishes it passes out through 

 the skull. 



The sympathetic nerve (sym.) is continued into the head and 

 becomes connected with some of the cerebral nerves. 



It will be noticed that there are facts in connection with the 

 cerebral nerves which suggest that they, like the spinal nerves, 

 have a segmental value, and indicate that the head of a Vertebrate, 

 like that of an Arthropod, is composed of fused metameres. For 

 instance, the nerves to the gills have a regular segmental arrange- 

 ment, and the conclusion is obvious that each visceral arch repre- 

 sents a metamere, the seventh, the ninth, arid the branchial 

 branches of the tenth being the corresponding segmental nerves. 

 But it has been shown that at an early period of development 

 the mesoderm of the head becomes divided into a number (9-19) 

 of distinct segments, like those which give rise to the myomeres 

 of the trunk and tail, and it is by no means certain that there is 

 any precise correspondence between this original segmentation of 

 the head and the segmentation of the pharynx which gives rise to 

 the gills and associated structures. It has been stated that the 

 first head-metamere gives rise to the superior, inferior, and internal 

 rectus muscles of the eye, the second to the superior oblique, and 

 the third to the external rectus. If this be so, the third, fourth, 

 and sixth are true segmental nerves, and the anomalous fact of 

 three out of ten nerves being devoted to the supply of the eye- 

 muscles is satisfactorily explained. It seems tolerably certain 

 that the third, fourth, sixth and twelfth nerves correspond to 

 ventral roots of spinal nerves they are all motor, and, except the 

 fourth, arise from the ventral region of the brain : the fifth, with 

 the exception of its motor root, and the seventh and eighth, ninth 

 and tenth appear to correspond to dorsal roots. 



Sensory Organs. The whole surface of the body forms an 

 organ of touch, but special tactile organs are more or less widely 

 distributed. End-buds consist of ovoidal groups of sensory cells 

 supplied by a special nerve : touch-cells (Fig. 735, A) are dermal 

 nerve-cells occurring at the termination of a sensory nerve : touch- 

 corpuscles (B) are formed of an ovoidal mass of connective tissue 

 containing a ramified nerve, the terminal branches of which end 

 in touch-cells : Pacinian corpuscles (C) consist of a terminal nerve- 



