110 NERVOUS SYSTEM OF VERTEBRATES. 



by rami from the V nerve. From this it would appear that the 

 hyoid segment had originally its own cutaneous innervation and 

 that when the operculum was formed the component in VII 

 disappeared and the V nerve supplied the parts of the hyoid 

 segment remaining exposed. 



The trigeminus gives small branches to the dorsal surface of 

 the head behind the eye, and in fishes it gives a larger ramusophthal- 

 micus superfacialis trigemini to the skin above the eye. It then 

 forms the rami maxillaris and mandibularis which supply the 

 skin of the upper and lower jaws and the lining of the stomodaeum. 



The ophthalmicus profundus nerve supplies the area in front 

 of the eye to the tip of the snout. In tailed amphibians the 

 maxillary nerve is greatly reduced and the profundus takes on 

 the innervation of the territory of the maxillaris. 



THE CENTRAL APPARATUS FOR CUTANEOUS IMPULSES. This 



consists of the dorsal horn of the gray matter in the spinal cord 

 and secondary tracts and centers connected with it, and of corre- 

 sponding structures in the brain. The cutaneous fibers have their 

 ganglion cells in the spinal ganglia. The central processes of 

 these ganglion cells enter the dorsal part of the cord and there 

 bifurcate in T- or Y-form. The two branches run one cephalad 

 and one caudad, forming the dorsal tracts of the cord. In man 

 it is known that the cephalic branch is the longer and this seems 

 to be the case in lower vertebrates as well. In man and mammals 

 the cephalic branches of fibers in the more caudal roots are pushed 

 toward the median plane by the incoming fibers of the successive 

 roots farther forward, so that the long cephalic branches come to 

 form a definite bundle (the mesial funiculus) mesial to the 

 fibers of the more cephalic roots (the lateral funiculus). Each 

 branch of a cutaneous fiber as it runs forward and backward 

 in the cord gives off fine collateral branches. These are distrib- 

 uted to the same and to the opposite side, to the ventral horn, 

 and to other parts of the cord as shown in Figs. 41, 52. The col- 

 laterals going to the ventral horn serve for the simplest reflexes. 

 The other collaterals spread the impulses widely through the 

 adjacent regions of the cord and so make possible more ample 

 responses. The ascending and descending branches end finally 



