IV. VERTEBRATA 473 



innervate the somatic musculature, the dorsal roots supply both the sensory 

 apparatus and the visceral muscles. 



One of the most striking facts in the question of the segmental nature of the 

 cranial nerves is the relation of these to the visceral arches, the trigeminal in front 

 of the mandibular, the facial in front of the hyoid arch, and the glossopharyngeal 

 and vagus with similar relations to the gill arches. 



Besides the nervous system of the body already outlined, the verteVjrates have 

 a special ner\'0us system supplying the viscera, the sympalhctic system, and in 

 this a special central organ consisting of right and left cords beneath the vertebral 

 column, in which ganglia are incorporated. The last of these ganglia lies at the 

 base of the caudal vertebrae, the most anterior at the beginning of the neck. 

 From the latter nerve cords extend into the head and are connected with ganglia 

 {otic, sphenopalatine). This system sends out nerves in the form of delicate 

 networks (plexus sympalhclici) which usually accompany the blood-vessels to 

 the vegetative organs (intestine, sexual apparatus, etc.). It is also connected 

 with the spinal nerves. 



The space between the central nervous s)^stem and the surrounding 

 skeleton is large in fishes and is tilled with a loose fatty tissue which, close 

 to the nervous mass, is thickened to a richly vascular envelope {mcninx) 

 of the brain and spinal cord. In the groups from amphibia to birds 

 this primitive meninx is divided by the development of a subdural lympli 

 space into two layers, the dura mater and the pia mater. The pia, which 

 in mammals differentiates a delicate arachnoid membranehy the for'msition 

 of numerous lymph spaces, is next to the nervous system and is the 

 vascular layer, while the dura is a tougher membrane wdiich approaches 

 and fuses with the periosteum lining the skull in proportion to the extent 

 that the brain fills the cranial cavity. In the ver- 

 tebral column the dura and the periosteum remain 

 distinct. 



Regarding the sense organs of the vertebrates 



we stand on firmer ground than with the invertc- ~J^ 



brates, since their great similarity to those of man 



supports the ideas of their functions derived from 



studies of their structure. Of the sense organs of 



the skin, the tactile organs of mammals consist of 



,., ,. , ., . , Fig. 52S. — Tactile 



nerve nbres which extend into the epidermis, lose corpuscle from bird's 



the medullary sheath, and branch among the epi- to"gue- H, outer envel- 

 , ' . ... ope; A'H", its nuclei; iV, 



thelial cells where they may terminate in special tac- nerve; S, partitions. 



tile discs (Elmer's organs of snout of pig and mole). 

 Besides these interepithelial nerve endings are others in the subepithelial 

 mesoderm (tactile corpuscles) (fig. 528), wdiich are specially modified and 

 receive special names (Krause's and Grandry's corpuscles). They consist 

 of a few or many cells, probably of epithelial origin, enclosed in a con- 

 nective tissue envelope. The nerve fibres branch within the organ, the 



