IV. VERTEBBATA. 53T 



Besides the nervous system of the body already outlined, the vertebrates 

 have a special nervous system supplying the viscera, the sympathetic 

 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 corda 

 extend into the head and are connected with ganglia (otic, sphenopalatine). 

 This system sends out nerves in the form of delicate networks (plexus 

 sympathetici) which usually accompany the blood-vessels to the vegeta- 

 tive organs (intestine, sexual apparatus, etc.). It is also connected with 

 the spinal nerves. 



Regarding the sense organs of the vertebrates we stand on 

 firmer ground than with the invertebrates, since their great simi- 

 larity to those of man supports the ideas of their functions derived 

 from studies of their structure. The tactile organs make an ex- 

 ception, since only in land animals, and not in fishes, do they 

 resemble those of man. These organs, in all forms above fishes, 

 have the peculiarity that the nerves do not end in epithelial cells, 

 but in special tactile cells of the derma, which either lie isolated in 

 the connective tissue (Amphibia, reptiles), or, grouped together,, 

 produce tactile corpuscles (birds, mammals, 

 fig. 571). These are oval bodies and are im- 

 bedded in special papillae of the derma. In 

 form and position they are much like the 

 Vater-Pacinian corpuscles, which are distin- 

 guished by their histological structure (fig. 78) 

 and, since they also occur in internal organs 

 (mesentery of cat), are of problematic function. v >JV^T 



Besides these mesodermal nerve endings there FIG. 571. Tactile cor- 

 are present in all vertebrates intraepithelial tongife. 

 nerve branchings which are best seen in the cfeff 6 ) 

 cornea of the eye and in animals, like pigs and Petitions, 

 moles, with sensitive snouts. Even here the finest nerve twigs do 

 not end in epithelial cells, but in small knobs between them. 



Fishes lack tactile cells, tactile corpuscles, and end bulbs; 

 hence the skin is provided with sense organs in which a sensory 

 epithelium occurs. The dermal nerves pass into the epidermis 

 and end in oval corpuscles, which, while imbedded in a stratified 

 epithelium, consist of a single layer of sense cells. According to 

 structure, nerve hillocks and nerve-end buds are distinguished. 

 The first are the specific organs of the lateral line, to be men- 

 tioned later, of fishes and branchiate amphibians and amphibian 

 larvae, and therefore appear to subserve special and important sensa- 



