Characteristics of the Vertebrata. xxxvii 



and, with the exception of the ElasmohrancMi, in the auditory 

 org-ans. 



Tactile sensibility is possessed in a greater or less degree by 

 the entire cutaneous system. Special tactile organs are developed 

 in many Vertebrata around the region of the mouth, and in some 

 upon the extremities ; and we find in Fishes and in the larvae of 

 Amphibia, an additional set of tactile organs in the structures 

 which constitute the system of the lateral line, and are distributed 

 over the walls of the bend as well as along the sides of the trunk. 

 Taste may be localized either in the tongue, or in the throat, or 

 in both ; or is probably absent altogether, where the epithelium 

 covering this region becomes indurated or spinous. The nerves, 

 as opposed to the nerve-centres of Vertebrata, are developed in the 

 middle, and not in the uppermost layer of the embryo; they are 

 divisible into dorsal, latero-motor, and splanchnic sets, accordingly 

 as they are distributed to the structures formed by the dorsal 

 laminae, by the ventral laminae, and by the visceral factor into 

 which that middle laj^er divides itself. The sympathetic nervous 

 system is not contained within the cranio-spinal canal, and its 

 branches are mainly, though not exclusively, distributed to the 

 viscera of organic life. It consists, firstly, of bilaterally symme- 

 trical chains of ganglia arranged on either side of the thoracico- 

 abdominal, of the cervical, and occasionally, as in osseous Fishes, 

 also of the caudal vertebrae : and, secondly, of certain great prae- 

 vertebral plexuses, partly lodged in the substance, but for the most 

 part placed upon the exterior of the viscera they supj)ly. With 

 the first of these divisions are to be ranked four pairs of ganglia 

 developed in connection with branches of the fifth cranial nerve, 

 and in relation with the cephalic parietes. The entire sympathetic 

 system is developed out of the middle layer of the embryo, and the 

 greater part, if indeed not the whole of its first division, is a 

 dependency of the similarly developed cerebro-spinal nerves with 

 which it is connected functionally and anatomically in adult life, 

 and which must be taken into account when the nervous systems of 

 Vertebrata and Invertebrata are compared with each other. Certain 

 ganglia developed upon the posterior roots of these spinal nerves 

 in the intervertebral foramina, and upon the roots of certain 

 cranial nerves, in or close to certain cranial canals, resemble the 

 sympathetic ganglia in structure ; but from their very obvious and 



