SENSE ORGANS. 



121 



The sense organs present the following arrangement. The anterior 

 is the olfactory organ, which consists of a pit usually paired, excep- 

 tionally unpaired (Cyclostomes); the nerves which pass to these pits 

 arise from the fore-brain and are often swollen at their origin into 

 special lobes (olfactory lobes). In aquatic animals which breathe by 

 gills the nasal cavity consists with rare exceptions (Myxine) of a 

 blind sac. In all lung-breathing Vertebrates, on the contrary, it 

 communicates with the cavity of the mouth by the nasal passages, 

 and serves for the entrance and exit of the pulmonary air. 



Next come the eyes with the optic nerves which arise from the 

 thalamencephalon and mid-brain. They are always paired (for the 

 structure of the eye vide 

 p. 73, vol. i.). In Am- 

 phioxus alone they are 

 represented by an un- 

 paired pigment spot placed 

 on the anterior end of the 

 central nervous system. 



The auditory organ, the 

 nerve of which belongs to 

 the hind-brain (probably 

 derived from the sensory 

 root of a spinal-like cranial 

 nerve), is entirely absent in 

 Amphioxus. In its simplest 

 form it is a membranous 

 sac (membranous labyrinth) 

 containing fluid and oto- 

 liths. The posterior part 

 of this sac is usually prolonged into three semi-circular canals, while 

 the anterior part, which in many cases is separated as the saccule, 

 gives off a prolongation which forms the cochlea (fig. 578, S, C.). 



The sense of taste is located in the palate and the root of the 

 tongue. The organs of taste consist of peculiarly-modified groups of 

 epithelial cells (taste buds), and are supplied by a spinal-like cerebral 

 nerve (glossopharyngeal). The general sensibility, which is distri- 

 buted over the whole surface of the body, and the tactile sense are 

 also connected with the terminations of the sensory fibres of spinal 

 nerves. 



In addition to the cerebro-spinal nervous system there is (except 

 in Amphioxus and the Cyclostomes} a special visceral nervous system 



FIG. 578. Diagram of the auditory labyrinth (after 

 Waldeyer). I, of fish ; II, of bird ; III, of mammal 

 U, utricle with the three semicircular canals ; S, 

 saccule ; US, alveus communis ; C, cochlea ; L, 

 lagena ; Cr, canalis reuniens ; R, aquseductus vesti- 

 buli. 



