SENSE-ORGANS. 77 



To summarise the matter Gaskell has suggested that in the primitive 

 condition, both in brain and spinal cord, there were three rows of nerve 

 roots : (1) a dorsal containing somatic sensory fibres, (2) a ventral with 

 somatic motor fibres, and (3) a lateral row containing both visceral sensory 

 and visceral motor. This condition is modified in all existing forms in the 

 cord by the splitting of the lateral roots in such a way that the visceral 

 sensory roots have joined (1) and the visceral motor roots (2) ; whereas in 

 the brain the roots of the lateral row have persisted and the somatic sen- 

 sory roots (restricted to one) have joined them, the somatic motor roots 

 (three in number) remaining distinct. This scheme does not, however, 

 take account of the acvistico-lateral system. 



The Eyes have a flat cornea, and a large almost spherical 

 lens, the anterior part of which projects far out of the pupil. 

 Movable eyelids are present in Elasmobranchii, but are absent 

 from most other fishes. There are no laoymal glands. The 

 usual eye muscles are present. There is frequently a rete 

 mirabile, the choroid gland, on the ophthalmic artery as it enters 

 the eye near the entrance of the optic nerve. The processus 

 falciformis and campanula halleri are described below under 

 Teleostei. The eyes are much reduced and functionless in most 

 adult Marsipobranchs and some cave-dwelling and abyssal 

 Teleosteans. The Auditory Organ consists of the otocyst 

 or membranous labyrinth, which is embedded in tlie side walls 

 of the auditory region of the skull. It lies in a cavity which 

 is closed from the cranial cavity in most Elasmobranchs, but 

 communicates with the latter in Chimaera, and Teleosteans, 

 Ganoids and Dipnoi. It consists (Fig. 40) of a central 

 chamber, the vestihtde, and of three semicircular canals opening 

 into the vestibule. The vestibule is divided into two parts 

 by a constriction ; of these the upper is the utricle, the lower 

 the saccule. The semicircular canals open into the utricle 

 while the saccule in some fishes gives off from its posterior end 

 a process called the lagena, which is an incipient cochlea. In 

 Chimaera and the Squali the ductus endolymphaticus which 

 is given off by the saccule opens on the surface of the head.* 

 Both saccule and utricle contain a chalky mass of otoliths. When 

 the lagena is well marked its papilla acustica {pi) becomes 

 separate from the macula acustica sacculi and receives a separate 

 nerve (vide account of membranous labyrinth under Teleostei). 

 The membranous labyrinth enters into peculiar relations with 



* This is the remains of the aperture of invagination of the embryo. 



