158 ZOOLOGY SECT. 



all these fibres originate in a centre in the medulla, the acustico- 

 lateral centre, common to them and the fibres of the auditory nerve. 



The spinal cord is a cylindrical cord which extends from the 

 foramen magnum, where it is continuous with the hind-brain, 

 backwards throughout the length of the neural canal, enclosed by 

 the neural arches of the vertebrae. As in the Craniata in general 

 (see p. 96), it has dorsal and ventral longitudinal fissures and a 

 narrow central canal, and gives origin to a large number of paired 

 spinal nerves, each arising from it by two roots. 



Organs of Special Sense. The olfactory organs are rounded 

 chambers enclosed by the cartilage of the olfactory capsules of 

 the skull, and opening on the exterior by the nostrils on the 

 ventral surface of the head. The interior has its lining membrane 

 raised up into a number of close-set ridges running out from a 

 median septum. The fibres of the olfactory nerves terminate in 

 cells of the epithelium covering the surface of these ridges. 



The eye has the general structure already described as character- 

 ising the Craniata in general (p. 106). The sclerotic is cartilaginous, 

 the choroid has a shining metallic internal layer or tapetum cellu- 

 losum, and the lens is spherical. There are the usual eye-muscles, 

 the two obliques situated anteriorly, the four recti posteriorly, not em- 

 bracing the optic nerve. The eyelids are represented by stiff folds. 



The ear consists only of the membranous labyrinth (Fig. 798), 

 equivalent to the internal ear of higher Craniata, the middle and 

 outer ear being absent. The membranous labyrinth consists of 

 the vestibule and three semicircular canals. The former, which is 

 divided into two parts by a constriction, communicates by a narrow 

 passage the endolym.phatic duct or aqueductus vestibuli with the 

 exterior, in the position already mentioned. Of the three semi- 

 circular canals, the anterior and posterior are vertical and the 

 external horizontal, as in Craniata in general. Each has an ampulla, 

 that of the anterior and external canals situated at their anterior 

 ends, and that of the posterior canal, which is the largest of the three 

 and forms an almost complete circle, at its posterior end. In 

 the fluid (endolymph) in the interior of the vestibule are suspended, 

 in a mass of gelatinous connective-tissue, numerous minute 

 calcareous particles or otoliths, giving it a milky character. 



The sensory canals of the integument running along the lateral 

 line and over the head contain special nerve-endings (neuromasts), 

 and doubtless function as organs of some special sense (see p. 105). 

 The same probably holds good of a number of unbranched canals 

 (ampullary canals) arranged in groups situated on the anterior por- 

 tion of the trunk and on the head, and being particularly numerous 

 in the neighbourhood of the snout. These are dilated internally 

 into vesicles, the ampullce, provided with special nerve-endings. 



Urinogenital Organs. In the female there is a single ovary (Figs. 

 827, 835' ov.), an elongated soft, lobulated body, lying a little to 



