IX CRANIAL NERVES, SENSE-ORGANS 337 



all these nerves are connected with a common nerve-centre lying in the 

 medulla oblongata and in close relation to the centre of the auditory- 

 nerve. Physiologically they form a group of nerves by themselves, and 

 it is quite conceivable that, as some zoologists believe, the nerve-fibres 

 composing them formed originally either an independent cranial nerve 

 or a portion of the auditory but have in the course of evolution come 

 to be subdivided up and incorporated in other cranial nerves. 



We may take it that in the Vertebrata as in other groups the skin 

 was primitively provided with sensory cells scattered throughout its 

 ectoderm : and in this phylum as elsewhere we find special aggregations 

 of such sensory cells to- form sense-organs devoted to the perception 

 of some special type of impression from the outer world. The 

 simplest of these organs of special sense in the Dogfish is the olfactory 

 organ or organ of smell. 



Olfactory Organ. — This organ commences in the embryo as a 

 slightly thickened patch of ectoderm on the ventral surface of the 

 head on each side. This thickening becomes gradually sunk down 

 beneath the general surface, its opening becoming narrowed but never 

 completely closed. The ectoderm hning the cavity so formed becomes 

 crowded with sensory cells and a great increase in the area of the sensory 

 epithelium is obtained by its projecting into the cavity in the form of 

 a number of thin, almost leaf-like, folds. In Scyllium, though not in 

 Acanthias, the olfactory organ is connected with the mouth by a deep 

 groove, -the edges of which almost meet. 



Eye. — While the olfactory organ is relatively simple the eye on the 

 other hand reaches the utmost extreme of complexity in its structure, 

 and this fact is not surprising if the marvellous function performed by 

 the eye is borne in mind — the formation in the first place of an optical 

 picture of the external world and then the translating of this into 

 symbols composed of nerve impulses which can be duly interpreted by 

 the brain so as to form a correct mental picture. 



The eye of the vertebrate is of the camera type, comprising the two 

 essential and primary components : a lens for the formation of the image 

 and a sensitive surface — the retina — for the reception of that image. 



The lens (Fig. 141, Z) is composed of a mass of metamorphosed cells, 

 the protoplasm of which is converted into somewhat horny material of 

 glassy transparency and is in the Dogfish as in many other creatures in- 

 habiting water — a comparatively highly refracting medium — practically 

 spherical in shape. 



The retina (Fig. 141, R), the function of which is the translation of 



z 



