172 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



the corresponding vagus. x The sympathetic ganglia supply the 

 viscera and vascular system. 



Histologically, the nervous system is essentially made up of 

 the two usual elements, 'nerve-cells (ganglion cells) and nerve fibres. 

 In the brain and spinal cord these elements are respectively 

 aggregated into what are known as grey and white matter. 

 The former exists in the spinal cord and bulb as an axial core, in 

 the prosencephalon and cerebellum as an external crust. Masses 

 of it are also found in the optic lobes, side-walls of the thalam- 

 encephalon, basal part of the prosencephalon, and in the olfactory 

 lobes. 



10. The chief sense organs of the dogfish are those connected 

 with the skin, the olfactory sacs, the ears, and the eyes. All 

 of these essentially consist of end-organs connected with nerve- 

 fibres. 



The skin is abundantly provided with groups (end-buds) of 

 projecting sense-cells, probably of tactile nature, and it also 

 possesses lateral line organs and sensory tubes. The indistinct 

 groove termed lateral line in describing the external characters 

 corresponds pretty much in position with a tube which underlies 

 the skin and opens to the exterior at intervals. It is lined by 

 epithelium, many of the cells of which are mucus-secreting goblet 

 cells, while others are sense-cells (hair-cells), each provided with 

 a slender hair-like process. 



The sensory tubes (jelly tubes) which open on the snout under- 

 lie the skin of the head and each of them ends in a rounded 

 sac (ampulla) divided into compartments, and partly lined by 

 hair-cells. These tubes contain a gelatinous substance, secreted 

 by the lining epithelium. The ampullae are supplied by the 

 ophthalmic branches of the fifth and seventh cranial nerves, the 

 fibres of which run to the hair-cells. 



The large olfactory sacs, contained in the olfactory capsules 

 opening ventrally by the nostrils, have their lining raised into 

 numerous transverse folds, the epithelium of which contains 

 elongated olfactory cells, supplied by the olfactory nerve. 



The ear (Fig. 48) on each side is a membranous sac (labyrinth) 

 of complicated shape contained within the auditory capsule, and 

 surrounded by a large lymph-space. It is lined by epithelium, 

 part of which is sensory, and is filled with fluid in which are 

 suspended numerous calcareous particles. The membranous 



