^34 THE CELL AND MAMMALIAN HISTOLOGY 



first into funiculi and then into larger groups, the whole nerve 

 being covered by an epineurium comparable to the dura mater. 

 In the connective tissue of a nerve run blood vessels, lymphatics 

 and nerves — for even nerves have nerves — and there is often 

 much fat. 



SENSE ORGANS 



In some parts of the body, such as the muscles and skin, 

 sensory nerve fibres end in contact with cells of other tissues 

 (muscle fibres, epithelia) which are in some ways sense cells, but 

 it is perhaps safest to use this term only for a nerve cell which is 

 specially modified to form part of a sense organ. In the olfactory 

 membrane (Fig. 418) a number of nerve cell bodies are scattered 

 amongst the epithelial cells, and their axons run inwards as the 

 fibres of the olfactory nerve. In the retina (Fig. 419) things are 

 more complicated. On the outside a pigmented epithelium pre- 

 vents light from reaching the sensitive elements, or rods and cones, 

 except from the proper direction. Each rod or cone is a process 

 of a nerve cell, the bodies of which make the outer nuclear layer. 

 From these run rod and cone fibres, which meet the dendrons 

 of bipolar cells in the outer molecular or outer synapse layer. 

 A bipolar is a cell with two processes. The bodies of the bipolars, 

 which come next, make the inner nuclear layer, and next there 

 comes the inner molecular or inner synapse layer, where processes 

 of the bipolars meet dendrons of another set of cells, the bodies 

 of which make the seventh or ganglionic layer. From this the 

 axons run tangentially as nerve fibres which leave the eyeball 

 as the optic nerve. As a result of the way in which the eye 

 develops (Fig. 420) the layers are said to be inverted ; the sen- 

 sitive elements are on the outside, light has to travel through 

 two other layers of neurons to reach them, and the optic nerve 

 has to leave by what is, in effect, a hole in the retina, so that it 

 makes a blind spot which is insensitive to light. Between the rods 

 and cones and their bodies is connective tissue, the outer limiting 

 membrane, and inside the nerve fibres more connective tissue 

 making the inner limiting membrane. The two sets are connected 

 by fibres of Miiller. In the inner synapse layer and amongst the 

 nerve fibres are capillaries. 



In the fovea, where rays from an object at which we are 

 looking are normally focused, there are no rods, while the greater 

 the distance from this the more of them there are, until at the 



