THE CRANIAL NERVES. 527 



a centripetal direction, carrying nerve impulses from the olfactory cells 

 to the cerebrum (Fig. 239). 



Histologic and embryologic methods of research have shown that 

 some of the fibers in the olfactory tract are centrifugal in direction. 

 They originate in the olfactory cortical areas, pass toward the 

 periphery as far as the anterior commissure, where they cross to 

 become the dorsal root, enter the olfactory tract, and finally terminate 

 in the bulb. This tract serves to connect the cortex with the bulb of 

 the opposite side, and carries impulses from the cortex to the bulb. 

 The two opposite cerebral olfactory areas are also united by com- 

 missural fibers which decussate at the anterior commissure. 



Functions. The olfactory nerves, including the olfactory tract, 

 are channels of communication between the olfactory region in the 

 nose and the cerebral cortex'. The stimulus to its excitation is the 

 impact and chemic action of gaseous or volatile organic matter 

 on the dendrites of the olfactory cells. The eiiergy set free develops 

 nerve impulses which, traveling through the entire olfactory tract 

 to the cortex, evoke the sensation of odor. The sensitiveness of the 

 olfactory end-organ to the action of many substances is remarkable, 

 responding, for example, to the T.TO~i,woif a gram of oil of roses and 

 to the -2.T6ir.~o~o "o f a g ram of mercaptan. 



Division or destruction of the olfactory path at any point is fol- 

 lowed by an abolition of the sense of smell on the corresponding side. 

 Destructive lesions of the hippocampal and uncinate gyri are fol- 

 lowed by similar results. 



SECOND PAIR. THE OPTIC. 



The second cranial nerve, the optic, consists of centrally coursing 

 axons_ofjoeurons, which connect the essential part of the organ of 

 vision, the retina, , with sensory end-nuclei or ganglia situated at the 

 basejqf diec^rebrum. 



Origin. The axons which constitute the optic nerve have 

 their origin in the ganglionic cells in the anterior part of the jxiina. 

 Through their dendrites tEese cells are brought into relation pos- 

 teriorly with successive layers of cells which collectively constitute the 

 retina. Though the retina is said to consist of ten or eleven layers, 

 it may be reduced practically to three, viz. (Fig. 240) : 



1. The layer of ^visual cells. 



2. The layer of r blpolar^cells. 



3. The layer of ganglionic cells. 



The visual ceils presehr peripherally modified dendrites, known 

 as the rods^and_cpnes; centrally they give off an axon which after 

 a short course Terminates in an end-tuft. The bipolar cells also 

 possess dendrites and an axon; the former interlace with the end- 



