Cranial Nerves- 57 



translucent like a hydrocele. * In Cardinal's case this is said to have 

 been observed when the sun was shining behind him.' (Fagge.) 



Disease of the cerebellum is often associated with pain at the lower 

 and back part of the head, and with head-ache. There is also inco- 

 ordination of movement, so that the patient staggers ; he is often, 

 moreover, attacked with vomiting. 



THE CRANIAL NERVES 



No. i. The olfactory is shown by development to be a diver- 

 ticulum from the cerebral hemisphere ; in fetal life it is hollow and 

 communicates with the lateral ventricle. 



The external root of the nerve springs from the fissure of Sylvius, 

 near the anterior perforated space. This area is close to Broca's 

 region (p. 49), and so it comes about that aphasia is often associated 

 with an impaired sense of smell in the left nostril. The olfactory 

 filaments descend through the cribriform plate in groups for the mucous 

 membrane of the upper part of the septum and the roof, and of the 

 superior and middle turbinated bones. Odorous particles best reach 

 this area when the air in which they are suspended is ' sniffed ' up. 

 When, in nasal catarrh, swollen mucous membrane blocks the upper 

 part of the passages, and the sense of smell is lost, the patient has to 

 breathe by the floor of the nose or the open mouth. 



The sense of smell may be entirely lost after a fracture extending 

 across the anterior fossa of the skull, or as the result of malignant 

 disease in the ethmoid region. 



No. 2. The filaments of the optic nerve pass out through the 

 commissure into the optic tracts, by which they arise from the optic 

 thalamus and the corpora quadrigemina. The tracts wind over the 

 crura cerebri. 



The nerve passes out through the optic foramen in a special sheath 

 of dura mater, part of which blends with the orbital periosteum, whilst 

 the rest runs on to the sclerotic. The fibres of the nerve pass through 

 the lamina cribrosa of the sclerotic and spread out into the retina ; 

 those fibres which turn outwards, however, do not spread upon the 

 surface until they have reached the vertical meridian of the eye-ball. 



In the optic commissure the most anterior fibres are inter-retinal 

 and the most posterior inter-cerebral ; some fibres pass from the 

 thalamus to the retina of the same side ; and, lastly, others pass 

 across from the thalamus of one side to the retina of the other. This 

 decussation is needed because the outer half of one retina works in 

 harmony with the inner half of the other. When we look to the 

 right, for instance, the image falls upon the inner part of the right 

 retina and the outer part of the left. (For the anatomy of the eye see 

 p. 82.) 



