28 LABORATORY OUTLINE OF NEUROLOGY 



grayish-white retina. The retina in preserved specimens is apt 

 to be partly disintegrated and pulled loose from the choroid 

 coat. In the outer half notice the lens and observe its attach- 

 ment by a delicate suspensory ligament attached to the margin 

 of the eyeball in front of the retina. The space behind the lens 

 is filled with vitreous humor (corpus vitreum); the space in 

 front of the lens and behind the cornea is filled with aqueous 

 humor. The choroid coat is extended in front of the lens to 

 form the iris. 



14. Look up the structure of the human eye (see Section 

 135) and compare with that of the dogfish. Note the dif- 

 ference in the shape of the lens in the two cases. In 

 mammals the suspensory ligament is attached to a muscular 

 ridge, the ciliary process, whose ciliary muscles control the 

 accommodation of the lens. In the dogfish the ciliary muscles 

 are feebly developed and there is little, if any, power of ac- 

 commodation. 



15. Cranial nerves. — There are ten pairs of cranial nerves (the 

 XI and XII human nerves are not separately represented). 

 The spinal nerves are much alike, each pair repeating the same 

 functional pattern; but no two cranial nerves have the same 

 functional composition. Accordingly, in studying the cranial 

 nerves it is necessary to determine for each pair of nerves the 

 functional composition of each of its roots and the precise per- 

 ipheral and central connections of the fibers of each functionally 

 distinct root. This has been done for a sufficient number of 

 vertebrate types to establish a typical vertebrate pattern of 

 cranial nerve components. These functionally defined compo- 

 nents are classified in four major groups, somatic sensory and 

 motor and visceral sensory and motor, each of which may be 

 further subdivided. For the discussion of the principles and 

 mode of application of this classification (which is fundamental 

 to an understanding of the following sections), see Herrick, '15, 

 Chap. IX, and Johnston ('06), Chap. V. 



16. The nerve components of the dogfish have been carefully 

 studied microscopically by Dr. H. W Norris and Miss S. P. 

 Hughes, who have very kindly prepared for us the accompany- 

 ing drawings (Figs. 4 and 5) from a detailed account to be pub- 

 ashed shortly ('18). The systems of nerve components men- 



