l^ERVOUS SYSTEM OF A TWO-HEADED PIG EMBRYO 399 



Cranial nerves 



Superficial origin and peripheral distribution of the nerves of 

 the outer {normal) sides of the teratological head. As in the study 

 of the brain, so in that of the cranial nerves, the region between 

 members A and B is the area of greatest interest. But an 

 accurate interpretation of the nerves in this median region 

 requires first a careful study of the origin and distribution of those 

 on the outer sides of the head where the picture conforms to 

 normal conditions with striking exactness. Obviously, the 

 investigation has necessitated the identification of many struc- 

 tures other than nerves that would serve as anatomical land- 

 marks. Some of these have been labeled in the accompanying 

 figures. Owing to the dearth of descriptive anatomy of normal 

 embryos at the age of this teras, much of the interpretation has 

 had to be made from gross anatomy of human adult and the 

 descriptions of older embryos of a few of the lower mammals. 

 Each interpretation has been rigorously checked by constant 

 references to cross and longitudinal section series of normal 

 pig embryos, especially those of 20, 22, and 25 mm. in length, 

 and a few human ones. 



The entire series of twelve pairs of nerves is present in normal 

 linear arrangement, and each may be traced to such an extent 

 as to leave no doubt of its identification. For convenience, the 

 descriptions are given for the nerves of the left moiety of head 

 B, the aspect shown in figure 4, but it should be remembered that 

 they hold true also for the right moiety of head A, since nor- 

 mality is undisturbed in the outer half of each head member. 

 For this reason the description of each nerve is very brief and 

 given primarily for comparison with its mate in the median region. 

 The central connections of the nerves are treated under a separate 

 heading. The associated sympathetic ganglia have already been 

 considered (p. 388). 



I. Nervus olfactorius. The paired olfactory bulbs and primary 

 olfactory nerves are normal for each head member. The fila 

 olfactoria are readily identifiable in the cross-sections (figs. 11 

 to 14). 



