S30 THE HUMAN EMBEYO. 



The nerves of the first set, i.e. the motor nerves, have 

 localised centres of origin in the brain : the nerves of the second 

 set, or sensory nerves, are not definitely localised in the brain, 

 except by the points at which the fibres enter the brain. The 

 two groups of nerves arise independently, as in the case of the 

 spinal nerves. They may retain their independence, forming 

 purely motor, or purely sensory nerves ; or they may become more 

 or less closely associated with one another to form nerves of 

 mixed, motor and sensory, function. 



The course of the cranial nerves in the early stages of their 

 development is curiously straight (Fig. 227) ; their main direc- 

 tion, like that of the spinal nerves, being at right angles to the 

 axis of the head, or brain, at their points of origin. This initial 

 course is liable to disturbance through shifting relations of the 

 parts with which the nerves are in connection, or through 

 growth of the skeletal or other neighbouring parts. Thus the 

 facial nerve is at first straight, but, owing to the telescoping of 

 the hinder visceral arches within the anterior ones, its course 

 becomes much modified (Fig. 227, Vii). 



In many instances some further explanation is required : 

 thus the glossopharyngeal nerve extends forwards in front of its 

 proper territory, in order to reach the circumvallate papillae of 

 the tongue ; while the facial nerve extends forwards to the fore- 

 head. An interesting case is the extension of the pneumogastric 

 nerve to the heart, lungs, and stomach. The posterior limit of 

 the head may be taken as indicated by the hinder border of the 

 second branchial arch ; or, in the adult, by the boundary line 

 between the thyroid and cricoid cartilages, if Callender and His 

 are right in regarding the thyroid cartilage as developed from 

 the cartUage of the second branchial arch. In any case, the 

 heart, lungs, and stomach are, in the adult, far behind the head 

 region. It must be remembered, however, that the heart origi- 

 nally lies between the ventral ends of the visceral arches, and that 

 the lungs arise from the floor of the pharynx, so that both heart 

 and lungs really lie within the proper area of distribution of the 

 pneumogastric nerve. The stomach, however, does not do so, 

 and in order to reach it the pneumogastric nerve must pass 

 beyond the limits of its own territory. 



In describing the cranial nerves individually it will be con- 



