534 THE HUMAN EMBRYO. 



runs almost directly forwards, lying to the inner side of the 

 Gasserian ganglion, and reaches the external rectus muscle 

 early in the fifth week. The nerve is drawn, though not named, 

 in Fig. 227, as a thin band emerging from the ventral surface of 

 the brain immediately below the ganglion of the facial and 

 auditory nerves, and running horizontally forwards towards the 

 hinder border of the eye. 



The motor root of the trigeminal, or fifth cranial nerve, lies at 

 a level slightly ventral to the motor roots of the facial, glosso- 

 pharyngeal, and pneumogastric nerves, but clearly belongs to the 

 lateral rather than to the ventral series of roots. It lies to the 

 inner side of the Gasserian ganglion, and in close relation with 

 this, and is, in its early stages, slightly anterior to this in position. 



The fourth cranial nerve (Fig. 227, iv), though leaving the 

 brain on the mid-dorsal surface, is stated by His to arise from a 

 group of neuroblasts on the ventral surface of the isthmus, or 

 constricted neck between the hind- and mid-brains. These roots 

 lie close to the mid-ventral plane, and clearly belong to the 

 ventral series. From this origin the fibres of the fourth nerve 

 run up, in the sides of the brain, to its dorsal surface, cross those 

 of the opposite side in the mid-dorsal plane, and finally emerge 

 from the brain as the definite nerves. 



Though very slender, the fourth nerves are of considerable 

 length by the early part of the fifth week (Fig. 227), already 

 reaching to the level of the eye. 



The fourth nerve has long been a source of trouble to 

 morphologists. Professor His' observations on its development 

 in human embryos will, if confirmed and extended to other 

 Vertebrates, throw a very welcome light on the problem, show- 

 ing that, in spite of the peculiar position at which it leaves the 

 brain, the fourth nerve really belongs to the category of ventral 

 or motor roots. 



The third cranial nerve (Fig. 227, in) arises from a group 

 of neuroblasts in the floor of the mid-brain, rather further apart 

 than the other ventral roots, but belonging to the same series. 



Group B, The nerves included in this series arise from 

 groups of neuroblasts, not in the brain, but in the ganglia, i.e. 

 they are developed in the same manner as the dorsal or sensory 

 roots of the spinal- nerves. 



