532 



THE HUMAN EMBBYO. 



ever, to correspond to the ganglion described by Froriep in 

 sheep embryos as forming a true dorsal root to the hypoglossal 

 nerve. 



It is probable, therefore, that the hypoglossal nerve is to be 

 regarded as formed by the ventral roots of one or more nerves 

 equivalent to spinal nerves ; and of which the dorsal root, or 



Fig. 228. — Transverse section across the medulla oblongata of a, Human 

 Embryo, lettered by Professor His, Ko, and estimated as thirty-one days 

 old. The embryo is the same one as that represented in Pig. 227, and 

 the section passes through one of the roots of the hypoglossal nerve, and 

 through both the motor and sensory roots of the pneumogastric nerve. 

 (From His.) x 40. 



X.M, motor root of pneumogastric nerve. X.S, sensory root of pneumogastiic nerve. 

 XII, root of hypoglossal nerve. 



roots, are represented in man by the rudimentary Froriep's 

 ganglion alone. 



The spinal accessory, or eleventh cranial nerve (Fig. 227, xi), 

 arises by a number of roots, formed by outgrowths from groups 

 of neuroblasts in the side of the medulla oblongata ; the roots 

 lying at a level dorsal to that of the hypoglossal roots, and at the 

 junction of the ventro-lateral and dorso-lateral regions of the 

 medulla {cf. Fig. 228). > 



The roots of the spinal accessory nerve are very numerous. 

 At the beginning of the fifth week (Fig. 227) the most posterior 



