108 Development of Occipital Nerves in llumaii Embryos 



sory. No motor fibres are represented as running from tlie accessory 

 to the larynx, the absence of such fibres having been well established by 

 the work of Onodi, 02, and others, and the clinical observations of 

 Seijfer, 03. Fibres of the accessory doubtless join the trunk of the vagus, 

 but they are omitted here for sake of simplicity. 



The Hypoglossal Nerve can first be made out in embryos at the end of 

 the third week, at which time it consists of loose fibre strands which can 

 be traced between the occipital myotomes springing from the ground plate 

 of the neural tube and extending a short distance in the mesenchyma 

 (see Fig. 1). These rootlets are formed in three or four segmental 

 groups and develop in the same line with the ventral roots of the cervical 

 nerves. During the fourth week they grow forward and fuse in a common 

 trunk. At the end of the first month this trunk lias passed around the 

 ganglion nodosum, and curves around the sinus cervicalis mesially and 

 orally to reach the anlage of the floor of the mouth. A week later its 

 principal branches of distribution are indicated. 



As the hypoglossus crosses the ganglion nodosum it gives off the ramus 

 descendens, which is first definitely seen in embryos 1.0 cm. long. Mall, 

 91, and Piper, 00, report its absence in embryos 7.0 mm. and 6.8 mm., 

 respectively. His, 88, pictures a long r. descendens in Br3 (6.9 mm.). 

 In a reconstruction of the same embryo, made since then by the author 

 (see Fig. 4), this is not seen. There is, however, on the opposite side 

 (Fig. 5) a slight indication of a beginning branch. At the time the 

 descendens is developed the opportunity for communication between the 

 hypoglossus and the upper two or three cervical nerves already exists; 

 that is to say, the terminal fibres of the latter end in brush-like tufts in 

 close contact with the former. The amount of interchange of fibres 

 cannot be accurately traced, but it is evident that the character of the 

 descendens is dependent on the nature of the contribution of fibres from 

 the cervical nerves. The course in the development of this cervical 

 anastomosis is as follows (compare Figs. 3, 4, 6, 9, and 11) : The fibres 

 of the hypoglossal and the upper cervical nerves start out perpendicularly 

 from the neural tube, and due to the curve of the latter they come together 

 like spokes in a wheel, and then grow along adjacent to each other into 

 the premuscle tissue of Froriep's schulterzungenstrang ; when the forma- 

 tion of the nerve sheaths begins, adjoining fibres become thereby more or 

 less bound together, and as the individual tongue and hyoid muscles draw 

 apart these nerves are led out into an open plexus, the adult arrangement 

 of which and its variations has been described by Holl, 77. The exact 

 formation of this anastomosis must depend on the position of the fibres 

 at the time the sheaths are formed. This introduces a variabilitv wliich 



