CAEPEXTEE: DEVELOPMENT OF THE OCULOMOTOE NEETE. 155 



c. Histology. 

 1. Oculomotor Nerve. 



The trunk of the oculomotor nerve is made up of both large and small 

 meduUated neuraxons. Some of the former may reach the size of 15 

 micra in diameter, while some of the latter may measure only 3 micra. 

 Between these two extremes all intermediate sizes are to be found. 

 The trunk of the nerve is composed mainly of comparatively large neu- 

 raxons, among which a few small ones are interspersed, but at its lateral 

 periphery a zone of small neuraxons occurs (Plate 7, Fig. 21). This 

 zone is represented by the shaded portions of the diagrams shown in 

 Plate 2, Figure 3, which represent cross-sections of the oculomotor at 

 various levels along its course. The unshaded portion of the nerve 

 trunk is that in which large neuraxons predominate. At A is shown 

 the conditions which obtain in the nerve proximal to its branches. Dia- 

 grams B and C represent, respectively, sections through the origin of 

 the branch to the dorsal rectus, and through the origin of the ventral 

 ramus. It will be noticed that both draw their neuraxons from the un- 

 shaded portion of the nerve trunk. A photomicrograph of a cross-sec- 

 tion of the branch to the dorsal rectus muscle is given in Plate 7, 

 Figure 22. While the branch is mainly made up of neuraxons of large 

 size, a certain number of smaller ones is also present. It is probable 

 that the differences in size of the neuraxons correspond with the differ- 

 ences in the degree of development of the muscle fibres to which they 

 are distributed. It has been pointed out by C. J. Herrick ('99) that 

 large neuraxons of the eye-muscle nerves of Menidia are connected with 

 large muscle fibres, and those of lesser calibre with small muscle fibres. 

 In the eye muscles of the hen, fibres of varying sizes also occur. In a 

 later paper (C. J. Herrick, :02), the writer just cited has advanced the 

 opinion that differences in the calibre and niedullation of neuraxons 

 frequently signify nothing more than a correlation with the degree of 

 functional development of the peripheral end-organs. 



The neuraxons which pass into the ciliary ganglion are those which 

 form the peripheral zone, shown by the shading in the diagrams of 

 Plate 2, Figure 3. They are of small calibre. Distal to the ganglion 

 the ciliary nerve is likewise entirely made up of small neuraxons (D), 

 the medullary sheaths of which are, however, well developed. 



2. Abducent Nerve. 

 The elements of the abducent nerve, when seen in cross-section, appear, 

 for the most part, as large meduUated neuraxons. As in the oculomotor 



