112 A. M. PATEESON. 



Each root of the nerves divides into two unequal hranches — the 

 dorsal root beyond the ganglion, the ventral root directly. Of these 

 hranches the smaller is superior, the larger inferior in both cases. 

 The larger branches unite to form the main trunk of the nerve, 

 or the inferior primary division • the smaller branches combine to 

 form the superior primary division. This is directed upwards and 

 outwards, and sub-divides as it passes towards the surface. 



In Chicks at four days six hours, the condition of the nerves in 

 the trunk between the limbs is slightly more advanced, but presents 

 no change of any note. In the regions of the limbs, however, the 

 plexuses are now formed. In transverse sections through the 

 embryo, the nerves are found, on entering the limbs, to divide into two 

 fairly well-defined strands, separated by a central mass of mesoblast, 

 which is in active growth, and preparing to form the cartilaginous 

 basis of the limb. The nerves, in fact, spread out around this 

 central core, and arrange themselves into two sets, one dorsal, the 

 other ventral. These dorsal and ventral branches of the nerves 

 only pass a short distance into the limbs, and are not so well 

 defined as in embryos a few hours older. But even now the process 

 of plexus formation may be seen. 



When longitudinal vertical (sagittal) sections are made continuously 

 through the body, it is seen that the nerves to the limbs, besides 

 forming the dorsal and ventral branches above mentioned, unite 

 with adjacent nerves at the root of the limb to form a well-defined 

 plexus. In the Fowl three main trunks form the brachial plexus,* 

 the first thoracic and the last two cervical nerves, with, in addition, a 

 small branch from the more anterior cervical nerve. When sagittal 

 sections are made, the limbs being divided transversely at their 

 roots (Fig. 9, a.), the axillary artery (art.) and vein (v.), with the 

 three main nerves (N., 1, 2, 3), are divided just outside the body 

 cavity (B. C), and as they lie in the body wall (B. W.), before their 

 entrance into the limb bud, and below the terminations of the 

 muscle-plates. In successive sections from within outwards, these 

 nerves can be traced to their terminations in the limbs. They first 

 spread out, and approach one another, as described ; in doing so 

 they unite with adjacent nerves, so that the next step in the pro- 



* Macartney, Art. "Birds" ("Bees' Cyclopaedia "). 



