DEVELOPMENT OF FOURTH DAY 513 



The Myelencephalon : There is also little change in this region, but 

 later the roof becomes thinner and blood-vessels push their way into 

 the opening now called the fourth ventricle, as the posterior choroid 

 plexus, while the ventral and side-walls become floor and lateral walls 

 of the medulla. 



THE GANGLIA OF THE CRANIAL NERVES (Fig. 282, B) 



Along the neural crests already discussed, various ganglia are 

 formed. The largest on the fourth day is known as the Gasserian 

 ganglion of the fifth cranial nerve. (The fifth is also called the trigemi- 

 nal nerve.) It lies ventral and lateral, as well as opposite to the most 

 anterior neuromere of the myelencephalon. It forms the sensory nerve 

 fibers which grow from the brain mesially and distally into the mouth 

 and face region. This fifth cranial nerve is divided into three great 

 branches: the ophthalmic, the maxillary, and the mandibular. 



The first branch, the ophthalmic, can be seen on the fourth day 

 extending toward the eye, while the other two are just beginning to 

 grow toward the mouth angle. 



Just anterior to the auditory vesicle a mass of neural-crest cells is 

 developing into what is to become the facial or seventh cranial nerve 

 and the acoustic or eighth cranial nerve. This cell mass divides on the 

 fourth day to form the geniculate ganglion of the seventh and the 

 acoustic ganglion of the eighth nerve. 



Caudad to the auditory vesicle, the ganglion of the glossopharyngeal 

 or ninth cranial nerve can be seen, and the ganglion of the vagus or tenth 

 nerve may just be observed. The ninth can be seen in whole mounts, 

 the tenth probably cannot. 



THE SPINAL CORD 



Throughout the spinal cord there is a compressed, slit-like lumen 

 known as the central canal. Just as the ganglia of the cranial nerves 

 make their appearance on the fourth day, so, too, do the spinal nerves. 



It requires special methods of staining to study the growth of the 

 nerve fibers from the neuroblasts, but the development of the spinal 

 nerve roots can be studied in ordinarily stained slides. 



It is important to understand that in the adult there will be two 

 roots to each spinal nerve (Fig. 290), one ventral, which is motor in 

 function, and one dorsal, which is sensory in function. Both of these 

 unite lateral to the spinal cord. Immediately distal to this union there 

 is a branch extending to the sympathetic nerve cord. This branch is 

 known as the ramus communicans, and extends ventrad. 



Before the union of dorsal and ventral nerve roots takes place a 

 spinal ganglion or dorsal ganglion is seen lying in the dorsal roots. This 

 ganglion is formed from the neural crests, and grows toward the cord, 

 thus forming the dorsal root, but there are also fibers growing away 



