DEVELOPMENT OF PERIPHERAL NERVOUS SYSTEM 823 



the primitive neurocoel, special cavities in the brain arise, as follows (see 

 figure 357A): 



( 1 ) The telencephalon is made up of the anterior part of the prosen- 

 cephalon and two telencephalic vesicles. Each vesicle ultimately gives 

 origin to a cerebral lobe. The cavities of the telencephalic vesicles 

 are known as the first and second ventricles. 



(2) The cavity of the posterior, median portion of the telencephalon and 

 that of the diencephalon form the third ventricle. 



(3) The roof of the original mesencephalon may give origin to hollow, 

 shallow outpushings, but the cavity of the mesencephalon itself be- 

 comes a narrow passageway and is known as the cerebral aqueduct 

 or the aqueduct of Sylvius. 



(4) The cavity of the rhombencephalon is called the fourth ventricle. 



b. The Formation of Cervical and Pontine Flexures 



In addition to the primary or cephalic flexure previously described (p. 812) 

 other flexures may appear in the developing vertebrate brain, especially in 

 higher vertebrates. The cervical flexure develops at the anterior portion of 

 the spinal cord, as it joins the myelencephalon. It involves the caudal portion 

 of the myelencephalon, and the anterior part of the cord. It bends the entire 

 brain region ventrally (see figure 357D and E). The latter flexure is absent 

 in fishes, is present to a slight degree in the early neural tube of the amphibia, 

 and is pronounced in reptiles, birds and mammals. The third or pontine flexure 

 of the brain bends the brain dorsally. It arises in the mid-region of the 

 rhombencephalon, in the area between the myelencephalon and the meten- 

 cephalon. It appears later in development than the cephalic and cervical 

 flexures, and is found only in higher vertebrates. 



c. Later Development of the Five-part Brain 



The various fundamental regions of the five-part brain develop difi'erently 

 in diff'erent vertebrates. Figure 357B-G and H-O illustrates the changes 

 of the regions of the primitive five-part brain in the shark, frog, bird, dog, 

 and human. For detailed discussion of the function of the various parts of 

 the brain of the vertebrate, see Ranson, '39. 



D. Development of the Peripheral Nervous System 



1. Structural Divisions of the Peripheral Nervous System 



The peripheral nervous system integrates the peripheral areas of the body 

 with the central nervous system. It is composed of two main parts, 



( 1 ) the cerebrospinal system of nerves and 



(2) the autonomic system. The latter is associated intimately with the 

 cerebrospinal system. 



