266 THE RHOMBENCEPHALON. 



but there appears to be nothing analogous to the long association 

 fibers found in the cerebrum. 



RHOMBENCEPHALON. 

 SECTION II. THE PONS (VAROLII). 



The pons and medulla form the ventral part of the rhomben- 

 cephalon, the cerebellum being its dorsal portion. By a trans- 

 verse indentation of its roof, the posterior brain-vesicle is parti- 

 ally divided into an upper vesicle, the metencephalon, and a lower 

 vesicle, the myelencephalon; the. latter is the embryonic medulla, 

 the former gives rise to the cerebellum and the pons. The pons 

 is developed from the floor of the metencephalon (Fig. 79). It 

 is so named because it forms the connecting link, or bridge, be- 

 tween the mid-brain, above, and the cerebellum and medulla 

 oblongata, below; between the medulla and cerebellum, and be- 

 tween the two cerebellar hemispheres (Fig. 85). 



In shape the pons is roughly cylindrical. It has a broad basal 

 or ventral part, the pars basilaris pontis, and a narrower dorsal 

 portion, the pars dor sails pontis (Fig. 87). 



Size. The pons is about one inch long. It is a little broader 

 than long, and measures an inch, dorso-ventrally. 



Position. It rests in the anterior end of the groove which 

 extends from the foramen magnum to the dorsum sellae, and 

 lies between and ventral to the hemispheres of the cerebellum. 

 Superiorly, it joins the mid-brain; and, below, it is continuous 

 with the medulla oblongata. 



Surfaces of the Pons. The pons has four surfaces, viz., supe- 

 rior (attached); inferior (attached); anterior (free), and posterior 

 (partially free); and two borders, namely, right and left lateral, 

 continuous with the brachium pontis of the cerebellum. 



The superior and inferior surfaces are made by section, and 

 are directly continuous with mid-brain above and the medulla 

 below. 



Anterior Surface (Tuber annulare). The anterior surface 

 of the pons (Fig. 85) looks forward and slightly downward and 

 rests on the sphenoid bone behind the sella turcica. It is divided 



