804 



THE NER VE 8 YKTEM 



three primary divisions of the brain tube (a) medulla oblongata, (b) pons, (c) mid- 

 brain, (d) thalamic division of fore-brain. In this brain stem lie the majority of the 

 ganglionic masses enumerated above, together with the nerve tracts uniting the 

 various cell nests in (presumable) automatic coordination as well as the great 

 nerve tracts connecting the spinal gray with the cerebral hemispheres, the thalami, 

 cerebellum, and the ganglia of the medulla oblongata (including the cranial nerve 

 nuclei), and still other tracts connecting the medulla oblongata with the cerebral 

 hemispheres, the cerebellum, and the special ganglia of the pons and mid-brain. 



Parts Derived from the Hind-brain (Rhombencephalon). 



Morphology. 



External 



The Medulla Oblongata (oblongata; spinal bulb; postoblongata of Wilder; 

 myelencephalon). The medulla oblongata is the continuation cephalad of the 

 spinal cord, the transition lying at the level of the foramen magnum and marked 

 by the decussation of the pyramids. Its cephalic limit is sharply defined ventrad 

 by the rounded margin of the pons, while its dorsal surface is sunk into the cere- 

 bellar vallecula. The length of the medulla oblongata along its ventral surface 

 is f to 1 inch (20 to 25 mm.) ; its maximum width at the pontile end is f inch (17 

 to 18 mm.), and half as much at its transition into the spinal cord; its maximum 



SEMILUNAR GANGLION 

 OF TRIGEMINAL NERVE 



FACIAL NERVE 



N. INTERMEDIUS 

 ACOUSTIC NERVE 



GLOSSOPHARYNGEAL NERVE 

 VAGUS NERVE 



SPINAL ACCESSORY NERVE 

 HYPOGLOSSAL NERVE 



I. CERVICAL NERVE 



BASILAR GROOVE 



POSTPONTILE RECESS 



DECUSSATION 

 OF PYRAMIDS 



VENTRAL FISSURE 



Pin. 632. Ventral view of pons and medulla oblongata, showing the attachments of certain cranial nerves on 

 one side. * The inter-radicular pons tract or corpus pontobulbare, described in the text. 



thickness is about 15 mm. (| inch). Its expansion as it approaches the pons 

 gives it the form of a truncated cone. The ventral surface rests upon the basilar 

 groove of the occipital. 



Fissures. The ventral and dorsal fissures of the cord are continued upon 

 the medulla oblongata, making it a bilaterally symmetrical structure. The 

 ventral or ventromedian fissure (flasura mediaua anterior} at the level of the foramen _ 

 magnum is interrupted by a number of obliquely intercrossing fibres, called the 



