202 ELEMENTS OF MAMMALIAN ANATOMY. 



and other fibers connect these nuclei with the cortex of 

 the cerebrum and the optic thalamus. 



The Medulla Oblongata.- -The medulla oblongata, or 

 myelencephalon, is the stem of the true brain and extends 

 from the point of origin of the first spinal nerve to the 

 pons Varolii. The cerebellum must be removed in order 

 to study carefully the features of the medulla, and as 

 it is cut away the student should notice just ventrad to 

 it the very thin roof of the fourth ventricle. Its caudal 

 portion, composed of a reflection of the pia mater lined 

 with epithelium, takes the name of inferior medullary 

 velum, and the cephalic part, formed of pia and a thin 

 layer of nervous matter, is called the anterior medullary 

 -velum, or valve of Vieussens (Fig. 92). 



The fourth ventricle (Figs. 92 and 93) is an enlargement 

 of the canalis centralis of the cord. The clavae, or expan- 

 sions of the funiculi graciles, the three peduncles or crura 

 of the cerebellum, and the testes or colliculi inferiores 

 form its lateral boundaries. Its cavity is shallow and 

 pointed at either extremity. The floor, called fossa 

 rhomboidea, is formed by the continuation of the gray 

 matter of the spinal cord. Where the ventricle is widest 

 a tract of fibers, the stria medullaris, arises from the 

 midline and proceeds laterad to help form the auditory 

 nerve. 



Laterad of the clava a larger band of fibers, the 

 funiculus cuneatus, runs parallel with a more lateral 

 lying bundle, the funiculus cuneatus later alis, or funiculus 

 of Rolando. These three bands of fibers compose the 

 restiform body, or inferior peduncle of the cerebellum. 

 On the ventral aspect of the medulla may be seen the 

 anterior pyramids, composed of a band of fibers lying 

 on either side of the median line and apparently emerging 

 from the pons. They form a section of the crossed 



