ELEMENTS OF MAMMALIAN ANATOMY. 



ab. 



mi 



the olfactory nerve is seen adjacent to the median ventral 

 line cephalad of the anterior perforated space. The 

 olfactory lobes project from the cephalic ventral portion 

 of the cerebrum and give origin to the first pair of cranial 

 nerves. 



Internal Structure.- -The canalis centralis, a small canal 

 extending throughout the center of the spinal cord, 



enlarges in the region of the brain, 

 forming four cavities or ventricles 

 communicating with each other by 

 narrow channels. The brain is there- 

 fore to be considered as a hollow 

 structure. The first and second ven- 

 tricles, also known as lateral ventricles, 

 occupy the cerebral hemispheres (Fig. 

 91). The third and fourth ventricles 

 lie in the median line, and are there- 

 fore well seen in a sagittal section of 

 the brain (Fig. 92). 



The Ventricles of the Brain.- -The 

 fourth ventricle is visible on the dorsal 

 aspect of the medulla oblongata (Fig. 

 92). It is about three centimeters 

 long by one centimeter wide, and lies 

 ventral to the cerebellum. This ven- 

 tricle is merely an expansion of the 

 canalis centralis of the spinal cord. Its roof is very thin 

 and consists of two portions, one of which, the superior 

 medullary velum, sometimes called the valve of Vieussens, 

 covers the cephalic half of the ventricle ; the other portion 

 is the inferior medullary velum, lying over the caudal 

 half. The latter velum is composed of a fold of pia 

 mater tucked in between the cerebellum and medulla, 

 in addition to a layer of epithelial cells on the ventricular 



FIG. 91. DIAGRAM OP 

 VENTRICLES OF THE 

 BRAIN VIEWED 

 DORSALLY. 



ah, Anterior horn of 

 the right lateral ven- 

 tricle; as, aqueduct 

 of Sylvius ; cc, can- 

 alis centralis of the 

 spinal cord; 3 and 

 4, third and fourth 

 ventricles ; m, fora- 

 men of Monro ; mi, 

 middle horn or cor- 

 nu of the ventricle. 



