214 ELEMENTS OF MAMMALIAN ANATOMY 



is bounded by the lateral olfactory tract or lateral root of the 

 olfactory nerve, which presents the appearance of a band of 

 white libers extending from the olfactory lobe into the temporal 

 lobe. The medial or inner root of the olfactory nerve is seen 

 adjacent to the median ventral line craniad of the anterior 

 perforated space. The olfactory lobes project from the cranial 

 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 therefore to be considered as a hollow structure. The 

 first and second ventricles, also known as lateral ventricles, occupy 

 the cerebral hemispheres (Fig. 105). The third Sind fourth ven- 

 tricles lie in the median line, and are therefore well seen in a 

 sagittal section of the brain (Fig. 106). 



The Ventricles of the Brain. — The fourth ventricle is visible 

 on the dorsal aspect of the medulla oblongata (Fig. 106). It 

 is about three centimeters long by one centimeter wide, and 

 lies ventral to the cerebellum. This ventricle 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 Vieus- 

 sens, covers the cranial 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 aspect of the pia mater. 

 Some nervous matter in addition to the pia mater and epithe- 

 lium forms the superior velum. Two longitudinal vascular 

 fringes hanging from the roof of the ventricle on either side of 

 the mid-line, form the choroid plexus, which is merely a network 

 of blood-vessels carried by a reflected portion of the pia mater. 

 The choroid plexus appears to be the main source of the cere- 

 brospinal fluid. 



