THE BRAIN 



151 



cord. In many lower vertebrates the anterior part of the fourth 

 ventricle may extend as a metacoele into the cerebellum (fig. 174). 

 Telae Chorioideae. — The brain, as well as the spinal cord, receives 

 nourishment from blood-vessels distributed over the outer surface, 

 but, in addition, extensions of these outer vessels carry the roof and 

 floor plates of the fore- and hind-brains before them into the ventricles 

 of these regions. These foldings of the plates are called telae 

 chorioideae (figs. 158, 160) or chorioid plexuses, and by their means 

 nourishment passes by osmosis into the ventricles and so to the inner 

 surfaces of the brain. Usually the chorioid plexus of the fourth ven- 

 tricle is torn away in removing the envelopes of the brain, leaving a 

 large opening, the fossa rhomboidalis, into the cavity. 



Fig. 158. — Diagrammatic longitudinal section of brain, ac, anterior commissure 

 in lamina terminalis; aq, aqueduct; c, cerebrum; cb, cerebellum; c/>, chorioid plexus; cs, 

 corpus striatum; cv, cerebral ventricle; h, hypophysis; he, habenular commissure; 

 », infundibulum; ip, inferior chorioid plexus; m, mesencephalon; ml, myelencephalon ; 

 *, pinealis; pa, paraphysis; pc, posterior commissure; pe, parietal eye; v, anterior medul- 

 lary velum; vt, velum transversum with aberrant commissure; /// and IV, third and 

 fourth ventricles. 



Flexures. — Thus far the brain has been considered as if it were 

 a continuation in a straight line of the spinal cord. In fact, by 

 unequal growth of the two zones, it becomes flexed in the vertical 

 plane. In the lower vertebrates these flexures are prominent only in 

 the young and largely disappear in the adult. They are more devel- 

 oped in the higher vertebrates and persist throughout life. Most 

 constant of them is the primary flexure in the mid-brain, by which the 

 derivatives of the fore-brain are bent at a right angle (or even more) 

 to the axis of the rest (fig. 159). Second to appear is the nuchal 

 flexxure in the hinder part of the medulla oblongata which bends in the 

 same direction. The pontal flexure, in a plane with the cerebellum 

 tends to counteract the effect of the other two as it bends in the re- 

 verse way. Nuchal and pontal flexures are at best but weakly de- 

 veloped in the ichthyopsida where practically all flexures are obliter- 



