THE FORM AND SUBDIVISIONS OF THE BRAIN 27 



enter from the meningeal arterial network. Our preparations confirm 

 this observation and also the fact that the vascular pattern varies in 

 different parts of the brain. Both isolated loops and the capillary net 

 may be seen in the same field, as in the dorsal thalamus (fig. 44), or 

 one of these patterns may prevail, with few, if any, instances of the 

 other. In the tectum and dorsal tegmentum of the midbrain, for in- 

 stance, the tissue is vascularized by simple loops with only occasional 

 anastomosis (fig. 48), while in the underlying peduncle and isthmic 

 tegmentum the vascular network prevails, with occasional simple 

 loops. In the meninges and chorioid plexuses only the network has 

 been observed. 



The telencephalic and diencephalic chorioid plexuses have an 

 abundant arterial blood supply through the medial hemispheral 

 artery; but the elaborately ramified tubules of the paraphysis seem 

 to have no arterial supply or capillary net, the accompanying vessels 

 being exclusively venous sinusoids ('35, p. 342). The same seems to 

 be true of the endolymphatic sacs ('34c?, p. 543). The chorioid plexus 

 of the fourth ventricle has abundant arterial blood supply. In all 

 plexuses the capillaries unite into venules, which discharge into wide 

 sinusoids, which ramify throughout the plexus and have very thin 

 walls. All arterioles of the chorioid plexuses are richly innervated, 

 but it has not been possible to get satisfactory evidence of the sources 

 of these nerve fibers ('36, p. 343; '42, p. 255; Necturus, '336, p. 15). 



The enormous development of the chorioid plexuses and associated 

 endolymphatic organ of urodeles is apparently correlated with the 

 sluggish mode of life and relatively poor provision for aeration of the 

 blood. In the more active anurans the plexuses are smaller; but in the 

 sluggish mudfishes, including the lungfishes, with habits similar to 

 those of urodeles, we again find exaggerated development of these 

 plexuses. Existing species in the border zone between aquatic and 

 aerial respiration are all slow-moving and relatively inactive. The 

 enlarged plexuses and sinusoids give vastly increased surfaces for 

 passage of blood gases into the cerebrospinal fluid; and, correlated 

 with this, the brain wall is thin everywhere, to facilitate transfer of 

 metabolites between brain tissue and cerebrospinal fluid. Massive 

 thickenings of the brain wall occur in many fishes and in amniote 

 vertebrates, but not in mudfishes and urodeles. 



