288 Jourtial of Comparative Neurology and Psychology. 



are to be found all the intermediate gradations. The guinea pig, 

 mouse and rat possess plexuses that are smooth. Fig. i shows 

 the cross section of a lateral plexus of a one day old rat. It will 

 be noted that the surface is not entirely regular. There are pro- 

 jections and prolongations of the folds, but the typical villi are 

 absent. In the rabbit, the laminae are still more irregularly folded, 

 but they are not villous. Villi are scarce in the chicken, duck and 

 pigeon, but more abundant in the hog, while they reach a con- 

 siderable development in the horse, ox, and especially among por- 

 poises, crocodiles, and some of the selachians (Pettit 'o2-'o3). 

 In the sheep, villi are numerous along the free edge of the plexus, 



but they are thick and short. In man, 

 the villosities are also found but the type 

 is somewhat intermediate. Immamura 

 ('02) states that the human plexus con- 

 sists of two parts, a villous and a villus- 

 free portion. There seems, however, to 

 be much variation in this respect. 



The surface of the plexuses is much 

 greater than one would suppose. Faivre 

 ('54) has estimated it for the human 

 plexuses in the lateral ventricles. He 

 considers the average length as 7 centi- 

 ' ' ' meters, and the width as 2 centimeters. 



'^ By estimating 40 villi to each square cen- 



FiG. I. Cross section through the timcter, he calculatcs that the surface 



right ventricle of a one day old rat. ^^^jj ^^ increased aboUt four timeS, 



Magnification X 150. a, Plexus in , . ^ r • ' 



„-r,^.,vIo. K .,. A makmg a total 01 112 square centimeters 



ventricle; b, ependyma; c, nervous , o 1 



tissue. for the two plexuses. We might add that 



since the folds hang freely in the ventri- 

 cles, and are covered on all sides by the same epithelium and 

 villi, that twice this number, or 224 square centimeters, is more 

 nearly the total area of the free surface. 



The blood supply of the plexuses in man is well known. Two- 

 thirds is supplied by the anterior choroid branch of the internal 

 carotid which enters the plexus at the anterior end of the descend- 

 ing cornu. The remainder is supplied by the postero-lateral cho- 

 roid artery, a branch of the posterior cerebral. These arteries 

 break up into arterioles, the largest of which are visible to the 

 naked eye. After passing through the network of capillaries, the 



