CEREBRAL VENTRICLES IN THE PIG 237 



the pig it appears to be somewhat smaller than in the horse, 

 but in both these animals the opening is very large. The area 

 of fusion in the 260-mm. pig measures about 8.4 sq. mm., and 

 in the adult pig about 61.6 sq. mm. 



The lateral ventricles in pig, horse and man differ very greatly. 

 In man there is a posterior horn which is absent in the horse 

 and pig. But the latter both show an extension of the ventricle 

 into the olfactory lobe, whereas in man, the anterior horn ends 

 bluntly. In the horse the pedicle of the cavity of the olfactory 

 stalk is very long and markedly concave dorsally, it ends in an 

 elongated irregular ventricle. The pedicle in the pig is nearly 

 straight and much shorter. It terminates in a flattened expan- 

 sion which in the anterior end is compressed dorso-ventrally. 



In connection with the lateral ventricle it may be noted that 

 its chorioid plexus in the pig is not provided with villi. Meek 

 ('07) in his discussion of the general morphology of the plexus 

 writes: "Villi are scarce in the chicken, duck and pigeon, but 

 more abundant in the hog, while they reach a considerable devel- 

 opment in the, horse, ox, and especially among porpoises, croco- 

 diles, and some of the selachians (Pettit '02-'03)." Findlay ('99) 

 writes similarly of this plexus: ''The surface of the chorioid 

 plexus is beset with a large number of highly vascular villous 

 projections. These are of all sizes, and the largest may branch 

 and subdivide many times before the ultimate villi are formed." 

 The study of the brain of the pig has shown that the chorioid 

 plexus of the lateral ventricle first develops as an extension of 

 the velum transversum into the lateral ventricle. The free bor- 

 der rapidly becomes much longer than the attached part so that 

 it is early thrown into folds. These primary folds increase in 

 number and give rise to secondary folds, but as shown by means 

 of the binocular microscope, villi are not developed in this plexus. 



In pig embryos of 35 to 40 mm. and in all older stages, there 

 is a median dorsal recess behind the posterior commissure. This 

 postcommissural recess is shown by Retzius who labels it the 

 'incisura postcommissuralis.' It is shown also in Harvey's lateral 

 view. None of the other authors have an indication of it in 

 their figures. In the adult pig the recess is very prominent. 



