Comparative Anatomy of the Brain. 575 



:i:rmatosac. It appears to resemble the hypopt^sts, or pituitary gland, 

 which depends from the third ventricle in front of the optic lobes. 



These hypoaria and the haematosac are not found in either the lowest 

 or highest orders of fishes. In the Myxine, among the lowest, the an- 



FIG. 284. Brain of Pike-fish. Top view. 



01 Olfactory lobes lying above the Optic nerves, Op. 



H. Cerebral hemispheres. 



Mb Optic lobes. 



Cb . Cerebel 1 u in . 



terior pyramids swell out into a small protu- 

 berance beneath the optic lobes, and this is 

 perhaps an elementary form of the hypoaria. 

 But the sams thing is seen in the, in many re- 

 spects, advanced Lepidosiren. (Fig. 290.) It 

 may be elementary in this animal also, since 

 from its inactive habits its entire hind brain 

 is in a backward condition, but more likely in 

 both this and the Polypterus, which is another 

 salamandroid fish, these organs are rudimentary 

 from retrogression. In the Shark these col- 

 umns swell out laterally and form protuber- 

 ances on the sides of the crura cerebri, sepa- 

 rated from each other by the vascular matter forming the infundibulum 

 on the floor of the third ventricle, in advance of the base of the optic 

 lobes. These formations are to be regarded as modifications of the 

 hypoaria, which occur in most of the osseous fishes. Just behind the 

 nypoaria, in the osseous fishes, there is a band of transverse fibres cou- 

 rt * 



FIG. 284. 



FIG. 286. 

 FIG. 285. Brain of Skate. (Raia.) ( Underside.) 



a. Optic Nerves. 

 b. Cr 



FIG. 285. 



Jross fibres connecting the same, 

 c. Hypoaria. 



FIG- 286. .Brain, of Shark Carcharias. 



R Olfactory Lobes, with olfactory nerves at their extremities. 

 z Olfactory Crura. 



P.-Cerebral hemispheres. X.-Cerebral ' Crnra. 



O Optic Lobes. F-"a3?tf Gland, 



a-Cerebellum. /i.-Pituitary Gland. 



N. -Lateral bulbs from the anterior pyra- 2. Optic nerves. 



mids answering to the Hypoaria of the 5, 7, . Fifth, seventh and eighth pairs of 

 Cod, &c. nerves. 



necting together the two anterior pyramids. It is called the ansulate 

 commissure, and is to be regarded as the early intimation of the pons 



