THE BRAIN IN" THE EDENTATA. 353 



The Brain-stem. 



By the term " In-aiu-stem " we understand that part of the brain which remains after 

 the removal of those morphological excrescences, the cerebral hemispheres and the 

 cereljelluni. It is a term which is used merely as a convenience in description, and has 

 no claim to morphological accuracy. 



It is unnecessary to discuss in detail the featiu'cs of this part of the l)raiD, because 

 its morjihology is practically unchanged iu the main body of mammals, if we make 

 due allowance for purely functional modifications and variations in size. We shall 

 therefore consider this part of our subject in as brief a space as is consistent with 

 lucidity. 



The ventral surface of the thalamic region and mesencephalon forms in Orycteropus, 

 in common with most other mammals, a broad exposed area (tig. 1) extending from the 

 optic tract in front to the prominent pons Varolii behind. The lateral part of the 

 subthalamic region is hidden by the anterior part of the eras cerebri, Avhich is itself 

 partially overlapped by the lohun pi/riformis posticus. The mesial border of the anterior 

 extremity of the crus cerebri is about 6 mm. from the mesial plane, but as it proceeds 

 backward it inclines obliquely inward, so tliat when it reaches the transverse strand of 

 the pons Varolii it is in close proximity to the crus cerebri of the other side. 



Thus a triangular interval is lett between the anterior extremities of the crura cerebri. 

 A large prominent mass of pale colour occupies this interval. The posterior part of this 

 mass consists of two large ellipsoidal bodies, each G mm. long and 3 mm. thick, wliich 

 are placed so close the one to the other that it is necessary to examine them very closely 

 to discover the slight median groove Avhicli indicates their line of separation. They are 

 the corpora mumuiillar/a. The interval between these bodies and the optic chiasma is 

 occupied in the mesial 2)laue by the thin floor of the third ventricle — the inftDitUbaliiin — 

 from which the pituitary body is suspended in the normal state. In this specimen of 

 Orycteropus, liowever, the pituitary body has Ijeen torn otf, leaving an elliptical aperture, 

 which opens into the cavity of the third ventricle (compare fig. I). 



Extending backward from the corpora mammillaria to the pons Varolii there is a deep 

 mesial cleft between the two crura cerebri. 



When the crus cerebri makes its tirst ajipearance upon the base of the brain by coming 

 from under cover of the optic tract it consists of a strand of fibres 10 mm. broad, but as 

 it extends obliquely backw^ard it becomes narrower and plumper, and when it disappears 

 under the fibres of the pons it is only 5 mm. broad, but 2'5 mm. thick. In many of the 

 other Edentates (figs. 6 and 9) the crvis cerebri is a more prominent strand, and this is 

 doubtless a result of the relatively greater size of the pallium than is tlie case in 

 Orycteroi)us. 



The interpeduncular cleft is occupied immediately in front of the jions Varolii by an 

 ellipsoidal nodule of grey substance — the corpus hiterpedunculare — iu all the Edentata 

 (figs. 1, 6, and 9). This body is 5 mm. long in Orycteropus, and of corresjoondingly 

 large dimensions in all tlie other Edentates. 



Along the lateral border of the cms cerebri in Orycteropus, parts of the oj)tic thalamus 



