THE BRAIN IN THE EDENTATA. 323 



former term, I was able to clearly establish the identity of the area pra;commissuralin 

 with those surface-areas corresponding to sejji/im lucidum and gyms snbcallosns (Zucker- 

 kandl) of the human brain. Taking into consideration the fact that these two areas are 

 merely parts of one and the same formation, the separation of which into two parts is 

 purely arbitrary in most mammals, it seemed desirable to retain the term " area pra-- 

 commissuralis " for the whole surface-region, even though the term may not be so apt 

 elsewhere as it is in the case of Ornithorlnjuchus and the Submammalia. 



The term " precommissural area " sufficiently explains the locality of the region to 

 which it is meant to be applied in all vertebrate brains. The term " ])recommissu7-at 

 body''' however, is not nearly so appropriate a title for the ganglionic mass whose 

 surface is the pi'ecommissural area, and in many cases it is singularly inappropriate. 

 By substituting for it the name '' paracommissioral body " we shall obtain a term which 

 may justly be applied to tliis important ganglionic mass in any vertebrate. Unlike 

 the term "precommissural area," which was introduced much earlier, the term "pre- 

 commissural body " has not yet been adopted by other writers, and hence there can be 

 no objection to the substitution of the more apt title " corpus paracommissurale " for the 

 ganglionic mass which I had previously called the " corpms prcecommissurale." 



The need for some definite name for this mass of grey matter which forms so prominent 

 a constituent of the brain in the E,eptilia, Amphibia, and Dipnoi has been felt for a 

 considerable time. The terms " septum lucidum" and "gyrus suhcallosus " are not onlv 

 meaningless but inaccurate when applied to any other brains than the most bighly 

 organized of the Mammalia, and their application to such brains as the Prototherian. 

 Saurian, or Amphibian cannot fail to give rise to great misconception. Nor can these 

 terms be applied with any more propriety to the Basypodidce. 



In the dissection of the lateral ventricle of Orycteropus (fig. 19) the paracommissural 

 body makes its appearance in tlie interval bet^reen the anterior commissure and the 

 corpus callosum as a very plump mass of grey matter w^hich forms a most marked contrast 

 to the surrounding areas because they are all lined wdth white meduUated tibres. The 

 posterior margin of this body is abrupt and rounded, and appears as though it were placed 

 on the lateral aspect of tiie fimbria. Anteriorly the paracommissural body graduallv 

 sliades away toward the tunnel in the olfactory peduncle. 



The paracommissural body is separated from the corpus striatum by the slit-like lateral 

 ventricle. It is unnecessary to enter into a description of tbe corpus striatum or lateral 

 ventricle in this place. These features of the brain do not appreciably dilfer from those 

 of common Rodents such as Lepus. 



The essential features of the paracommissural body are similar in all the Edentates. 

 The shape of the body, however, is modified considerably by the growth of a corpus 

 callosum, and hence it is very variable in the ditferent families. These changes in shape 

 are best appreciated from a study of the mesial surface of the hemispliere, Avhere the 

 paracommissural body enters into the constitution of the w^all of the brain formin"- the 

 precommissural area. 



In the Sloths (figs. 17, 18, and 23 B) we see the typical arrange.-ncnt of the precom- 



