40G ANATOMICAL TECHNOLOGY. 



1062. Unequal Morphical Value of the Segments. So far as appears in the second 

 column of the Table, the six segments are equal primary divisions. 



In one sense this is certainly not the case. According to most accounts (Reichert, A, 

 II, 11 ; Mihalkovics, A, 21 ; Spitzka, C, 27 ; Huxley, A, 56 ; Quain, A, II, 750), the 

 embryonic brain consists of three primary vesicles ; of these the middle is developed into 

 the mesencephalon, while each of the others is again divided into secondary vesicles, from 

 which the other five regions are formed. Balfour, however (A, II, 345), admits only two 

 primary vesicles. 



Aside from the verbal distinctions indicated in the Table, most of the differences are 

 due to the unequal estimates placed by writers upon the several segments from an embryo- 

 logical point of view. 



Anatomically, there seems to be no objection to the arrangement here adopted, for 

 while the cerebellum and hemispheres preponderate in the higher animals, the optic lobes 

 are larger in some " fishes," the olfactory lobes are enormous in some sharks and placed 

 at a considerable distance from the prosencephalon, and in a skate (Torpedo) the largest 

 part of the brain is the medulla, 



For practical purposes, then, the six segments may be regarded as coordinate divisions. 



1063. Advantages of Using the Regmental Names. There are three advantages in 

 the use of terms designating the encephalic segments : 



(1) They indicate the segmental constitution of the brain. 



(2) Each designates a general region which may consist of several more or less distinct 

 parts. MesencepJialon, for example, includes not only the optic lobes, but the crura 

 eerebri, etc. 



(3) They are single words capable of inflection. 



1064. Names of the Cavitie.i.0n account of the peculiar condition of things in man 

 and the higher Mammals, certain portions of the general cavity have been recognized as 

 such, while others have been called passages or ignored altogether. In modern times the 

 larger cavities have been usually called ventricles, from the Latin ventriculus. 



The incongruity of the anthropotomical designations of the encephalic cavities has 

 been pointed out by Owen (A, I, 294, note), and the senior author (f>, 125, 14:, 539). 



The canalis centralis expands into a cavity which, although the first of the series, is 

 called the fourth ventricle. The more or less distinct cavities corresponding to the cere- 

 bettum and the optici are not called ventricles at all, and the second is known by either of 

 the following phrases : aquceductiis Kylcii and iter a tertio ad ventriculum quartum. The 

 diencephalic cavity is the third ventricle. The two " lateral " ventricles are rarely men- 

 tioned as the first and second, but since the numbers must be understood in order to account 

 for the third &nd fourth, the student desires, in vain, to know which is the first and which 

 the second. In point of fact, if the enumeration is begun at the cephalic end of the series, 

 the lateral ventricles are the third and fourth, since, in most air-breathing Vertebrates, 

 there are well-developed ventricles in the Lobi oifaetoru. Finally, a "fifth ventricle " is 

 mentioned, which is not only at a great distance from the fourth, but has no normal con- 

 nection with the other ventricles, and is, in fact, no part of the series. 



It is hardly possible to imagine less appropriate and consistent appellations for a series 

 of essentially similar cavities. 



Ventriculum is objectionable because of its length, because its use is apparently estab- 

 lished in connection with the cardiac cavities, and because, as a Latin word, it is not readily 

 combined with Greek prefixes. No one of these objections is fatal, but combined they may 

 be regarded as warranting the use of another word if such can be found. 



