410 ANATOMICAL TECHNOLOGY. 



the latter in the greater difEerentiation of the segments, and from both in the sabglobnlar 

 form of the hemispheres. 



All that is shown, however, might really exist in a brain without contravention of our 

 general knowledge of the structure and development of the organ ; hence, even If consid- 

 erable modification of detail should be required, such diagrams would still be useful as 

 an elementary introduction to the study of the brain. 



At the outset the student will do well to regard the diagrams as purely geometrical 

 figures or as representing hoUow masses of wood or iron. The shading is conventional, 

 and intended to be uniform excepting with the three nerves of special sense in Fig. 110. 

 Xo attempt is made to indicate the difference between the true nervous parts and those 

 {conarium and hypophysis) which may consist of very difEerent material, between the 

 fibrous and cellular portions of the nervous tissue, or between the longitudinal and trans- 

 verse fibers of the former. 



The idea of diagrams like Kg. 110, 111 was derived from the very clear and suggestive 

 views of the typical brain given by Huxley (A, Fig. 19, 20) ; the only diagrams known to us 

 comparable with those in Fig. 112 were published by the senior author (23, PL III), but 

 tbe membranes were not included. The transections of the frog's brain given by Stieda 

 (22, Taf. XVIII) are of actual sections and likewise omit the membranes. 



§ 1072. Fig, 110 — Horizontal Section of the entire Neuron (cerebro-spinal axis). 

 Copied, with slight modification, from the X. T. Medical Journal, March 21, 1885. p. 326, 

 Fig. 2 (Wilder, GS). Compare the Table of Names on page 400£?. 



Neurocosle includes the entire cavity of the neuron ; the caudal, longer and narrower 

 portion, the cavity of the myel or spinal cord, is called myeioc<de, comprising the tubular 

 syringoc(Ble (central canal) and the more or less expanded rlmmbocaie of the "lumbar 

 ealargement ;" the rhombocoele is not constant in adult vertebrates and may not always 

 appear in embryos, but its existence in certain mammalian embryos (dog, Quain, 9th 

 ed., II, Fig. 648), and its persistence in Birds (Owen, II, 117), seem to warrant its inclusion 

 in the present scheme. 



The encephalocaie, or general brain cavity, is here regarded as including five divisions 

 corresponding with the five, commonly accepted, encephalic segments or encephalomeres ; 

 the difficulties and questions respecting the determination of the number and limits of 

 the encephalomeres, and their designation, have been set forth by the senior author {58, 

 63, 327). 



Three of the encephalocceles are represented as simple, although it is by no means 

 certain that both the epicoele and metaccele do not present lateral protrusions, and the 

 optic vesicles may bear that relation to the diacoele, as suggested by the senior author 

 (.56', 146). Tbe mesocoele of man and other mammals presents but slight indications of 

 a triple constitution, but in Birds and Frogs the lateral divisions are very distinct (Wilder, 

 63, Fig. 4-6). The prosocoelian lateral protrusions (procoeles) are shown in their sim- 

 plest condition, and without the greater extension cephalad which masks their true rela- 

 tions in most vertebrates. The cavity of the olfactory lobe (rhinoccele rhc.) is made subor- 

 dinate to the procoele, although this relation is reversed in lamprey-eels, and may not really 

 hold good in other vertebrates. The width of tbe terma is exaggerated for the sake of 

 distingidshing fully the aula or mesal division of the prosocoale, and in accordance with 

 the representations of Balfour (II, Fig. 2"i7) and KolHker (Fig. 311, 312). Even, however, 

 were no ^vidth assigned to the terma, the aula would merely be reduced from a quadrangle 

 to a triangle, and still have an appreciable extent. 



S 1073. Fig. Ill — Mesal aspect of the right lialf of an ideal, amphibian brain, exclu- 

 sive of tbe cephalic portion of the hemisphere and the olfactory lobe. Of course, only the 



