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| [[File:Mark_Hill.jpg|90px|left]] This historic 1948 book by Herrick describes the development of the lizard, tiger salamander (''Ambystoma tigrinum'').
| [[File:Mark_Hill.jpg|90px|left]] This historic 1948 book by Herrick describes the development of the lizard, tiger salamander (''Ambystoma tigrinum'').
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Modern Notes: {{lizard}} | {{neural}}
root; cellular organisms; Eukaryota; Opisthokonta; Metazoa; Eumetazoa; Bilateria; Coelomata; Deuterostomia; Chordata; Craniata; Vertebrata; Gnathostomata; Teleostomi; Euteleostomi; Sarcopterygii; Tetrapoda; Amniota; Sauropsida; Sauria; Lepidosauria
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'''Modern Notes:''' {{lizard}} | {{neural}}


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{{Herrick1948 TOC}}
{{Historic Disclaimer}}
{{Historic Disclaimer}}
=The Brain of the Tiger Salamander=
=The Brain of the Tiger Salamander=


Ambystoma tigrinum  
Ambystoma tigrinum  




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Money for the prosecution of the work and for financing its publication was liberally supplied by the Dr. Wallace C. and Clara A.  
Money for the prosecution of the work and for financing its publication was liberally supplied by the Dr. Wallace C. and Clara A. Abbott Memorial Fund of the University of Chicago.  
Abbott Memorial Fund of the University of Chicago.  
 
 
 
 


Part I. General Description and Interpretation


==Contents==  
==Contents==  


PART I. GENERAL DESCRIPTION AND INTERPRETATION
# [[Book - The brain of the tiger salamander 1|Salamanders and Their Brains]]
 
## The salamanders
I. Salamanders and Their Brains
## The scope of this inquiry
 
## The plan of this book
The salamanders, 3. — The scope of this inquiry, 4. — The plan of
## Sources and material
this book, 6.— Sources and material, 10.— Development of the brain,
## Development of the brain
11. — The evolution of brains, 13
## The evolution of brains
 
# [[Book - The brain of the tiger salamander 2|The Form and Subdivisions of the Brain]]
II. The Form and Subdivisions of the Brain 18
## Gross structure
 
## Ventricles
Gross structure, 18.— Ventricles, "24.— Meninges, chorioid plexuses,
## Meninges, chorioid plexuses, and blood vessels
and blood vessels, 'id
# [[
 
III. Histological Structure
 
General histology, 38.— The neuropil, '29.— The ventrolateral
peduncular neuropil, 3,3
 
IV. Regional Analysis 4
 
The subdivisions, spinal cord to pallium, 41.— The commissures, ;5;5.
^Conclusion, 56
 
V. Functional Analysis, Central and Peripheral 57
 
The longitudinal zones, 57. — The sensory zone, 58. — The motor
zone, 60. — The intermediate zone, 64.— The functional systems, 65
 
VI. Physiological Interpretations
 
Apparatus of analysis and synthesis, 70. — The stimulus-response
formula, 7^2. — Reflex and inhibition, 73.— Principles of localization
of function, 8'2
 
Vll. The Origin and Significance of Cerebral Cortex .... 91
The problem, 91. — Morphogenesis of the cerebral hemispheres,
9*2. — The cortex, 98. — Physiology and psychology, 106
 
VIII. General Principles of Morphogenesis
 
Morphogenic agencies, 109. — Morphological landmarks, 116. — The
future of morphology, 120
 
PART II. SURVEY OF INTERNAL STRUCTURE
 
IX, Spinal Cord and Bulbo-spinal Junction
 
The spinal cord and its nerves, 125. — The bulbo-spinal junction,
129
 
X. Cranial Nerves
 
Development, 131. — Survey of the functional systems, 132
 
XI. Medulla Oblongata
 
Sensory zone, 153. — Intermediate zone, 156. — Motor zone, 157. —
Fiber tracts of the medulla oblongata, 158.— The lemniscus
sy.stems, 162
 
XII. Cerebellum
 
Brachium conjunctivum, 176.— The cerebellar commissures, 177.—
Proprioceptive functions of the cerebellum, 178
 
XIII. Isthmus 1'^^
 
Development, 179.— Sensory zone, 181.— Intermediate zone, 182.
—Motor zone, 182.— White substance, 186.— Isthmic neuropil, 187.
— Physiological interpretation, 189
 
XIV. Interpeduncular Nucleus
 
Comparative anatomy, 191.— Histological structure, 193.—
Afferent connections, 197.— Efferent connections, 201.— Interpretation, 202. — Conclusion, 210
 
XV. Midbrain
 
Development, 212.— Sensory zone, 214.— Intermediate zone, 215.
— Motor zone, 216
 
XVI. Optic and Visual-motor Systems 219
 
Optic nerve and tracts, 219.— Tectum opticum, 222.— Tectooculomotor connections, 226.— Visual functions, 227
 
XVII. Diencephalon
 
General features, 230.— Development, 231.— Epithalamus, 234.—
Dorsal thalamus, 236.— Ventral thalamus.— 239.— Hypothalamus,
241
 
XVIII. The Habenula and Its Connections 247
 
The di-telencephalic junction, 247.— Fornix, 254.— Stria terminalis,
255.— Stria medullaris thalami, 256.— Fasciculus retroflexus, 261
 
XIX. The Cerebral Hemispheres 265
 
Subdivisions of the hemisphere, 265.— The olfactory system, 266
 
XX. The Systems of Fibers 270
 
The basal forebrain bundles, 271.— The tegmental fascicles, 275.—
Fasciculus tegmentalis profundus, 286
 
XXI. The Commissures ^^^
 
General considerations, 289.— The dorsal commissures, 292.— The
ventral commissures, 294
 
 
Bibliography
 
Illustrations
 
Abbreviations for All Figures
 
 
==Part I  General Description and Interpretation==
 
 
CHAPTER I  SALAMANDERS AND THEIR BRAINS
 
 
 
THE SALAMANDERS
 
SALAMANDERS are shy little animals, rarely seen and still more
rarely heard. If it were not so, there would be no salamanders at
all, for they are defenseless creatures, depending on concealment for
survival. And yet the tiger salamander, to whom this book is dedicated, is appropriately named, for within the obscurity ol its contracted world it is a predaceous and voracious terror to all humbler
habitants.
 
 
This salamander and closely allied species have been found to be
so well adapted for a wide range of studies upon the fundamental
features of growth and differentiation of animal bodies that during
the last fifty years there has been more investigation of the structure,
development, and general physiology of salamanders than has been
devoted to any other group of animals except mankind. The reason
for this is that experimental studies can be made with these amphibians that are impossible or much more difficult in the case of
other animals. This is our justification for the expenditure of so much
hard work and money upon the study of the nervous system of these
insignificant little creatures.
 
 
The genus Ambystoma is widely distributed throughout North
America and the tiger salamander, A. tigrinum, is represented by
several subspecies. The individuals vary greatly in size and color,
and the subspecies have different geographical distribution, with
some overlap of range (Bishop, '43). The subspecies, A. tigrinum
tigrinum (Green), ranges from New York southward to Florida and
westward to Minnesota afid Texas. It has a dark-brown body crossed
by bright-yellow stripes, as shown in the lower figure of the Frontispiece. The species probably was named for these tiger-like markings,
not for its tigerish ferocity. The upper figure of the Frontispiece is an
adult of a western form, with less conspicuous markings. Other subspecies range as far to the northwest as Oregon and British Columbia.
Several other species of Ambystoma are found in the same areas as
A. tigrinum.
 
 
Zoological names. — The approved names of the genus and larger groups to which
reference is here made, as given in a recent official list (Pearse, '36), are as follows:
 
Salienta, to replace Anura
 
Caudata, to replace Urodela
 
Ambystoma, to replace Amblystoma
Ambystoma (or Siredon) maculatum has priority over A. punctatum. The names,
Anura, Urodela, and Amblystoma, are used throughout this text because they are
so commonly found in current literature that they may be regarded as vernacular
terms.
 
 
THE SCOPE OF THIS INQUIRY
 
From the dawn of interest in the minute structure of the human
brain, it was recognized that the simpler brains of lower vertebrates
present the fundamental features of the human brain without the
numberless complications which obscure these fundamentals in
higher animals. This idea motivated much research by the pioneers
in neuroanatomy, and it was pursued systematically by L. Edinger,
H. Obersteiner, Ramon y Cajal, C. L. Herrick, J. B. Johnston,
Ariens Kappers, and many others. In 1895, van Gehuchten wrote
that he was engaged upon a monograph on the nervous system of the
trout, "impressed by the idea that complete information about the
internal organization of the central nervous system of a lower vertebrate would be of great assistance as our guide through the complicated structure of the central nervous system of mammals and of
man." The few chapters of this monograph which appeared before
his untimely death intensify our regret that he was not permitted to
complete this work. Van Gehuchten's ideal has been my own inspiration.
 
 
Our primary interest in this inquiry is in the origins of the structural features and physiological capacities of the human brain and
the general principles in accordance with which these have been developed in the course of vertebrate evolution. This is a large undertaking. What, then, is the most promising approach to it.'^ My
original plan was, first, to review all that has been recorded about the
anatomy and physiology of the nervous systems of backboned animals, to arrange these animals in the order of their probable diverse
specialization from simple to complex in the evolutionary sequence,
then to select from the series the most instructive types and subject
them to intensive study, in the expectation that the principles underlying these morphological changes would emerge.
 
 
So ambitious a plan, however, is far too big to be encompassed
within the span of one man's lifetime. The fact-finding research is
 
 
 
SALAMANDERS AND THEIR BRAINS 5
 
extremely laborious and exacting; and, during the fifty years which
have elapsed since my project was formulated, the descriptive literature has increased to enormous volume. This literature proves to be
peculiarly refractory to analysis and interpretation. Until recently
this vast accumulation of factual knowledge has contributed disappointingly little to the resolution of the fundamental problems of
human neurology. Nevertheless, the method is sound, and this slow
growth is now coming to fruition, thanks to the conjoint labors of
specialists in many fields of science. What no individual can hope to
do alone can be done and has been done in co-operative federation,
as illustrated, for instance, by the Kappers, Huber, and Crosby
team and their many collaborators.
 
 
Traditionally, comparative neurology has been regarded as a subdivision of comparative anatomy, and so it is. But it is more than
this. The most refined methods of anatomical analysis cannot reveal
the things that are of greatest significance for an understanding of the
nervous system. Our primary interest is in the behavior of the living
body, and we study brains because these organs are the chief instruments which regulate behavior. As long as anatomy was cultivated
as a segregated discipline, its findings were colorless and too often
meaningless.
 
 
Now that this isolationism has given way to genuine collaboration
among specialists in all related fields — physiology, biochemistry, biophysics, clinical practice, neuropathology, psychology, among others
— we witness today a renaissance of the science of neurology. The
results of the exacting analytic investigations of the specialists can
now be synthesized and given meaning. The task of comparative
anatomy in this integrated program of research is fundamental and
essential. The experimentalist must know exactly what he has done
to the living fabric before he can interpret his experiment. In the past
it too often happened that a physiologist would stab into a living
frog, take his kymograph records, and then throw the carcass into
the waste-jar. This is no longer regarded as good physiology. Without
the guidance of accurate anatomical knowledge, sound physiology is
impossible; and, without skilful physiological experimentation, the
anatomical facts are just facts and nothing more.
 
 
Early in my program the amphibians were selected as the most
favorable animals with which to begin a survey of the comparative
anatomy of the nervous system. Time has proved the wisdom of this choice, and the study of these animals has been so fruitful that by
far the larger part of my research has been devoted to them.
 
 
In this work it was my good fortune to be associated with the late
G. E. Coghill, whose distinguished career pointed the way to an
original approach to the problems of the origins and growth of the
nervous organs and their functions. The record of phylogenetic history spans millions of years and is much defaced by time; but the
record of the embryonic development of the individual is measured
in days and hours, and every detail of it can be watched from moment to moment. The internal operations of the growing body are not
open to casual inspection, but Coghill showed us that the sequence
of these changes can be followed.
 
 
He selected the salamanders for his studies for very good reasons,
the same reasons that led me to take these animals as my own point
of departure in a program of comparative neurology. My intimate
association with Coghill lasted as long as he lived, and the profound
influence which his work has had upon the course of biological and
psychological events in our generation has motivated the preparation of a book devoted to his career ('48). This influence, though
perhaps unrecognized at the time, was doubtless largely responsible
for my persistent efforts to analyze the texture of the amphibian
nervous system, for his studies of the growth of patterns of behavior
and their instrumentation in young stages of salamanders brought to
light some prmciples which evidently are applicable in phylogenetic
development also.
 
 
While Coghill's studies on the development of salamanders were in
process, we were impressed by the importance of learning just how
these processes of growth eventuate in the adult body. This was my
job, and so we worked hand in hand, decade after decade, for forty
years. Progress was slow, but our two programs fitted together so
helpfully that my original plan for a comprehensive study of the
comparative anatomy of the nervous system was abandoned in favor
of more intensive study of salamanders.
 
 
THE PLAN OF THIS BOOK
 
The preceding details of personal biography are relevant here because they explain the motivation and plan of this book. The significant facts now known about the internal structure of the brain of
the tiger salamander in late larval and adult stages are here assembled. The observation's on this and allied species previously recorded by the writer and many others are widely scattered, often in
fragmentary form, and with confusing diversity in nomenclature and
interpretation. As observations have accumulated, gaps in knowledge
have been filled, early errors have been corrected, the nomenclature
has been systematized, and now, with the addition of considerable
new observation here reported, the structure may be viewed as a
whole and interpreted in relation with the action system of the living
animal. Many of my observations during the last fifty years confirm
those of others; and, since references to these are given in the papers
cited, this account is not encumbered with them except where they
supplement my own experience or deal with questions still in controversy. I here describe what I myself have seen, with exceptions
explicitly noted. This explains the disproportionate number of references in the text to my own papers.
 
 
Two genera of urodeles have been studied intensively to find out
what is the instrumentation of their simple patterns of behavior. All
observations on the brain of the more generalized mudpuppy,
Necturus, were assembled in a monograph ('336) and several followmg papers. The present work is a similar report upon the brain of the
somewhat more specialized tiger salamander. The original plan was
to follow this with an examination of the brain of the frog, for which
abundant material was assembled and preliminary surveys were
made; but this research, which is urgently needed, must be done by
others.
 
 
In this book the anatomical descriptions are arranged in such a
way as to facilitate interpretation in terms of probable physiological
operation. Though the amount of experimental evidence about the
functions of the internal parts of the amphibian brain is scanty, there
is, fortunately, a wealth of such observation about the brains of other
animals; and where a particular structural pattern is known to be
colligated with a characteristic pattern of action, the structure may
be taken as an indicator of the function. The reliability of this method depends upon the adequacy of our knowledge of both the function
and the structure. The present task, then, is an assembly of the
anatomical evidence upon which the interpretations are based.
 
 
The first part of this work is written to give biologists and psychologists an outline of the plan of organization of a generalized
vertebrate brain and some insight into the physiological principles
exemplified in its action. These eight chapters, with the accompanying illustrations, can be read independently of the rest of the book.
 
 
 
 
Part II is written for specialists in comparative neurology. It
covers the same ground as the first part, reviewing each of the conventional subdivisions of the brain, giving details of the evidence
upon which conclusions are based, with references to sources and
much new material. This involves some repetition, which is unavoidable because all these structures are interconnected and in action
they co-operate in various ways. Many structures must be described
in several contexts and, accordingly, the Index has been prepared
with care so as to enable the reader to assemble all references to every
topic.
 
 
The most important new observations reported in Part II relate to
the structure and connections of the isthmus (chap, xiii), interpeduncular nucleus (chap, xiv), and habenula, including analysis of
the stria medullaris thalami and fasciculus retroflexus (chap, xviii).
In chapter xx a few of the more important systems of fibers are described, including further analysis of the tegmental fascicles as enumerated in the paper of 1936 and references to other lists in the literature. The composition of the commissures of the brain is summarized
in chapter xxi. The lemniscus systems are assembled in chapter xi,
and other tracts are described in connection with the structures with
which they are related.
 
 
Since most neurologists are not expert in the comparative field,
where the nomenclature is technical and frequently unintelligible
except to specialists, the attempt is made in Part I to present the
salient features with a minimum of confusing detail and, so far as
practicable, in terms of familiar mammalian structure. This is not an
easy thing to do, and no clear and simple picture can be drawn, for
the texture of even so lowly organized a brain is bafflingly complicated and many of these structures have no counterparts in the human body. Homologies implied by similar names are rarely exact,
and in many of these cases the amphibian structure is regarded as the
undifferentiated primordium from which the mammalian has been
derived. This is emphasized here because homologies are usually
defined in structural terms and because organs which are phylogenetically related are regarded as more or less exactly homologous,
regardless of radical changes in their functions. Thus the "dorsal
island" in the acousticolateral area of the medulla oblongata of
Necturus is regarded as the primordium of the dorsal cochlear nucleus of mammals, despite the fact that Necturus has no recognizable
rudiment of a cochlea or cochlear nerve. This is because, when the cochlear rudiment and its nerve appear in the frog, the tissue of the
"dorsal island" receives the cochlear nerve with radical change in the
functions performed (p. 138).
 
 
The histological texture of these brains is so different from that of
mammalian brains that the development of an intelligible nomenclature presents almost insuperable difficulty — a difficulty exacerbated
by the fact that in the early stages of the inquiry it was necessary to
apply descriptive terms to visible structures before their relationships were known. With increase of knowledge, errors were corrected,
and unsuitable names were discarded; but terms already in use are
still employed so far as possible, even though they are in some cases
cumbersome and now known to be inappropriate. In all these descriptions I have consistently used the word "fissure" to designate visible
furrows on the external surface of the brain and "sulcus" for those on
ventricular surfaces. Attention is called to the list of abbreviations
(p. 391) and to previous lists there cited where synonyms are given.
In all my published figures of brains of urodeles the intent has been
to use the same abbreviations for comparable structures. This intention has been approximately realized, but there are some inconsistencies, in most of which the differences express a change in emphasis rather than a correction of errors of observation.
 
 
Many well-defined tracts of fibers seen in fishes and higher animals
are here represented in mixed collections of fibers of diverse sorts,
here termed "fasciculi," or they may be dispersed within a mixed
neuropil. The practice here is to define as a "tract" all fibers of like
origin or termination, whether or not they are segregated in separate
bundles. The customary self-explanatory binomial terminology is
used wherever practicable — a compound word with origin and termination separated by a hyphen. But, since a single fiber of a tract
may have collateral connections along its entire length, the fully
descriptive name may become unduly cumbersome ('41a, p. 491).
Thus, in accordance with strict application of the binomial terminology, tractus strio-tegmentalis would become tractus striothalamicus et peduncularis et tegmentalis dorsalis, isthmi et trigemini. The chemists seem to be able to manipulate similar enormities
even without benefit of hyphens or spaces, but not many anatomists
are so hardy. Few of the named tracts are sharply delimited, and all
of them are mixtures of fibers with different connections. Any
analysis is necessarily somewhat arbitrary.
 
 
Simple action systems of total-pattern type, wherever found (cyclostomes, primitive ganoid fishes, urodeles), are correlated with
a histological texture of the brain which is characteristic and probably primitive (chap. iii).
 
 
The external configuration of the urodele brain also is generalized,
much as in a human embryo of about 6 weeks. In the next chapter
special attention is directed to this comparison to assist the reader in
identifying familiar parts of the human brain as they are seen in the
simplified amphibian arrangement.
 
 
In our comparison of the amphibian brain with the human, two
features are given especial emphasis, both of which are correlated
with differences in the mode of life of the animals in question, that is,
with the contrast between the amphibian simplicity of behavior with
stereotyped total patterns of action predominating and the human
complexity of movement in unpredictable patterns. The correlated
structural differences are, first, in Amblystoma the more generalized
histological texture to which reference has just been made, and notably the apparent paucity of provision for well-defined localization
of function in the brain; and, second, the preponderant influence of
motor patterns rather than sensory patterns in shaping the course of
differentiation from fishlike to quadrupedal methods of locomotion
and somatic behavior in general.
 
 
SOURCES AND MATERIAL
 
The material studied comprises gross dissections and serial sections
of about five hundred specimens of Amblystoma from early embryonic to adult stages. About half these brains were prepared by the
Golgi method and the remainder by various other histological procedures. Most of these are A. tigrinum, some are A. maculatum
(punctatum), and a few are A. jeffersonianum. In early developmental stages some specific differences have been noted in the embryological papers of 1937-41, but no systematic comparative study
has been made. The late larval and adult brains under consideration
in this book are of A. tigrinum. In former papers there are comments
on this material and the methods of preparation ('25, p. 436; '35a,
p. 240; '42, p. 193).
 
 
In the study of these sections the cytological methods of Nissl and
others are less revealing than in more highly differentiated brains
because of the unspecialized structure of the nervous elements. Some
modifications of the method of Weigert which decolorize the tissue
sufficiently to show the myelinated fibers and also the arrangement of cell bodies prove to be most useful for general orientation. Other
details can then be filled in by study of reduced silver preparations
and especially of Golgi sections. A favorable series of transverse
Weigert sections (no. IIC; see p. 3'21) has been chosen as a type or
standard of reference, and the median section as reconstructed from
this specimen (fig. 2C) has been used as the basis for many diagrams
of internal structure. For reference to published figures of this brain
and other details concerning it see page 321. Figures 2A and B are
similar diagrams of the median section of the specimen from which
figures 25-36 were drawn. The topography shown in these median
sections is the basis for the descriptive terms used throughout this text.
 
 
Except for scattered references to details, the only systematic
descriptions of the brain of Amblystoma are in my papers, Bindewald's ('14) on the forebrain, and Larsell's ('20, '32) on the cerebellum. Mention should also be made of Roofe's account ('35) of the
endocranial blood vessels and Dempster's paper ('30) on the endolymphatic organ.
 
 
Kingsbury's admirable paper on Necturus in 1895 may be taken as
a point of departure for all further investigation of the brains of
urodeles, including my monograph of 1933 and several preceding and
following papers. Some of the more important descriptions of the
brains of other urodeles are cited in the appended bibliography,
notably the following: Salamandra (Kuhlenbeck, '21; Kreht, '30),
Proteus (Kreht, '31; Benedetti, '33), Cryptobranchus (Benzon, '26),
Gymnophiona (Kuhlenbeck, '22), Siren (Rothig, '11, '24, '27), and
several other urodeles in Rothig's later papers, Hynobius, Spelerpes,
Diemyctylus (Triturus), Cryptobranchus, Necturus.
 
 
For the Anura the excellent description of the frog by E. Gaupp in
1899 laid a secure foundation for all subsequent work, and the time is
now ripe for a systematic restudy of this brain with the better methods now available and the correlation of the histological structure
with physiological experiments specifically designed to reveal the
action of this structure. Aronson and Noble ('45) have published an
excellent contribution in this field.
 
 
DEVELOPMENT OF THE BRAIN
 
No comprehensive description of the development of the brain of
Amblystoma has been published. The difficulties met in staging
specimens by criteria defined by Harrison, Coghill, and others I
have discussed elsewhere ('48, chap. x). Griggs ('10) described with excellent illustrations the early stages of the neural plate and neural
tube. Baker ('27) illustrated dorsal and ventral views of the open
neural plate, and Baker and Graves ('32) described six models of
the brain of A. jeffersonianum from 3 to 17 mm. in length. Burr
('22) described briefly the early development of the cerebral hemispheres.
 
 
Successive stages of the brain of A. punctatum have been illustrated by Coghill and others (some of which I have cited, '37, p. 391,
and '38, p. 208), and at the Wistar Institute there are other models
of the brains of physiologically tested specimens. Coghill's papers
include a wealth of observation on the development of the mechanisms of the action system, and these were summarized in his London
lectures, published in 1929. His reports were supplemented by a series
of papers which I published from 1937 to 1941, but these fragmentary
observations (of the younger stages particularly) were based on inadequate material and are useful only as preliminary orientation for
a more systematic investigation. In Coghill's papers there are accurate projections of all mitotic figures and neuroblasts of the central
nervous system in nonmotile, early flexure, coil, and early swimming
stages and the arrangement of developing nerve fibers of the brain in
the last-mentioned stage (Coghill, '30, Paper IX, fig. 4). On the basis
of these data he divided the embryonic brain in front of the isthmus
into sixteen regions, each of which is a center of active and characteristic differentiation. These regions are readily identified in our
reduced silver preparations of these and later stages. Using a modification of this analysis, I have distinguished and numbered twentytwo such regions in the cerebrum and cerebellum ('37, p. 392), and
the development of each of these can be followed through to the
adult stage. In my papers of 1937-39 some salient features of these
changes are recorded; but this account is incomplete, and more
thorough study is urgently needed. In the present work some details
only of this development are given in various contexts as listed in the
Index under "Embryology."
 
The most detailed description of the development of the urodele
brain is the paper by Sumi ('26) on Hynobius. Soderberg ('22) gave a
brief description of the development of the forebrain of Triturus
(Triton) and a more detailed account of that of the frog, and Rudebeck ('45) has added important observations.
 
 
The successive changes in the superficial form of the brain can be
interpreted only in the light of the internal processes of growth and differentiation. The need for a comprehensive study of the development of the histological structure of the brain of Amblystoma, including the differentiation of the nervous elements and their fibrous
connections, is especially urgent in view of the very large number of
experimental studies on developmental mechanics which have been
in progress for many years and will probably continue for years to
come. Amblystoma has proved to be an especially favorable subject
for these studies, and in many of them a satisfactory interpretation
of the findings cannot be achieved without more complete knowledge
than we now possess of the development of both the nervous tissues
and other bodily organs.
 
 
THE EVOLUTION OF BRAINS
 
The nervous systems of all vertebrates have a common structural
plan, which is seen most clearly in early embryonic stages and in the
adults of some primitive species. But when the vertebrate phylum is
viewed as a whole, the nervous apparatus shows a wider range of
adaptive structural modifications of this common plan than is exhibited by any other system of organs of the body. In order to understand the significance of this remarkable plasticity and the processes
by which these diverse patterns of nervous organization have been
elaborated during the evolutionary history of the vertebrates, it is
necessary to find out what were the outstanding features of the
nervous system of the primitive ancestral form from which all higher
species have been derived.
 
 
Since the immediate ancestors of the vertebrate phylum have been
extinct for millions of years and have left no fossil remains, our only
recourse in this search is to examine the most generalized living
species, compare them one with another and with embryonic stages,
and so discover their common characteristics. This has been done,
and we are now able to determine with a high degree of probability
the primitive pattern of the vertebrate nervous system.
 
 
The most generalized living vertebrates (lampreys and hagfish)
have brains which most closely resemble that of the hypothetical
primordial vertebrate ancestor. The brains of the various groups of
fishes show an amazing variety of deviations from the generalized
pattern. The paleontological record shows that the first amphibians
were derived from one of the less specialized groups of fishes; and
there is evidence that the existing salamanders and their allies have
preserved until now a type of brain structure which closely resembles that of the most primitive amphibians and of the generahzed fishes
ancestral to them.
 
 
The internal texture of the brains of the generalized amphibians
which are described in this work closely resembles that of the most
primitive extant fishes; but the brain as a whole is organized on a
higher plane, so that it can more readily be compared with those of
reptiles, lower mammals, and man. For this reason the salamanders
occupy a strategic position in the phylogenetic series. This examination has brought to light incipient stages of many complicated human structures and some guiding principles of both morphogenesis
and physiological action that are instructive.
 
 
When the first amphibians emerged from the water, they had all
the land to themselves ; there were no living enemies there except one
another. During aeons of this internecine warfare they carried protective armor; but in later times, during the Age of Reptiles, these
more efficient fighters exterminated the clumsy armored amphibians.
The more active frogs and toads survived, and so also did the sluggish salamanders and their allies, but only by retiring to concealment
in sheltered places.
 
 
In Devonian times, probably about three hundred million years
ago, various species of fishes made excursions to the land and acquired structures adapted for temporary sojourn out of water. Some
of the primitive crossopterygian fishes went further and, after a fishlike larval period, experienced a metamorphosis into air-breathing
tetrapods. They became amphibians. These were fresh-water species,
and the immediate cause of this evolutionary change was extensive
continental desiccation during the Devonian period. While their
streams and pools were drying up, those fishes which had accessory
organs of respiration in addition to the gills of typical fishes, were
able to survive and, through further transformations, become airbreathing land animals. An excellent summary of the paleontological
evidence upon which the history of the evolution of fishes has been
reconstructed has been published by Romer ('46).
 
 
Two prominent features of this revolutionary change involved the
organs of respiration and locomotion, with corresponding changes in
the nervous apparatus of control. These systems of organs are typical
representatives of the two major subdivisions of all vertebrate bodies
and their functions — the visceral and the somatic. The visceral functions and the visceral nervous system will receive scant consideration in this work, for the material at our disposal is not favorable for the
study of these tissues. Here we are concerned primarily with the
nervous apparatus of overt behavior, that is, of the somatic adjustments.
 
 
The most important change in these somatic adjustments during
the critical evolutionary period under consideration is the transition
from swimming to walking. The fossil record of the transformation of
fins into legs is incomplete, but it is adequate to show the salient
features of the transformation of crossopterygian fins into amphibian legs (Romer, '46). In the individual development of every
salamander and every frog the internal changes in the organization
of the nervous system during the transition from swimming to walking can be clearly seen. And these changes are very significant in our
present inquiry because they illustrate some general principles of
morphogenesis of the brain more clearly than do any other available
data.
 
 
In fishes, swimming is a mass movement requiring the co-ordinated
action of most of their muscles in unison, notably the musculature of
the trunk and tail. The paired fins are rudders, not organs of propulsion. The young salamander larva has no paired limbs but swims
vigorously. This is a typical total pattern of action as defined by
Coghill. The adult salamander after metamorphosis may swim in the
water like the larva; and he can also walk on land with radically different equipment. Some fishes can crawl out on land, but the modified fins are clumsy and ineflBcient makeshifts compared with the
amphibian's mobile legs.
 
 
Quadrupedal locomotion is a very complicated activity compared
with the simple mass movement of swimming. The action of the four
appendages and of every segment of each of them must be harmoniously co-ordinated, with accurate timing of the contraction of many
small muscles. These local activities are "partial patterns" of behavior, in Coghill's sense. From the physiological standpoint there is
great advance, in that the primitive total pattern is supplemented,
and in higher animals largely replaced, by a complicated system of
co-ordinated partial patterns. This is emphasized here because it provides the key to an understanding of many of the differences between
the nervous systems of fishes, salamanders, and mammals. Motility,
and particularly locomotion, have played a major role in vertebrate
evolution, as dramatically told by Gregory ('43). This outline has
been filled in by Howell's ('45) interesting comparative survey of the mechanisms of locomotion, and I have elsewhere discussed ('48) Coghill's contributions to this theme.
 
 
In the history of vertebrate evolution there were four critical periods: (1) the emergence of the vertebrate pattern of the nervous
system from invertebrate ancestry; (2) the transition from aquatic
to terrestrial life; (3) the differentiation within the cerebral hemispheres of primitive cerebral cortex; (4) the culmination of cortical
development in mankind, with elaboration of the apparatus requisite
for language and other symbolic (semantic) instrumentation of the
mental life.
 
 
1 . The extinct ancestors of the vertebrates in early Silurian times
were probably soft and squashy creatures, not preserved as fossils.
Some of their aberrant descendants may be recognized among the
Enteropneusta, Tunicata, and Amphioxi ; but the first craniate vertebrates preserved as fossils were highly specialized, heavily armored
ostracoderms, now all extinct.
 
 
2. The salient features of the second critical period have been mentioned, and here the surviving amphibians recapitulate in ontogeny
many instructive features of the ancestral history.
 
 
3. Amphibians have no cerebral cortex, that is, superficial laminated gray matter, in the cerebral hemispheres. This first takes
definitive form in the reptiles, though prodromal stages of this differentiation can be seen in fishes and amphibians, a theme to which
we shall return in chapter vii.
 
 
4. The fourth critical period, like the first, does not lie within the
scope of this work, though study of the second and third periods
brings to light some principles of morphogenesis which may help us
to understand the more recondite problems involved in human cortical functions.
 
 
It is probable that none of the existing Amphibia are primitive in
the sense of survival of the original transitional forms and that the
urodeles are not only aberrant but in some cases retrograde (Noble,
'31; Evans, '44); yet the organization of their nervous systems is
generalized along very primitive lines, and these brains seem to me
to be more instructive as types ancestral to mammals than any others
that might be chosen. They lack the highly divergent specializations
seen in most of the fishes; and, in both external form and internal
architecture, comparison with the mammalian pattern can be made
with more ease and security. So far as structural differentiation has advanced, it is in directions that point clearly toward the mammalian
arrangement.
 
 
Amphibian eggs and larvae are readily accessible to observation
and experiment; they are easily reared; they tolerate experimental
operations unusually well; and, in addition, the amphibian neuromuscular system begins to respond to stimulation at a very early age,
so that successive stages in maturation of the mechanism are documented by changes in visible overt movement. The adult structure is
instructive; and, when the embryological development of this structure is compared with that of higher brains and with the sequence of
maturation of patterns of behavior, basic principles of nervous organization are revealed that can be secured in no other way. In the
absence of differentiated cerebral cortex, the intrinsic structure of the
stem is revealed. Experimental decortication of mammals yields
valuable information, but study of such mutilations cannot tell us all
that we need to know about the normal operations of the brain stem
and the reciprocal relationships between the stem and the cortex.
 
 
In brief, the brains of urodele amphibians have advanced to a
grade of organization typical for all gnathostome vertebrates, Amblystoma being intermediate between the lowest and the highest species
of Amphibia. This brain may be used as a pattern or template, that
is, as a standard of reference in the study of all other vertebrate
brains, both lower and higher in the scale.
 
 
 
 
CHAPTER II THE FORM AND SUBDIVISIONS OF THE BRAIN
 
GROSS STRUCTURE
 
REFERENCE to figures 1-5, 85, and 86 shows that the larger
; subdivisions of the human brain are readily identified in Amblystoma, though with remarkable differences in shape and relative
size. When this comparison is carried to further detail, the sculpturing of the ventricular walls shown in the median section is especially
instructive. It is again emphasized that the application of mammalian names to the structures here revealed rarely implies exact
homology; these areas are to be regarded as primordia from which
the designated mammalian structures have been differentiated. The
relationships here implied have been established by several independent lines of evidence: (1) The relative positions and fibrous connections of cellular masses and the terminal connections of tracts.
In so far as these arrangements conform with the mammalian pattern, they may be regarded as homologous. (2) Embryological evidence. The early neural tubes of amphibians and mammals are similar, and subsequent development of both has been recorded. On the
basis of Coghill's observations of rates of proliferation and differentiation in prefunctional stages, the writer ('37) gave arbitrary numbers to recognizable sectors of the neural tube in early functional
stages, and the subsequent development of each of these is, in broad
lines, similar to that of corresponding mammalian parts. (3) The
relationships of svipposed primordia of mammalian structures may be
tested by the comparative method. In an arrangement of animal
types which approximates the phylogenetic sequence from the most
generalized amphibians to man, there are many instances of progressive differentiation of amphibian primordia by successive increments up to the definitive human form.
 
 
Many pictures of the brains of adult and larval Amblystoma and
other urodeles have been published, some of which I have cited
('35a, p. 239). The most accurate pictures of the brain of adult A.
tigrinum are those of Roofe ('35), showing dorsal, ventral, and lateral aspects and the distribution of endocranial arteries and veins. The
outHnes of the brain were drawn from specimens dissected after
preservation for 6 weeks in 10 per cent formahn. One of these is
shown here (fig. 86 A). Figure IB is drawn from a dissection made by
the late Dr. P. S. McKibben, showing the sculpturing of the ventricular surfaces. Figures lA and 85 are drawn from a wax model in
which there is some distortion of the natural proportions. Not all the
differences seen in these pictures and in the proportions of sections
figured are artifact, for the natural variability of urodele brains is
surprisingly large (Neimanis, '31). Brains of larval stages have been
illustrated by many authors and in my embryological papers of
1937-39.
 
 
 
The somewhat simpler brain of the mudpuppy, Necturus, has
been described in a series of papers as completely as available material permits, and comparison with the more differentiated structure
of Ambly stoma is instructive. The sketches shown in figures 86B and
C illustrate the differences between the form of this forebrain and
that of Amblystoma. The monograph of 1933 contains a series of
diagrams ('33&, figs. 6-16) of the internal connections of the brain of
Necturus similar to those of Amblystoma shown here (figs. 7-24).
In 1910 I described the general features of the forebrain of A.
tigrinum, with a series of drawings of transverse Weigert sections,
no. lie, which has subsequently been used as the type specimen.
Though this paper contains some errors and some morphological
interpretations which I now regard as outmoded ('33a), most of the
factual description has stood the test of time, and additional details
and reports on other parts of the brain have been published in a
series of papers.
 
 
The most conspicuous external fissures of the brain of Amblystoma
are: (1) the longitudinal fissure separating the cerebral hemispheres;
(2) the deep stem-hemisphere fissure; (3) a wide dorsal groove separating the epithalamus from the roof (tectum) of the midbrain; (4)
the ventral cerebral flexure or plica encephali ventralis, which is a
sharp bend of the floor of the midbrain, where it turns downward and
backward into the "free part" of the hypothalamus; and (5) the
fissura isthmi, extending downward and forward from the anterior
medullary velum between midbrain and isthmus. The middle part of
the fissura isthmi is at the anterior border of the auricle, which is more
prominent in the larva than in the adult ('14a, figs. 1-3). Here in the
adult it lies near the posterior border of the internal isthmic tissue, some distance posteriorly of the ventricular sulcus isthmi; but, like
the latter, it really marks the anterior border of the isthmus, as will
appear in the description of the development of the isthmic sulcus
(p. 179).
 
 
The obvious superficial eminences on the dorsal aspect of the brain
are the small cerebellum, the dorsal convexity of the roof of the
midbrain (tectum mesencephali) , the habenular nuclei of the epithalamus, and the two cerebral hemispheres. Posteriorly of the
habenulae in the early larvae is the membranous pineal evagination,
which in the adult is a closed epithelial vesicle detached from the
brain except for the few fibers of the parietal nerve. The lateral aspect
of the thalamus, midbrain, and isthmus is a nearly smooth convexity,
posteriorly of which is the high auricle, composed of tissue which is
transitional between the body of the cerebellum and the acousticolateral area of the medulla oblongata. This auricle contains the primordia of the vestibular part of the cerebellar cortex (flocculonodular
lobe of Larsell), and most of its tissue is incorporated within the
cerebellum in mammals. On the ventral aspect there is a low eminence in front of the optic chiasma, which marks the position of the
very large preoptic nucleus, and a similar eminence behind the
chiasma formed by the ventral part of the hypothalamus. The latter
is in the position of the human tuber cinereum but is not exactly
comparable with it. Most of the hypothalamus is thrust backward
under the ventral cerebral flexure as the pars libera hypothalami.
The large pars glandularis of the hypophysis envelops the posterior
end of the infundibulum and extends spinal ward from it, not anteriorly as in man.
 
 
The primary subdivisions of the human brain as defined from the
embryological studies of Wilhelm His are readily identified in adult
Amblystoma, as shown in the median section (fig. 2A).
 
 
At the anterior end of each cerebral hemisphere is the very large
olfactory bulb, the internal structure of which shows some interesting
primitive features (p. 54; '246). The bulbar formation extends backward on the lateral side for about half the length of the hemisphere,
but on the medial side only as far as the anterior end of the lateral
ventricle (figs. 3, 4). Bordering the bulb is an undifferentiated anterior olfactory nucleus, and posteriorly of this the walls of the lateral
ventricle show early stages of the differentiation of the major subdivisions of the mammalian hemisphere — in the ventrolateral wall a
strio-amygdaloid complex, ventromedially the septum, and dorsally the pars pallialis. In the pallial part no laminated cortical gray is
differentiated, but there are well-defined pallial fields: dorsomedially,
the primordial hippocampus; dorsolaterally, the primordial piriform
lobe; and between these a primordium pallii dorsalis of uncertain
relationships.
 
 
The boundaries of the diencephalon, as here defined and shown in
figure 2A, are: anteriorly, the stem-hemisphere fissure and the posterior border of the anterior commissure ridge and, posteriorly, the
anterior face of the posterior commissure and the underlying commissural eminence and, more ventrally, the sulcus, s, which marks
the anterior border of the cerebral peduncle. The inclusion of the
preoptic nucleus is in controversy; but, whether or not this inclusion
is justifiable morphologically, its relationships with the hypothalamus are so intimate that it is practically convenient to consider these
parts together. The four primary subdivisions of the diencephalon as
I defined them in 1910 are: (1) the dorsal epithalamus, containing on
each side the habenula and pars intercalaris, the latter including the
pretectal nucleus; (2) pars dorsalis thalami, which is the primordium
of the sensory nuclei of the mammalian thalamus; (3) pars ventralis
thalami, the motor zone of the thalamus, or subthalamus; (4) hypothalamus. The mammalian homologies of these areas are clear, though their relative sizes and fibrous connections exhibit remarkable
differences.
 
 
The posterior boundary of the mesencephalon is marked by the
external fissura isthmi, the ventricular sulcus isthmi (fig. 2B, s.is.),
and ventrally in the floor plate a pit, the fovea isthmi [f.i.). These
are all more prominent in the larva than in the adult. This sector
includes the posterior commissure, the tectum mesencephali (primordial corpora quadrigemina) , the underlying dorsal tegmentum
(subtectal area), and the area surrounding the tuberculum posterius
at the ventral cerebral flexure, termed the "nucleus of the tuberculum
posterius." On embryological grounds and for convenience of description, this ventral area, which is bounded by the variable ventricular
sulcus s, is here called the "peduncle" in a restricted sense ('36, p.
298; '396, p. 582). This is a primordial mesencephalic structure which
is not the equivalent of the peduncle of human neurology. Amblystoma has nothing comparable with the human basis pedunculi, and
its "peduncle" is incorporated within the tegmentum of the human
brain. The III cranial nerve arises within the "peduncle" and
emerges near the fovea isthmi. The nucleus of the IV nerve is in the isthmus. In the human brain there are no definite structures comparable to the amphibian dorsal and isthmic tegmentum.
 
 
The isthmus is much more clearly defined than in adult higher
brains, it is relatively larger, and its physiological importance is correspondingly greater, as will appear later. It is bounded anteriorly
by the sharp isthmic sulcus and posteriorly by the cerebellum,
auricle, and trigeminal tegmentum. The so-called "pons" sector of
the human brain stem is named from its most conspicuous component, but this name is meaningless in comparative anatomy. In man
it is the pons and the sector of the stem embraced by it; but in no
two species of mammals is the part embraced by the pons equivalent;
and below the mammals the pons disappears entirely. The medulla
oblongata, on the other hand, is a stable structure, extending from
the isthmus to the spinal cord, and for it the shorter name "bulb" is
sometimes used, especially in compounds.
 
 
I outlined the development and morphological significance of the
urodele cerebellum ('14, '24), and this was followed by detailed descriptions of the development and adult structure of this region of
Amblystoma by Larsell ('20, '32), whose observations I have subsequently confirmed, including his fundamental distinction between
its general and its vestibular components.
 
 
Some features of the larval medulla oblongata and related nerves
have been described ('14a, '396) and, more recently ('446), additional details of the adult, particularly the structures at the bulbospinal junction. Much remains to be done to clarify the organization
of the medulla oblongata and spinal cord.
 
 
The cranial nerves and their analysis into functional components
(chap, v) were described by Coghill ('02). The embryological development of these components also has been extensively studied (chap.
x). The arrangement and composition of these nerves are fundamentally similar to those of man, with a few notable exceptions. The
internal ear lacks the cochlea, which is represented by a very primitive rudiment; a cochlear nerve, accordingly, is not separately differentiated. There is an elaborate system of cutaneous organs of the
lateral lines, whose functions are not as yet adequately known. These
are supplied by very large nerves commonly assigned to the VII and
X pairs, though it would be more appropriate to regard them as
accessory VIII nerves, for all these nerve roots enter a wide zone at
the dorsolateral margin of the medulla oblongata known as the "area
acusticolateralis." There is no separate XI cranial nerve, this being represented by an accessorius branch of the vagus. The XII nerve is
represented by branches of the first and second spinal nerves. The
first spinal nerve in some specimens has a small ganglion ; the second
nerve always has a large dorsal root and ganglion. In this connection
a passage in the comprehensive work on the anatomy of Salamandra
by Francis ('34, p. 134) is worthy of mention: "After making due
allowance for the absence of a lateralis component in the adult
salamander, the correspondence between the cranial nerves of this
animal and those of Ambly stoma is very close indeed."
 
The configuration and mutual relations of the gross structures just
surveyed can be seen only in sections, of which many, cut in various
planes, have been illustrated in the literature. Only a few selected
examples are included in the present work, with references in subsequent chapters to many others. For general orientation the following
figures may be consulted : a series of selected transverse sections from
the spinal cord to the olfactory bulb (figs. 87-100); a series of horizontal sections through the middle part of the brain stem (figs.
25-36); a few sagittal sections (figs. 101-4). Figures 6-24 show the
chief fibrous connections of each well-defined region of the brain
stem.
 
 
The diencephalon, mesencephalon, and isthmus have the form of
three irregular pyramids oppositely oriented (fig. 2A). The broad
base of the diencephalon extends from the anterior commissure to the
hypophysis, and the apex is at the epiphysis. The tectum forms the
base of the mesencephalic pyramid, and the apex is at the ventral tip
of the tuberculum posterius, which borders the ventral cerebral
flexure. The base of the pyramidal isthmus is formed by the massive
tegmentum isthmi of each side and the median interpeduncular
nucleus in the floor plate. It narrows dorsally into the anterior medullary velum between the tectum and the cerebellum.
 
 
The middle sectors of the brain stem — diencephalon, mesencephalon, and isthmus — contain the primordial regulatory and integrating apparatus controlling the fundamental sensori-motor systems of adjustment. The most important peripheral connections are
with the eyes, and these in most vertebrates play the dominant role
in maintaining successful adjustment with environment. From this
topographical feature it naturally followed that, during the course of
phylogenetic differentiation of the brain, the chief centers of adjustment of the other exteroceptive systems were elaborated in close
juxtaposition with the visual field in the midbrain and thalamus.
 
 
Here they are interpolated between the primary sensory and motor
apparatus of the medulla oblongata and spinal cord below and the
great olfactory field and suprasegmental apparatus of the cerebral
hemispheres above.
 
 
In all lower vertebrates the roof of the midbrain, the tectum, is the
supreme center of regulation of motor responses to the exteroceptive
systems of sense organs. The hypothalamus is similarly elaborated for
regulation of olfacto- visceral adjustments. The patterning of motor
responses for both these groups of receptors is effected in the cerebral
peduncle and tegmentum. In the region of the isthmus, between the
tectum and the primary vestibular area of the medulla oblongata and
above the tegmentum, the cerebellum was elaborated as the supreme
adjustor of all proprioceptive systems.
 
 
At the rostral end of the brain, within and above the specific
olfactory area of the cerebral hemisphere, there gradually emerged a
synthetic apparatus of control, adapted to integrate the activities of
all the other parts of the nervous system and to enlarge capacity to
modify performance as a result of individual experience. In the lowest vertebrates this "suprasensory" and "supra-associational" apparatus, as Coghill termed it, is not concentrated in the cerebral
hemispheres, but it is dispersed, chiefly in the form of diflfuse neuropil.
In the amphibian cerebral hemispheres this integrating apparatus is
more highly elaborated than elsewhere, with some local differentiation of structure. The hemispheres are larger than in fishes, and the
primordia of their chief mammalian subdivisions can be recognized.
A dorsal pallial part is distinguishable from a basal or stem part of
the hemisphere, though the distinctive characteristics of the pallium
are only incipient. There is no cerebral cortex, and, accordingly, the
mammalian cortical dependencies in the thalamus, midbrain, and
cerebellum have not yet appeared. The primordial thalamus is concerned chiefly with adjustments within the brain stem, though
precursors of the thalamic radiations to the hemispheres are present.
 
 
VENTRICLES
 
The lateral ventricles of the cerebral hemispheres have the typical
form except at the interventricular foramen, where the amphibian
arrangement is peculiar. The anterior and hippocampal commissures
do not cross as usual in or above the lamina terminalis, but in a more
posterior high commissural ridge ; and between these structures there
is a wide precommissural recess, into which the interventricular foramina open. This results in some radical differences from reptilian
and mammalian arrangements of the related fiber tracts and membranous parts, as elsewhere described (p. 291; '35). The third ventricle is expanded dorsally into the complicated membranous paraphysis and dorsal sac. Ventrally, the great elongation of the preoptic
nucleus gives rise to a large preoptic recess between the anterior commissure ridge and the chiasma ridge, and in front of the latter there is
a lateral optic recess (fig. 96), which in early larval stages extends
outward as far as the eyeball, as a patent lumen of the optic nerve
('41), an arrangement which persists in the intracranial part of the
nerve of adult Necturus ('41a). In the hypothalamus the ventricle is
dilated laterally ('35a, p. 253; '36, figs. 10-14), and posteriorly it is a
wide infundibulum with membranous roof and thin but nervous floor
and posterior wall. The latter is the pars nervosa of the hypophysis
and is partly enveloped by the pars glandularis (figs. 2, 101; '35a, p.
254; '42, p. 212 and figs. 56-65; Roofe, '37). The aqueduct of the
midbrain is greatly expanded dorsoventrally. Its ventral part is contracted laterally by the thick peduncles and tegmentum, and the
dorsal part is dilated as an optocoele. The sulcus lateralis mesencephali marks its widest extent, and tectal structure reaches far below this
sulcus. The fourth ventricle is of typical form except anteriorly,
where the wide lateral recess with membranous roof extends outward
and forward to cover the whole dorsolateral aspect of the auricular
lobe (figs. 90, 91 ; '24, p. 627). The rhombencephalic chorioid plexus is
elaborately developed in interesting relation with the peculiar endolymphatic organs of this animal ('35, p. 310). The ventricular systems of adult Triturus (Diemyctylus) and of larval and adult stages
of Hynobius have been described and illustrated with wax models
bySumi('26, '26a).
 
 
The ventricular surface of both larvae and adults is clothed with
very long cilia. These are not preserved in ordinary preparations and
in our material are seen only in Golgi sections, where their impregnation is erratic and local ('42, p. 196). They are most frequently seen
in the infundibulum and optocoele under the tectum. In the vicinity
of the posterior commissure the ciliated ependyma is thickened (subcommissural organ of Dendy), and to it the fiber of Reissner is attached ('42, p. 197). This thick, nonnervous fiber extends backward
through the ventricle to the lower end of the spinal cord and, like the
cilia, is apparently an outgrowth from the internal ependymal membrane.
 
 
MENINGES, CHORIOID PLEXUSES, AND BLOOD VESSELS
 
The meninges of Amblystoma were described in 1935. This account
shoukl be compared with that of Salamandra pubhshed in 1934 by
Francis, whose description was based on the investigation of Miss
Helen O'NeiU ('98), done under the direction of Wiedersheim and
Gaupp. In Amblystoma the meninges are intermediate between the
meninx primitiva of the lower fishes and those of the frog. Over the
spinal cord and most parts of the brain a firm and well-defined
pachymeninx, or dura, closely invests the underlying undifferentiated pia-arachnoid. The meninges of the frog have been described by
others, and recently Palay ('44) has investigated their histological
structure in the toad. The most interesting feature of these amphibian membranes is their intimate relation with the enormous
endolymphatic organ described by Dempster ('30) and the associated  blood vessels.
 
 
The vascular supply of these brains is peculiar in several respects.
The distribution of arteries and veins has been described by Roofe
('35, '38), and I have added some details from the adult ('35) and the
larva {'Md). The endocranial veins form a double portal system of
sinusoids of vast extent and unknown significance. Between the
cerebral hemispheres and the epithalamus the nodus vasculosus
(Gaupp) is permeated by a complicated rete of sinusoids, which receives venous blood from the entire prosencephalon— chorioid
plexuses, brain wall, and meninges. The efferent discharge from this
rete is by the two oblique sinuses, which pass backward across the
midbrain to enter a similar rete of wide, anastomosing smusoids
spread over the chorioid plexus of the fourth ventricle and the
lobules of the endolymphatic organs. This rete also receives the vems
from all posterior parts of the brain, meninges, and chorioid plexus.
The common discharge for all this endocranial venous blood is by a
large sinus, which emerges from the cranium through the jugular
foramen and joins the jugular vein. These membranous structures
are readily observable in the living animal without serious disturbance of normal conditions, and they provide unique opportunities or
experimental study of some fundamental problems of vascular
 
^\\' wSomted out by Craigie ('38, '38a, '39, '45) that within the
substance of this brain the penetrating blood vessels are arranged in
two ways-a capillary net of usual type and simple loops, which enter from the meningeal arterial network. Our preparations confirm
this observation and also the fact that the vascular pattern varies in
different parts of the brain. Both isolated loops and the capillary net
may be seen in the same field, as in the dorsal thalamus (fig. 44), or
one of these patterns may prevail, with few, if any, instances of the
other. In the tectum and dorsal tegmentum of the midbrain, for instance, the tissue is vascularized by simple loops with only occasional
anastomosis (fig. 48), while in the underlying peduncle and isthmic
tegmentum the vascular network prevails, with occasional simple
loops. In the meninges and chorioid plexuses only the network has
been observed.
 
 
The telencephalic and diencephalic chorioid plexuses have an
abundant arterial blood supply through the medial hemispheral
artery; but the elaborately ramified tubules of the paraphysis seem
to have no arterial supply or capillary net, the accompanying vessels
being exclusively venous sinusoids ('35, p. 342). The same seems to
be true of the endolymphatic sacs ('34c?, p. 543). The chorioid plexus
of the fourth ventricle has abundant arterial blood supply. In all
plexuses the capillaries unite into venules, which discharge into wide
sinusoids, which ramify throughout the plexus and have very thin
walls. All arterioles of the chorioid plexuses are richly innervated,
but it has not been possible to get satisfactory evidence of the sources
of these nerve fibers ('36, p. 343; '42, p. 255; Necturus, '336, p. 15).
 
 
The enormous development of the chorioid plexuses and associated
endolymphatic organ of urodeles is apparently correlated with the
sluggish mode of life and relatively poor provision for aeration of the
blood. In the more active anurans the plexuses are smaller; but in the
sluggish mudfishes, including the lungfishes, with habits similar to
those of urodeles, we again find exaggerated development of these
plexuses. Existing species in the border zone between aquatic and
aerial respiration are all slow-moving and relatively inactive. The
enlarged plexuses and sinusoids give vastly increased surfaces for
passage of blood gases into the cerebrospinal fluid; and, correlated
with this, the brain wall is thin everywhere, to facilitate transfer of
metabolites between brain tissue and cerebrospinal fluid. Massive
thickenings of the brain wall occur in many fishes and in amniote
vertebrates, but not in mudfishes and urodeles.
 
 
 
 
CHAPTER III HISTOLOGICAL STRUCTURE
 
GENERAL HISTOLOGY
 
IN AMPHIBIAN brains the histological texture is generalized,
exhibiting some embryonic features; and it is at so primitive a
level of organization as to make comparison with mammals difficult.
Most of the nerve cells are small, with scanty and relatively undifferentiated cytoplasm. There are some notable exceptions, such as
the two giant Mauthner's cells of the medulla oblongata and related
elements of the nucleus motorius tegmenti. With the exceptions just
noted, Nissl bodies are absent or small and dispersed.
 
 
Almost all bodies of the neurons are crowded close to the ventricle
in a dense central gray layer, with thick dendrites directed outward
to arborize in the overlying white substance (figs. 9, 99). The axon
usually arises from the base of the dendritic arborization, rarely from
its tip, and sometimes from the cell body; it may be short and much
branched or very long, with or without collateral branches. The
ramifications of the short axons and of collaterals and terminals of
the longer fibers interweave with dendritic arborizations to form a
more or less dense neuropil, which permeates the entire substance of
the brain and is a synaptic field. Some of the nerve fibers are
myelinated, more in the peripheral nerves, spinal cord, and medulla
oblongata than in higher levels of the brain. Both myelinated and
unmyelinated fibers may be assembled in definite tracts, or they may
be so dispersed in the neuropil as to make analysis difficult. The arrangement of recognizable tracts conforms with that of higher brains,
so that homologies with human tracts are in most cases clear. These
tracts and the gray areas with which they are connected provide the
most useful landmarks in the analysis of this enigmatic tissue.
 
 
In the gray substance there are few sharply defined nuclei like
those of mammals, but the precursors of many of these can be recognized as local specializations of the elements or by the connections of
the related nerve fibers. In most cases the cells of these primordial
nuclei have long dendrites, which arborize widely into surrounding
fields (figs. 9, 24, 61, 66), so that the functional specificity of the nucleus is, at best, incomplete. This arrangement facilitates mass
movements of "total-pattern" type, but local differentiations serving
"partial patterns" of action (Coghill) are incipient. Localized reflex
arcs are recognizable, though in most cases these are pathways of
preferential discharge within a more dispersed system of conductors
(chap. vi).
 
 
Tissue differentiation is more advanced in the white substance
than in the gray. The most important and diversely specialized
synaptic fields are in the alba, and this local specialization is correlated with differences in the physiological properties of the nervous
elements represented. This means, as I see it, that functional factors
must be taken into account in both ontogenetic and phylogenetic
differentiation and that in the long view the problems of morphogenesis are essentially physiological, that is, they resolve into questions of
adaptation of organism to environment (chap. viii). This is the reason
why in this work the histological analysis is made in terms of physiological criteria, even though these criteria are, in the main, based on
indirect evidence, namely, the linkage of structures in functional
systems of conductors.
 
 
The nonnervous components of this tissue comprise the blood vessels, ependyma, and a small number of cells of uncertain relationships
which are regarded as undifferentiated free glial cells or transitional
elements ('34, p. 94; '336, p. 17). The ependymal elements everywhere span the entire thickness of the brain wall with much free
arborization. They assume various forms in different regions, and
their arrangement suggests that they are not merely passive supporting structures, though if they have other specific functions these are
still to be discovered. For illustrations see figures 63, 64, 70, 79, and
81.
 
 
More detailed descriptions of the histological structure of urodele
brains may be found in earlier papers ('14a, p. 381; '17, pp. 232,
279 ff.; '335, pp. 16, 268; '33c; '33cf; '34; '34a,- '346; '42, p. 195; '44a).
In the olfactory bulbs of Necturus ('31) and Amblystoma ('246) we
find an interesting series of transitional cells between apparently
primitive nonpolarized elements and typical neurons, as described
on page 54.
 
 
THE NEUROPIL
 
In the generalized brains here under consideration the neuropil is
so abundant and so widely spread that it evidently plays a major
role in all central adjustments, thus meriting detailed description.
 
 
 
Only the coarser features of this tissue are open to inspection with
presently available histological technique. In my experience its texture is best revealed by Golgi preparations, and very many of them,
for the erratic incidence of these impregnations may select in different specimens now one, now another, of the component tissues —
blood vessels, ependyma, dendrites, or axons. In each area of neuropil
these components are independent variables, and in most of these
areas axons from many sources are so intricately interwoven that the
tissue can be resolved only where fortunate elective impregnations
pick out one or another of the several systems of fibers in different
specimens. It is difficult to picture the neuropil either photographically or with the pen, and the crude drawings in this book and in the
literature give inadequate representations of the intricacy and delicacy of its texture.
 
 
A survey of the neuropil of adult Amblystoma as a whole has led
me to subdivide it for descriptive purposes and somewhat arbitrarily
into four layers ('42, p. 202). From within outward, these are as
follows:
 
1. The periventricular neuropil pervades the central gray so that
every cell body is enmeshed within a fabric of interwoven slender
axons (figs. 106, 107). This persists in some parts of the mammalian
brain as subependymal and periventricular systems of fibers.
 
 
2. The deep neuropil of the alba at the boundary between gray and
white substance knits the periventricular and intermediate neuropil
together, and it also contains many long fibers coursing parallel with
the surface of the gray. The latter are chiefly efferent fibers directed
toward lower motor fields (fig. 93, layer 5; '42, figs. 18-21, 24, 29-45,
47).
 
 
3. The intermediate neuropil in the middle depth of the alba contains the largest and most complicated fields of this tissue. It is very
unevenly developed, in some places scarcely recognizable and in
others of wide extent and thickness. Its characteristics are especially
well seen in the corpus striatum (figs. 98, 99, 108, 109), thalamus
('396, fig. 81; '42, figs. 71, 81), and tectum opticum (figs. 93, layer 2,
101; '42, figs. 26, 30, 32, 79-83). Many of the long tracts lie within
this layer and have been differentiated from it. Most of the specific
nuclei of higher animals, including the outer gray layers of the
tectum, have been formed by migration of neuroblasts from the central gray outward into this layer. Here we find much of the apparatus of local reflexes and their organization into the larger, innate
patterns of behavior.
 
 
4. The superficial neuropil is a subpial sheet of dendritic and axonal
terminals, in some places absent, in others very elaborately organized. Here are some of the most highly specialized mechanisms of
correlation in the amphibian brain, from which specific nuclei of
higher brains have been developed. Notable examples are seen in the
interpeduncular neuropil (chap, xiv) and the ventrolateral neuropil
of the cerebral peduncle described in the next section. This neuropil
seems to be a more sensitive medium for strictly individual adjustments (conditioning) than the deeper neuropil, but of this there is no
experimental evidence. This hypothesis is supported by the fact that
in higher animals cerebral cortex develops within this layer and
apparently by neurobiotactic influence emanating from it.
 
 
In the first synapses observed in embryogenesis numerous axonic
terminals converge to activate a single final common path (Coghill,
'29, p. 13), This is the first step in the elaboration of neuropil. As
differentiation advances, neurons are segregated to serve the several
modalities of sense and the several systems of synergic muscles, and
these systems are interconnected by central correlating elements. In
no case are these connections made by an isolated «hain of neurons in
one-to-one contact between receptor and effector. The central terminals of afferent fibers from different sense organs are widely spread
and intermingled. Dendrites of the correlating cells branch widely in
this common receptive field, and the axons of some of them again
branch widely in a motor field, thus activating neurons of the several
motor systems. This arrangement is perfectly adapted to evoke mass
movement of the entire musculature from any kind of sensory stimulation, and this is, indeed, the only activity observed in early
embryonic stages.
 
 
It is the rare exception rather than the rule for a peripheral sensory
fiber to effect functional connection directly with a peripheral motor
neuron. One or more correlating elements are interpolated; and, as
differentiation advances, the number of these correlating neurons is
enormously increased in both


z.lim.lat., zona limitaus lateralis
z.lim.med., zona limitans medialis


(1) to (10), tegmental fascicles
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Herrick CJ. The Brain of the Tiger Salamander (1948) The University Of Chicago Press, Chicago, Illinois.

Online Editor  
Mark Hill.jpg
This historic 1948 book by Herrick describes the development of the lizard, tiger salamander (Ambystoma tigrinum).


root; cellular organisms; Eukaryota; Opisthokonta; Metazoa; Eumetazoa; Bilateria; Coelomata; Deuterostomia; Chordata; Craniata; Vertebrata; Gnathostomata; Teleostomi; Euteleostomi; Sarcopterygii; Tetrapoda; Amniota; Sauropsida; Sauria; Lepidosauria

Modern Notes: lizard | neural

Part I. General Description and Interpretation 1. Salamander Brains | 2. Form and Brain Subdivisions | 3. Histological Structure | 4. Regional Analysis | 5. Functional Analysis, Central and Peripheral | 6. Physiological Interpretations | VII. The Origin and Significance of Cerebral Cortex | VIII. General Principles of Morphogenesis Part 2. Survey of Internal Structure 9. Spinal Cord and Bulbo-spinal Junction | 10. Cranial Nerves | 11. Medulla Oblongata | 12. Cerebellum | 13. Isthmus | 14. Interpeduncular Nucleus | 15. Midbrain | 16. Optic and Visual-motor Systems | 17. Diencephalon | 18. Habenula and Connections | 19. Cerebral Hemispheres | 20. Systems of Fibers | 21. Commissures | Bibliography | Illustrations | salamander

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The Brain of the Tiger Salamander

Ambystoma tigrinum


By C. Judson Herrick

Professor Emeritus of Neurology The University of Chicago

The University Of Chicago Press Chicago • Illinois


Preface

This work reports the results of a search, extending over fifty years, for the fundamental plan of the vertebrate nervous system as revealed in generalized form in the amphibians. In these small brains we find a simplified arrangement of nerve cells and fibers with a pattern of structural organization, the main features of which are common to all vertebrates. From this primitive and relatively unspecialized web of tissue it is possible to follow the successive steps in progressive elaboration as the series of animals from salamanders to men is passed in review.


This is a record of personal observation, not a compilation of the literature. It is presented in two parts, which differ in content and method of treatment of the data.


The first part gives a general over-all view of the structure without details, followed by physiological interpretation and discussion of some general principles of embryologic and phylogenetic morphogenesis. This part, with the accompanying illustrations, can be read independently of the histological details recorded in the second part.


The second part presents the evidence upon which conclusions are based, drawn from my own previously published descriptions, to which references are given, together with considerable new material. This part is designed for specialists in comparative neurology and as a guide for physiological experiments. The second part supplements the first, to which the reader must make frequent reference.


Grateful acknowledgment is made to many colleagues for generous assistance and criticism, and particularly to Doctors Elizabeth C. Crosby, Davenport Hooker, Olof Larsell, Gerhardt von Bonin, Ernst Scharrer, and W. T. Dempster. In the preparation of the manuscript invaluable help was given by Miss Anna Seaburg.


I am indebted to Dr. Paul G. Roofe for permission to copy one of his pictures, shown here as figure 86A. The American Museum of Natural History, New York, generously furnished the two photographs, taken from life, shown at the bottom of the Frontispiece. These are copyrighted by the Museum. One of them has been previously published by the Macmillan Company in Hegner's Parade of the Animal Kingdom (p. 289). The photograph of the midlarval stage was kindly suppHed by Professor Sherman C. Bishop of the University of Rochester. The upper figure is from G. K. Noble's Biology of the Amphibia, courtesy of the McGraw-Hill Book Company (copyrighted, 1931). Figures 1, 2C, and 86-113 are reproductions of figures previously published by the author in the Journal of Comparative Neurology and used here by courtesy of the Wistar Institute of Anatomy and Biology, publishers of that Journal. The other figures are originals prepared for this work.


Money for the prosecution of the work and for financing its publication was liberally supplied by the Dr. Wallace C. and Clara A. Abbott Memorial Fund of the University of Chicago.

Part I. General Description and Interpretation

Contents

  1. Salamanders and Their Brains
    1. The salamanders
    2. The scope of this inquiry
    3. The plan of this book
    4. Sources and material
    5. Development of the brain
    6. The evolution of brains
  2. The Form and Subdivisions of the Brain
    1. Gross structure
    2. Ventricles
    3. Meninges, chorioid plexuses, and blood vessels
  3. Histological Structure
    1. General histology
    2. The neuropil
    3. The ventrolateral peduncular neuropil
  4. Regional Analysis
    1. The subdivisions, spinal cord to pallium
    2. The commissures
    3. Conclusion
  5. Functional Analysis, Central and Peripheral
    1. The longitudinal zones
    2. The sensory zone
    3. The motor zone
    4. The intermediate zone
    5. The functional systems
  6. Physiological Interpretations
    1. Apparatus of analysis and synthesis
    2. The stimulus-response formula
    3. Reflex and inhibition
    4. Principles of localization of function
  7. The Origin and Significance of Cerebral Cortex
    1. The problem
    2. Morphogenesis of the cerebral hemispheres
    3. The cortex
    4. Physiology and psychology
  8. General Principles of Morphogenesis
    1. Morphogenic agencies
    2. Morphological landmarks
    3. The future of morphology
  9. Spinal Cord and Bulbo-spinal Junction
    1. The spinal cord and its nerves
    2. The bulbo-spinal junction
  10. Cranial Nerves
    1. Development
    2. Survey of the functional systems
  11. Medulla Oblongata
    1. Sensory zone
    2. Intermediate zone
    3. Motor zone
    4. Fiber tracts of the medulla oblongata
    5. The lemniscus systems
  12. Cerebellum
    1. Brachium conjunctivum
    2. The cerebellar commissures
    3. Proprioceptive functions of the cerebellum
  13. Isthmus
    1. Development
    2. Sensory zone
    3. Intermediate zone
    4. Motor zone
    5. White substance
    6. Isthmic neuropil
    7. Physiological interpretation
  14. Interpeduncular Nucleus
    1. Comparative anatomy
    2. Histological structure
    3. Afferent connections
    4. Efferent connections
    5. Interpretation
    6. Conclusion
  15. Midbrain
    1. Development
    2. Sensory zone
    3. Intermediate zone
    4. Motor zone
  16. Optic and Visual-motor Systems
    1. Optic nerve and tracts
    2. Tectum opticum
    3. Tectooculomotor connections
    4. Visual functions
  17. Diencephalon
    1. General features
    2. Development
    3. Epithalamus
    4. Dorsal thalamus
    5. Ventral thalamus
    6. Hypothalamus
  18. The Habenula and Its Connections
    1. The di-telencephalic junction
    2. Fornix
    3. Stria terminalis
    4. Stria medullaris thalami
    5. Fasciculus retroflexus
  19. The Cerebral Hemispheres
    1. Subdivisions of the hemisphere
    2. The olfactory system
  20. XX. The Systems of Fibers
    1. The basal forebrain bundles
    2. The tegmental fascicles
    3. Fasciculus tegmentalis profundus
  21. The Commissures
    1. General considerations
    2. The dorsal commissures
    3. The ventral commissures
  22. Bibliography
  23. Illustrations
    1. Abbreviations for All Figures


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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Part I. General Description and Interpretation 1. Salamander Brains | 2. Form and Brain Subdivisions | 3. Histological Structure | 4. Regional Analysis | 5. Functional Analysis, Central and Peripheral | 6. Physiological Interpretations | VII. The Origin and Significance of Cerebral Cortex | VIII. General Principles of Morphogenesis Part 2. Survey of Internal Structure 9. Spinal Cord and Bulbo-spinal Junction | 10. Cranial Nerves | 11. Medulla Oblongata | 12. Cerebellum | 13. Isthmus | 14. Interpeduncular Nucleus | 15. Midbrain | 16. Optic and Visual-motor Systems | 17. Diencephalon | 18. Habenula and Connections | 19. Cerebral Hemispheres | 20. Systems of Fibers | 21. Commissures | Bibliography | Illustrations | salamander

Reference

Herrick CJ. The Brain of the Tiger Salamander (1948) The University Of Chicago Press, Chicago, Illinois.

Cite this page: Hill, M.A. (2024, April 26) Embryology Book - The brain of the tiger salamander. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_brain_of_the_tiger_salamander

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