Figure 1 is a stylized drawing of the right thalamus as seen from above and behind, (the left thalamus is an anatomic mirror image). A thin slice has been taken out about 2/3 of the way back from the anterior (front) end in order to show some of the internal structure. What figure 1 shows is what modern anatomists call the "dorsal thalamus” because of its embryonic origin. In reality, the dorsal thalamus is enshrouded by the reticular nucleus (R), a thin shell of cells which are of ventral embryonic origin. The R is not shown in this figure because it would hide the dorsal thalamus from view. Anatomists of the Renaissance were unaware of both the embryology and the microscopic detail; they applied the term "thalamus" to the entire assembly including R, and this is what neurologists usually mean when they use the word thalamus. [The word "thalamus" is Latin for bridal couch; the older anatomists enjoyed a rather randy vocabulary].

      Fig. 1 shows, in white, the internal lamina. [There are other laminae (layers of fibers) outside of the thalamus but are not relevant here.] The lamina consists of millions of nerve fibers going to or from the nearby neurons. To the right of the lamina are the lateral and ventral parts; they are largely concerned with specific sensory or motor functions and not further discussed in this chapter. Toward the anterior end the lamina bifurcates, the lateral branch continuing on toward the cortex. The medial branch dives under the collections of neurons to which most of its fibers are headed. 

     Each thalamus (the right thalamus is shown here) contains many sub-parts called nuclei.  There are, in each thalamus, about 40 nuclei according to E.G.Jones (and about 80 according to Hassler). We do not need to bother with most of these nuclei (they are unrelated to the text) so we do not show the subdivisions of the lateral and anterior parts; and we do not label most of the nuclei, with four exceptions: MD, Pl, LD and CM. 

       The medial dorsal nucleus (MD) is most of the medial thalamus. The MD is almost certainly important for emotion because of its large input from the amygdala of the same hemisphere. And MD is likely important for thoughts about the future, as its output is to the prefrontal cortex; more specifically the large celled part of MD projects to the orbitofrontal cortex which is an important part of the so-called  "emotional brain".

       On top of MD is the rather thin nucleus called Lateral Dorsal (LD); it is included by E.G. Jones (1985) in   his discussion of the Anterior nuclei. These have been implicated in both memory and emotion by various authors and may be involved in other processes since they project to the anterior cingulate gyrus which itself has been implicated in a wide variety of cognitive, volitional and emotional processes. Note that, in the human, the LD is neither lateral nor dorsal; it acquired its name from its position in more primitive brains such as rabbit, cat, or the most primitive primates like the bushbaby.

        At the posterior end of the dorsal thalamus there is a quite large nucleus, having several subdivisions; it is called the Pulvinar (Pl). [the word "pulvinar" means pillow.] The pulvinar is very widely connected to almost all of the cortex known to respond to external stimuli of whatever nature; it is particularly large in primates where it is thought to help in determining salience of information, and therefore plays a role in directing attention.

        The ILN are shown in black, surrounding the MD; in reality the nerve cells of the IN are not all in a discrete band, as shown here, because many of them are dispersed within the lamina. The subdivisions of the ILN have been given a variety of names, which are unnecessary in this introductory chapter. Because of their arrangement in a shell around the MD, they were lumped together by Grunthal {1934) as the “nucleus circularis”.  This nicely descriptive usage was followed by a half dozen anatomists subsequently   but is no longer current. 

The centromedian nucleus (CM), more commonly called centrum medianum in Latin or centre mediane in French, is visible where the slice was taken out. The CM is much larger in monkeys than in most mammals and is even bigger in humans. It projects to the motor cortex, which is one of the reasons for thinking that it is probably essential to volition. It has long been customary to consider CM to be part of the ILN because it is located in the lamina (as shown on the cut surface in Fig. 1). Moreover, it has a major output to the striatum, which is a defining characteristic   of the ILN. At present, in simple terms, we can think of CM as the motor part of ILN and the rest of ILN (the veil surrounding MD) as sensory; in other words, roughly speaking, this reflects the difference between the intentional and perceptual aspects of C. [This stylized drawing was done by Michael Stern, using Photoshop, with advice from JEB, referring to the coronal sections in Fig.6 of Jones,!998c and the coronal sections of Figs. 1-8 in Munkle et al, 2000}

Abbreviations:

• ANT= anterior part made up of several nuclei.
• CM= centrum medianum (aka centre mediane).
• ILN = intralaminar nuclei of thalamus.
• LAT = the lateral part of the thalamus, with many nuclei.
• LD = the lateral dorsal nucleus.
• MD = Mediodorsal nucleus.
• Pl = Pulvinar
• R  = Reticular nucleus of thalamus (not shown in Fig. 1)