The morphology and distribution of cells which do not conform to the conventional pyramidal pattern have been investigated in rapid Golgi, Golgi-Kopsch and Golgi-Cox preparations from cortical areas 3, 1 and 2 of juvenile and mature squirrel monkeys. The material has been analyzed qualitatively and quantitatively by means of a computer program which permits cells to be rotated so as to display their three-dimensional architecture. Nine non-pyramidal types are identified of which one is a rare giant cell and another, forming a major proportion of the cells in layer VI, is considered to be a modified form of pyramidal cell. Of the other seven types, two have horizontally distributed axons, one essentially confined to layer II, the other sending long (up to 1 mm) branches anter-posteriorly through all layers. Two types have vertical axons. One, corresponding to the "double bouquet dendritique" cell of Cajal, is mainly situated in layer II or the upper part of layer III and has a cluster of large axon branches which descend to layers IV and V and which enclose and terminate on the apical dendrites of pyramidal cells. The other type is the only non-pyramidal cell which has a relatively high concentration of dendritic spines in the adult animal. Its soma lies in layer IV and it has several strongly recurrent, thick axonal branches ascending to layer II, also enclosing the apical dendrites of pyramidal cells. The dendritic field is not truly stellate but is drawn out into a pronounced ascending tuft which ascends into layer IIIb. The cell thus resembles a "star-pyramid" of Lorente de Nó. Nevertheless such cells have many features, notably the distribution of their axons and the distribution of dendritic spines which are identical to those of the well-known "spiny stellate" cell of the visual cortex. Conversely the same features both in these cells and in the spiny stellate cells of the visual cortex (which were also eamined) differ markedly from those of small pyramidal cells with somata of similar dimensions. The three remaining non-pyramida cell types have locally ramifying axons which appear to terminate predominantly on pyramidal cells. In one, the axon forms smoothly curving arcades in layer III, in another it is intensely tangled in layer IV and in the third it is bush-like in layers II-IV. continued.

The posterior nuclear complex of the thalamus in rhesus, pigtailed and squirrel monkeys consists of the combined suprageniculate-limitans nucleus and an ill defined region of heterogeneous cell types extending anteriorly from the dorsal lobe of the medial geniculate body towards the posterior pole of the ventral nuclear complex. This region is referred to as the posterior nucleus. It is directly continuous with the ventroposteroinferior nucleus. The cortical projections of each of these nuclei, together with those of the adjacent ventral, pulvinar and medial geniculate complexes, have been studied by means of the autoradiographic tracing technique. The suprageniculate-limitans nucleus, the main input to which is the superior colliculus, projects upon the granular insular area of the cortex. The medial portion of the posterior nucleus projects to the retroinsular field lying posterior to the second somatic sensory area. There is clinical and electrophysiological evidence to suggest that the retroinsular area may form part of a central pain pathway. The lateral portion of the posterior nucleus which is closely related to certain elements of the medial geniculate complex, projects to the postauditory cortical field. The ventroposterioinferior nucleus, which may be involved in vestibular function, projects to the dysgranular insular field. The principal medial geniculate nucleus can be subdivided into a ventral division that projects to field AI of the auditory cortex and a dorsal division that merges with the posterior nucleus; it is further subdivided into an anterodorsal component that projects to two fields on the superior temporal gyrus, together with a posterodorsal component in which separate cell populations project to areas lying anterior and medial to AI. The magnocellular medial geniculate nucleus, sometimes considered a part of the posterior complex, appears to project diffusely to layer I of all the auditory fields. The auditory fields are bounded on three sides by the projection field of the medial nucleus of the pulvinar which also extends into the upper end of the lateral sulcus to bound the fields receiving fibers from the posterior nucleus. The topography of the areas receiving fibers from the posterior, medial geniculate and pulvinar complexes, taken in conjunction with the rotation of the primate temporal lobe, permits all of these fields to be compared with similar, better known areas in the cat brain.

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The cells of origin of cortico-cortical and subcortical projections from the subfields of the somatic sensory area and from the motor cortex have been identified in cynomolgus and squirrel monkeys by the retrograde axonal transport method.The somata of the cells of origin of a particular fiber system have a specific laminar or sublaminar distribution. The somata of the majority of cortico-cortical cells lie in the supragranular layers. Those projecting to the opposite cortex are confined to the deeper half of layer I11 (layer IIIB). Ipsilateral cortico-cortical neurons lie mainly superficial to them in layers IIIA and 11, but in the second somatic sensory area (SII) and in area 2 of the first @I), small numbers are also found in layer V. Corticospinal cells lie in the deeper part of layer V and corticostriatal cells in the superficial part. Corticopontine, corticobulbar and corticorubral cells lie in between. The majority of corticothalamic cells lies in layer VI but a second, smaller population is found in the deep part of layer V.The cells giving rise to a particular set of efferent connections can be distinguished in terms of size and, with the exception of the corticospinal cells, their size does not vary greatly from area to area. In many cases, the size and laminar specificity indicates that cells sending axons to one site cannot have collateral branches projecting to another.In most of the fiber systems studied, labeled cells form single or multiple strips, 0.5-1 mm wide and oriented mediolaterally across the cortex. The strips appear in all of the subfields of the somatic sensory and motor areas and may form the basis of the clustering of like groups of efferent neurons demonstrable in physiological studies.It is an old belief (e.g., Campbell, '05; Bolton, '10) that the supragranular layers of the cerebral cortex are primarily concerned with "receptive and associative" functions and that the infragranular layers are primarily "corticofugal and commissural." This belief rested mainly upon the observation that the supragranular layers were affected earliest in cases of progressive cortical atrophy leading to dementia. Although early experimental work tended to support the idea that the infragranular layers give rise to certain efferent connections, there are many grounds upon which the concept of a simple dual organization has been discredited. Nevertheless, the question of the interrelationship between afferent fibers and efferent cells in the cortex remains largely unexplored with recently de- 391

Neurons in the cat and monkey cerebral cortex were stained immunocytochemically for glutamic acid decarboxylase (GluDCase; L-glutamate 1-carboxy-lyase, EC 4.1. GluDCase was co-localized with CCK, SRIF, or NPY not only in cell somata, but also in small punctate structures, which are likely to be axon terminals. From the data gained in previous electron microscopic studies, we postulate that neurons displaying GluDCase-and CCK-like immunoreactivity are a class separate from those displaying GluDCase-and SRIF-like immunoreactivity. NPY, however, is co-localized with SRIF immunoreactivity. These results imply that classes of cortical interneuron contain a conventional neurotransmitter (y-aminobutyric acid) and a neuromodulator (one of the peptides).