Intrinsic connections and cytoarchitectonic data of the frontal association cortex in the dog

Organization of intrinsic connections of the frontal association cortex (FAC) in dogs was studied using retrograde HRP-transport method. For cytoarchitectonic observations and measurements of thickness of the cortex and its particular layers, additional sections stained with Nissl method were examined. Organization of intrinsic connections showed that within the dog's FAC two main cortical zones could be distinguished - the dorsal and the ventral one. The dorsal zone involves dorsally situated areas on the lateral and medial aspects of the hemisphere, which belong to the prefrontal and premotor regions. The vientral zone consists only of prefrontal areas situated ventrally on both aspects of the hemisphere. Each of the zones is characterized by strong mutual intrinsic connections and weak connections with the other zone. At the border there is a transitional area in which connections from both dorsal and ventral zones overlap. The cytoarchitectonic observations indicated that the dorsal and ventral zones can be distinguished in the central and caudal, but not in the rostral FAC subregion. The dorsal zone is characterized by considerable thickness of the cortex, cortical layers III and V, and the presence in these layers of scattered, large pyramidal neurons. The ventral zone has thinner cortex and layers III and V, and their pyramidal neurons are more uniform in size. In none of the zones clearly defined granular layer IV was observed.