A quantitative comparison of the hominoid thalamus: II. Limbic nuclei anterior principalis and lateralis dorsalis.

Structures in the limbic system are commonly thought to be similar in form and function in all mammalian brains. In the study reported here, two thalamic limbic nuclei, N. anterior principalis and N. lateralis dorsalis, were compared among a group of extant hominoids. The nuclear volumes, neuronal densities, numbers of neurons per nucleus, and columes of neuronal perikarya were measured. Humans have much larger nuclei but the nuclei constitute a similar proportion of the whole thalamus as found in the other hominoids. Whereas the human limbic nuclei were observed to have a decrease in the densities of nerve cells compared with those of the other hominoids, this difference is less than that found in most other thalamic nuclei. Consequently the estimated number of neurons is much higher for humans. The total number of neurons best separates the human limbic nuclei from those of the other hominoids. This preliminary study suggests that during hominid evolution neurons were preferentially added to the limbic nuclei of the thalamus.