Left, systemic administration into the dorsal lymph sac. See text for further details. An appreciation of the phylogenetic classes of animals and differences between species from one class to another is essential for a rational exploration of animal welfare and ethical issues for biomedical research. Comparative neurology of pain. Even this primordial cortex in reptiles is scant and lacking the complex laminar structure seen in mammals.
Amphibians simply have a brain without a cerebral cortex.
The phylogeny of the medial pathway correlated with motivational-affective aspects of pain is similar whereby in amphibians the most rostral projection reaches to a diffuse olfactory area with little organization of neurons. In mammals, the most rostral target of this pathway is the highly-organized limbic cortex. Again, the beginning of even a rough laminar organization of the limbic area does not appear until class Reptilia . The amphibian brain does not contain a limbic cortex. Cortical tissue, whether in limbic or cerebral regions, is a highly complex and laminated structure which is a relatively recent development in the evolution of the nervous system.