Results of experiments in which 9 branchial and hyoid muscles were electrically stimulated are presented. Upon contraction of the m. protractor pectoralis the pectoral girdle is protracted and makes contact with the lower pharyngeal jaw resulting in the elevation of the lower pharyngeal jaw. The force of the m. protractor pectoralis contributes to the biting force of the m. levator externus 4 and m. levator posterior during biting and in this way increases the total biting force that can be exerted on prey. Galis, F. 1999a. Developmental constraints, Hox genes and Cancer.
Mammals have seven cervical vertebrae, a number that remains remarkably constant.
In vertebrates, Hoxgenes are involved in the development of the skeletal axis and the nervous system, among other things. A review of the veterinary literature shows that cancer rates appear lower in birds and reptiles than in mammals. The low susceptibility to cancer in these classes probably prevents the deleterious pleiotropic effect of neonatal cancer when changes in cervical vertebral number occur. The coupling of functions is either a conserved trait that is also present in reptiles and birds, but without apparent deleterious effects, or the coupling is new to mammals due to a change in the functioning of Hox genes. The cost of the coupling of functions in mammals appears to be an increased risk for neural problems, neonatal cancer, stillbirths, and a constraint on the variability of cervical vertebral number.