20 Field Museum of Natural History — Zoology, Vol. XL 



Ideal section of a Mammal. 



a, Olfactory nerves; b, optic nerve; c, brain; d, ear conch or pinna; e, epiglottis; f, oesophagus; g, 

 thyroid glands; h, vertebral column or back-bone; i, spinal cord; j, trachea; k, lungs; 1, heart; m, 

 diaphram; n, liver; o, pancreas; p, stomach; q, spleen; r, kidney; s, intestines; t, bladder; u, scrotum 

 and testes; v, anus. (After Owen.) 



The Heart — The heart in all mammals is divided into four chambers 

 known as auricles and ventricles, but the shape is more or less 

 variable in different forms. Each n auricle communicates with the 

 ventricle of the same side, the opening being provided with valves 

 which permit the blood to pass only in the right direction, viz., from 

 the auricle to the ventricle. Briefly stated, the circulation of the 

 blood is accomplished as follows: It enters the right auricle, which 

 is supplied by means of the venae cavae, and thence to the ventricle 

 of the same side, from whence it is forced into the lungs through 

 the pulmonary artery and there absorbs the necessary oxygen and 

 is relieved of carbon dioxide. From the lungs by means of the 

 pulmonary veins it enters the left auricle and passes through the 

 left ventricle into the aorta, passing out again through the large 

 arteries (such as the ''carotid" and ''brachial"), which arise from 

 it, and thence into the general system, where, after passing through 

 the capillaries, it is conducted by a diversity of veins to the large 

 superior and inferior venae cavae and thence back again to the right 

 auricle. 



There is considerable variation in mammals in the mode in 

 which different arteries arise from the aorta. In most cases the 

 right brachial and right carotid have a common origin known as 

 the innominate artery in human anatomy. The other two may 

 extend from this, as in the Ungulates, or may arise separately from 

 the aorta, as in Man, or as is common in many species, the left 

 carotid may arise from the "innominate" and the left brachial 



