ANATOMICAL CONSIDERATIONS. 23 



necessity for such repeated checks. If the conservation of the brain 

 was the only end to be attained, might not that object have been assured 

 by a single set of valves ? Such may seem a feasible objection ; but to 

 prevent the return current was, as nature appears to have conceived, 

 best done by repeated assistance of the onward flow; consequently, 

 these numerous valves anticipate the possibility of regurgitation in any 

 degree, and provide repeated checks to pressure from the supported 

 column of heavy venous blood. 



There remains, however, another provision to be explained. The 

 return current has hitherto been spoken of, as though the upward flow 

 of fluid was its natural tendency. Still, every person must have per- 

 ceived the necessity, when liquid was to be propelled in that direction, 

 of something resembling a forcing pump. Such an apparatus nature 

 has provided. The head of a healthy animal is depressed only when 

 eating or when drinking. During the performance of either function, 

 muscles are contracting which compress the soft coats of the veins, and 

 thus help to drive the circulation against gravity. 



Thus, during feeding, the head is maintained in a depressed attitude 

 for hours together ; and, throughout that space, a most powerful agent 

 is in operation. The lower jaw, while the quadruped chances to be 

 thus engaged, is in constant motion, being opened and closed either in 

 biting or in chewing. When the jaw sinks, the muscles of mastication 

 are relaxed, and the venous blood rushes from the cranium into the 

 sinuses. But when the bone is raised by those strong motor agents 

 which render the bite of a horse so fearful an infliction, the current from 

 the brain is for a moment checked, and the contents of the maxillary 

 sinuses are energetically propelled up the jugulars. During the first 

 half of the action, the valves are in operation, having all the strength 

 necessary for the perfect performance of their allotted function; but, 

 during the latter part, they are forced against the sides of the vessels by 

 the contractive masticatory influence, and cease to act in any way upon 

 the internal current of the blood. 



Notwithstanding the strong conviction emphatically asserted by the 

 fixed position of the nation's mangers, the author must be obstinate 

 enough to disregard human authority, when he has an opportunity of 

 studying the living book, written by the unerring hand of nature. 

 Yalves, though generally present in veins, are never discovered where 

 the position of the vessel or other reason would render such provisions 

 unnecessary. The Great Creator often makes one thing to serve more 

 than one use ; but never creates when His work can answer no profit- 

 able purpose. 



The use of veins is simply that of conduits, to convey the refuse 



