262 COMPARATIVE PHYSIOLOGY. 



It is important in considering this subject to have clear no- 

 tions of the structure of the blood-vessels. It will be borne in 

 mind that, while muscular elements are perhaps not wholly 

 lacking in any of the arteries, they are most abundant in the 

 smallest, the arterioles, which by their variations in size are best 

 fitted to determine the quantity of blood reaching any organ. 

 It is well known that nerves derived chiefly from the sympa- 

 thetic system pass to blood-vessels, though their exact mode of 

 termination is obscure. As the result of the section and stimu- 

 lation of certain nerves the following inferences have been 

 drawn in regard to the nerves supplying blood-vessels. 



1. There are vaso-motor nerves of two kinds — vaso-constrict- 

 ors and vaso-dilators — which may exist in nerve-trunks either 

 separately or mingled. Examples of the former are found in 

 the cervical sympathetic, splanchnic, etc., of the latter in the 

 chorda tympani, nerves of the muscles and nervi erigentes 

 (from the first, second, and third sacral nerves), while the sci- 

 atic seems to contain both. 



2. Impulses are constantly passing from the medullary vaso- 

 motor center along the nerves to the blood-vessels, hence their 

 dilatation after section of the nerves. The nerves are traceable 

 to the spinal cord, and in some part of their course run, as a 

 rule, in the sympathetic system. 



3. Impulses pass at intervals to the areas of distribution of 

 vaso-dilators along these nerves, the effect of which is to dilate 

 the vessels through their influence, as in other cases, on the 

 muscular coat. 



It is inferred that there are vaso-motor centers in the 

 spinal cord which are usually subordinated to the main center 

 in the medulla, but which in the absence of the control of the 

 chief center in the medulla assume an independent regulating 

 influence. 



There is a nerve with variable origin, course, etc., in differ- 

 ent mammals, but in the rabbit given off from either the vagus, 

 the superior laryngeal, or by a branch from each, which, run- 

 ning near the sympathetic nerve and the carotid artery, reaches 

 the heart, to which it is distributed. This is known as the de- 

 pressor nerve. 



From stimulation of the central end of this nerve results 

 follow which warrant the conclusion that impulses can by it 

 reach the vaso-motor center in the medulla, and interfere with 

 (inhibit) the outflow of efferent, constrictive, or tonic impulses, 



