520 THE ARTERIES. 



arteries, these two nucleated layers are enveloped by a thin tunic of con- 

 nective tissue. 



(Some authorities state that the walls of the very finest capillaries are 

 merely composed of closely-adhering cells, without any basement membrane, 

 which only becomes apparent in tubes of a large diameter.) 



Vessels and nerves The arteries are provided witli vessels termed vasa 

 vasorum, which are furnished either by the arteries themselves, or by neigh- 

 bouring vessels. These vasa vasorum form a superficial network with quad- 

 rilateral meshes, and a deep plexus whose principal branches are helicoidal. 

 The majority of anatomists believe that this plexus does not extend beyond 

 the external tunic. 



The lymphatic vessels maintain intimate relations, in certain regions, 

 with the capillaries. In the brain and spleen there has been discovered, 

 around the arterial capillaries, a vessel that completely envelops them, 

 and which has been named the lymphatic sheath. 



The nerves, designated vasa motors, accompany the vessels and penetrate 

 the muscular tunic, for which they are naturally destined. These vaso- 

 inotor filaments join the branches of the capillary plexuses, and form, at the 

 points where they meet each other, ganglionic enlargements, from which arise 

 the fibres of Remak, the termination of which is unknown. 



ANOMALIES IN THE ARTERIES. In their arrangement, the arteries very 

 often present anomalies which the surgeon should be guarded against. 

 These usually belong to their number, their point of origin, and their 

 volume. In a purely anatomical and physiological point of view, however, 

 these anomalies are of no moment ; as it matters little whether the blood 

 comes from one source rather than another, or that a collateral vessel be- 

 comes the principal at the expense of the parent trunk, provided its rela- 

 tions are not altered, and the principle of immutability of connections is 

 maintained. 



PREPARATION OF THE AKTERIES. This requires two successive operations: 1, In- 

 jection; 2, Dissection. 



Injection of the arteries. The object to be attained in injecting these vessels, is to 

 introduce into their interior a solidifiable substance which will 'cause them to assume the 

 volume and conformation they presented during life, when they are filled with blood. 



Tallow, coloured by lamp-black, is the most convenient and general injecting mate- 

 rial. Sometimes a solution of gelatine, with the addition of a certain quantity of plaster 

 of Paris, is used ; but this is seldom employed in the French schools. A copper or 

 brass syringe, and a canula with a stop-cock to fit on its extremity, are the only instru- 

 ments necessary to propel these matters into the arteries. 



The following are the details of the operation, when it is desired to make a general 

 injection : The animal being placed on a table, the carotid artery is exposed by an 

 incision in the jugular channel, and opened longitudinally. A ligature is applied above 

 the opening, and the tube, with the stop-cock, is firmly fixed in the cavity of the 

 artery towards the heart by a second ligature. The injection, previously prepared, is 

 tuken up by the syringe, which is fitted into the tube, and the piston pushed, in order 

 to drive the contents of the instrument into the arterial canals. 



To perform the operation successfully, the following precautions are to be attended 

 to: 1. Inject the vessels of an animal killed by effusion of blood, and yet warm. 2. If 

 snet is employed, and which is always to be recommended, make it so hot that the finger 

 can scarcely endure it. When it is colder than this it solidifies too quickly, and when 

 hotter it shrivels up the sigmoid valves, passes into the left ventricle, and from thence 

 into the auricle and pulmonary veins; an accident generally attributed to the too power- 

 ful force applied to the piston of the syringe. 3. Do not make any undue pressure on 

 the piston, though this does not strain the sigmoid valves so frequently as is believed. 

 4. Cease injecting when the arteries react, by their elasticity, on the piston, so as to 

 drive it back in the syringe. 



In order to insure the retention of the injected matter in the arteries, and prevent 

 the sigmoid valves being forced a cork may be introduced into the aorta througii the 



