THE VEINS 467 



three coats, the serous and cellular being very similar in structure, but the 

 fibrous is very much thinner and devoid of muscular fibres. A feature 

 peculiar to the veins is the existence of valves, which are sometimes single, 

 at others double, and occasionally arranged in threes and fours around 

 the interior of the large veins. They vary in numbers, and are altogether 

 absent in the pulmonary veins, in the vense cavse, and the vena portse. 



The anterior vena cava is made up of the jugular vein, the pectoral, 

 vertebral, axillary, and cervical veins, and the vena azygos. The jugular 

 vein, which is that usually selected for bleeding, returns the blood from the 

 brain, jaws, and neck, along each side of which it lies, separated from the 

 carotid artery in the upper part of the neck by a layer of oblique fibres 

 belonging to the levator humeri. In the lower half the vein becomes more 

 deeply seated, approaches more closely the carotid artery, and, entering the 

 chest with it, falls into the vena cava anterior between the first and second 

 ribs. Near its termination it receives the superficial brachial vein (the plate 

 vein), which passes up in front of the arm, along the anterior edge of the 

 flexor, and winding upwards in the hollow between the arm and sternum 

 joins the jugular vein. The vertebral and axillary veins correspond with 

 the arteries of the same name, the divisions of the latter contained within 

 the foot being described with that organ. The left axillary vein receives 

 the contents of the thoracic duct which opens into it close to its junction 

 with the vena cava. 



The posterior vena cava commences by the junction of the two common 

 iliac veins (each made up of an external and internal iliac corresponding 

 to the arteries of the name). It is then joined by the lumbar veins, the 

 spermatic and renal veins, after which, and close to its termination, the 

 hepatic and phrenic veins empty themselves into it. 



The pulmonary veins, commencing with eight trunks as they emerge 

 from the lungs, soon unite into four, in which number they enter the left 

 auricle. They carry arterial blood, and differ in this respect from all the 

 other veins of the body, as has been already mentioned. 



PHYSIOLOGY OF RESPIRATION 



The essence of the act of breathing consists in the absorption of oxj^gen 

 from the air, and the excretion of carbonic acid from the blood which is 

 circulated through it. In a state of rest this intei'change must go on with 

 regularity, for carbonic acid is constantly developed by the decay of the 

 tissues, arising from the peculiar necessities of the muscular and nervous 

 tissues, and by the conversion of the carbon of the food which appears to be 

 required for the development of heat. But when the muscles of the whole 

 body are called into play with unusual rapidity and force, the development 

 of carbonic acid is largely augmented, and thus, not only is there a necessity 

 for extra means of excreting the carbonic acid, but there is also a demand 

 for more oxygen to unite with the carbon, which is the result of the dis- 

 integi^ation of the muscular fibres employed. Hence the acts of respiration 

 are more complete and rapid during exercise than in a state of rest, and 

 while much more carbonic acid is given off, a greater volume of oxygen, is 

 absorbed from the air which is inspired. 



