888 ORGANS OF RESPIRATION. 



The following works on the spleen may be referred to : Gray, Structure and Use 

 of the Spleen, 1854; Sanders, in Goodsir's Annals; Busk and Huxley on the 

 Malpighian Bodies, in the Sydenham Society's translation of Kolliker's Histology ; 

 also Huxley in Micr. Jour., ii. p. 74; Billroth, in Zeitschr. f. Wissensch. Zool., xi. 

 p. 325 ; W. Miiller, Ueber d. fein. Bau der Milz, 1865. Stieda, in Virchow's Archiv, 

 xxiv. p. 540, an abstract of which is given in Medico-Chir. Rev., October, 1862. 



Development. The spleen appears in the foetus, about the seventh or eighth week, 

 on the left side of the dilated part of the alimentary tube or stomach, and close to the 

 rudiment of the pancreas. By the tenth week it forms a distinct lobulated body 

 placed at the great end of the stomach. After birth it increases rapidly in size ; and 

 in a child a few weeks old, it has attained the same proportional weight to the body 

 as in the adult. This organ is peculiar to vertebrate animals. 



ORGANS OF RESPIRATION. 



THE organs of respiration consist of the thorax (already described), the 

 larynx, the trachea, and the lungs. The larynx, affixed to the upper end of 

 the windpipe, is not only the entrance for air into the respiratory organs from 

 the pharynx, but also the organ of voice, and will be described later. 



THE TRACHEA AND BRONCHI. 



The trachea or windpipe, the common air-passage of both lungs, is an 

 open tube which commences at the larynx above, and divides below into 

 two smaller tubes, the right and the left bronchus, one for each lung. 



The trachea is placed in the middle plane of the body, and extends from 

 the lower border of the cricoid cartilage of the larynx, on a level with the fifth 

 cervical vertebra in the neck, to a place opposite the third dorsal vertebra in 

 the thorax, where it is crossed in front by the arch of the aorta, and at or 

 immediately below that point it bifurcates into the two bronchi. It usually 

 measures from four inches to four inches and a half in length, and from 

 three-quarters of an inch to one inch in width ; but its length and width 

 are liable to continual variation, according to the position of the larynx and 

 the direction of the neck ; moreover, it usually widens a little at its lower 

 end, and its diameter is always greater in the male than in the female. In 

 front and at the sides the trachea is rendered cylindrical, firm, and resistant 

 by a series of cartilaginous rings ; these, however, are deficient behind, so 

 that the posterior portion is flattened and entirely membranous. 



The trachea is nearly everywhere invested by a loose areolar tissue, 

 abounding in elastic fibres, and it is very movable on the surrounding parts. 

 Both in the neck and in the thorax, it rests behind against the oesophagus, 

 which intervenes between it and the vertebral column, and towards its lower 

 part projects somewhat to its left side. The recurrent laryngeal nerves 

 ascend to the larynx on each side in the angle between these two tubes. 



In the neck the trachea is situated between the common carotid arteries ; 

 at its upper end it is embraced by the lateral lobes of the thyroid body, the 

 middle part or isthmus of which lies across it just below the larynx. It 

 is covered in front by the sterno-thyroid and sterno-hyoid muscles, between 

 which, however, there is left an elongated lozenge-shaped interval in the 

 middle line : this interval is covered in by a strong process of the deep 

 cervical fascia, while, more superficially, another layer not so strong crosses 

 between the sterno-mastoid muscles. The inferior thyroid veins and the 

 arteria thyroidea ima, when that vessel exists (p. 340), also lie upon its 

 anterior surface ; whilst at the root of the neck, in the episternal notch, the 

 innominate artery and the left carotid pass obliquely over it as they ascend 

 to gain its sides. 



