258 ANATOMY OF THE PCETUS. 



small pouch of peritoneum will be found protruding at 

 that point, and into this the testicle projects from behind, 

 supported by a duplicature, or suspensory fold, of the peri- 

 toneum, called the mesorchium ; this pouch, under the 

 name of processus vaginalis peritonei, precedes the testicle 

 in its course through the inguinal canal, and enters the 

 scrotum in advance of the gland. The neck of this pouch, 

 by which it is connected with the general peritoneal cavity, 

 becomes gradually obliterated after birth, while the pouch 

 itself remains as an independent serous sac, under the 

 name of tunica vaginalis testis. 



The ovaries in the female are likewise placed in the lum- 

 bar region, and gradually descend to the pelvis. A pouch 

 of peritoneum, analogous to the processus vaginalis of the 

 male, accompanies the round ligament of the uterus for 

 a short distance into the inguinal canal, and is called the 

 canal of Nuck. 



On opening the thorax there is found in the anterior 

 mediastinum, and extending upward upon the trachea into 

 the neck, a narrow, elongated^ body, lobulated, pinkish in 

 color, and soft in texture ; this is the THYMUS GLAND. It 

 consists of two lateral lobes enveloped in an areolar cap- 

 sule. A central cavity, containing a milky fluid, exists in 

 each lobe; it has no duct or outlet. The thymus gland 

 increases in size for about two years ; it then dwindles, 

 and becoming converted into a fatty mass, at the age of 

 puberty has nearly disappeared. 



The LUNGS are small, and lie packed in the posterior 

 part of the thorax. Previous to respiration they consist 

 of a dense, gland-like substance. Subsequently to respira- 

 tion they are of a pinkish, spongy structure, expanding 

 and completely filling the pleural cavity. 



The HEART is well developed in point of size, but the 

 septum between the auricles is incomplete, being perforated 

 by a large opening called the foramen ovale. A communi- 

 cation will also be found between the left pulmonary artery 

 and the aorta, just beyond the origin of the brachio-cephalic 

 vessels, by a short trunk called the ductus arteriosus ; this 

 degenerates into a fibrous cord shortly after birth, and the 

 foramen ovale usually becomes obliterated. The Eus- 

 tachian valve is of large size, and seems to be a continua- 

 tion upward of the anterior wall of thmcnferior vena cava 

 toward the foramen ovale. 



The FCETAL CIRCULATION presents peculiarities. The 



